L

Lab

▶CIELAB for Color Image Encoding (CIELAB,8-Bit; Domain and Range, Uses)

Labradorescence

▶ Iridescence (Goniochromism)

Lambert, Johann Heinrich

Rolf G. KuehniCharlotte, NC, USA

Biography

Lambert was born on August 26, 1728 in the cityofMulhouse, then an enclave of Switzerland (nowpart of France). He was largely self-educated,going to school only until age 12. By age 17 heassumed the job of secretary to a newspaper pub-lisher in nearby Basel, Switzerland. He also beganto work as a private tutor. At age 20 he becametutor to three boys in the family of Count Petervon Salis in Chur, Switzerland, a position he heldfor 10 years. There he had access to the count’s

# Springer Science+Business Media New York 2016M.R. Luo (ed.), Encyclopedia of Color Science and TechnoloDOI 10.1007/978-1-4419-8071-7

large library and was able to travel widely inEurope with his charges. In 1755 he began topublish scientific articles on a number of subjects.In 1756 he traveled with his pupils to Göttingen inGermany where he met Tobias Mayer and waselected a member of the Konigliche Gesellschaftder Wissenschaften (Royal Society for the Sci-ences). In 1759 he published his work on lightmeasurement, Photometria [1], introducing hismathematical formula for the law of absorptionof light (Lambert’s law), described nonmathe-matically a few years earlier by Pierre Bouguer.In 1764 he followed an invitation by the Swissmathematician Leonhard Euler to come to Berlinwhere, after some initial difficulties, Frederic IIappointed him to a position in the Koniglich-Preussische Akademie der Wissenschaften(Royal Prussian Academy of Sciences). Lambertestablished an important position as mathemati-cian, physicist, astronomer, and philosopher. Healso had considerable interest in the art of paint-ing. Among many other achievements, Lambertwas the first to mathematically prove the irratio-nality of the number p. He died on September25, 1777 in Berlin, Germany [2].

Major Accomplishments/Contributions

In 1758 Tobias Mayer presented his public lectureon a three-dimensional color order system, of

gy,

Lambert, Johann Heinrich, Fig. 1 Lambert’s illustra-tion of his triangular color pyramid, 1772

826 Lambert, Johann Heinrich

which a report was published in GottingischeAnzeigen f€ur gelehrte Sachen (Göttingen reportson learned matters), read by Lambert. In 1768Lambert published an article Mémoire sur lapartie photométrique de l’art du peintre(Dissertation on the photometric component ofthe art of the painter) [3] in which he discussedthe effect of light on the appearance of coloredmaterials. Soon after and as a result of Mayer’spremature death, he began work on an implemen-tation of Mayer’s conceptual double pyramidalsystem, with assistance of the Prussian courtpainter Benjamin Calau (1724–1785). The resultwas published in 1772 as Beschreibung einermit Calauischem Wachse ausgef€uhrtenFarbenpyramide (Description of a color pyramidpainted with Calau’s wax) [4] containing a hand-colored abbreviated version of the conceptualcolor pyramid (Fig. 1) [3]. Lambert and Calauhad to solve a number of practical issues, forexample, they determined the relative strength ofthe pigments they used. When mixing the threeprimary paint samples, they obtained near-black

Lanthanoid Ion Color 827

colors. As a result, Lambert saw no need for thelower half of Mayer’s double pyramid. TheLambert-Calau pyramid is the first three-dimensional illustrated representation of a sys-tematically developed color solid.

Lanthanoid Ion Color, Table 1 Characteristic color oflanthanoid ions

References

1. Lambert, J.H.: Photometria, sive deMensura et GradibusLuminis, Colorum et Umbrae. Klett, Augsburg (1760)

2. Kraus, A.: Lambert, Johann Heinrich. In: NeueDeutsche Biographie, Berlin: Duncker & HumblotGmbH, vol. 13, pp. 437–439. (1982)

3. Lambert, J.H.: Mémoire sur la partie photométrique del’art du peintre. Mémoires de l’Académie des Sciencesde Berlin XXIV, 313–334 (1768)

4. Lambert, J. H.: Beschreibung einer mit CalauischemWachse ausgef€uhrten Farbenpyramide. Haude undSpener, Berlin (1772). English translation available atwww.iscc.org

Element Iona4foccupancyb Color

Lanthanum La3+ 4f0 Colorless

Cerium Ce4+ 4f0 Colorless

Ce3+ 4f1 Paleyellow

L

Land Retinex Theory

▶Retinex Theory

Praseodymium Pr4+ 4f1 Colorless

Pr3+ 4f2 Green

Neodymium Nd3+ 4f3 Lilac/violet

Promethium Pm3+ 4f4 Pink

Samarium Sm3+ 4f5 Pale

Lanthanide Ions

▶Lanthanoid Ion Color

yellow

Sm2+ 4f6 Red/green

Europium Eu3+ 4f6 Pink

Eu2+ 4f7 Red brown

Gadolinium Gd3+ 4f7 Colorless

Terbium Tb4+ 4f7 Colorless

Lanthanides

▶Lanthanoid Ion Color

Tb3+ 4f8 Pale pink

Dysprosium Dy3+ 4f9 Paleyellow

Dy2+ 4f10 Brown

Holmium Ho3+ 4f10 Yellow

Erbium Er3+ 4f11 Pink

Thulium Tm3+ 4f12 Green

Tm2+ 4f13 Green

Lanthanoid Ion Color

Richard J. D. TilleyQueen’s Buildings, Cardiff University,Cardiff, UK

Ytterbium Yb3+ 4f13 Colorless

Yb4+ 4f14 Colorless

Lutetium Lu3+ 4f14 ColorlessaLn3+ is the principal ionic state encounteredbThere is uncertainty about the exact f-orbital occupation inmany compounds

Synonyms

Lanthanide ions; Lanthanides; Rare earths; Rare-earth elements; Rare-earth ions

Definition

The lanthanoids (often designated Ln) are the15 elements with atomic numbers 57 (lanthanum)to 71 (lutetium).

Colors, Electron Configurations, andEnergy Levels

ColorMost of the lanthanoid ions exhibit rather palecharacteristic colors when introduced into trans-parent solids or in water solutions, the mostimportant being the Ln3+ state (Table 1). These

Lanthanoid Ion Color, Table 2 Ground state terms andlevels of principal lanthanoid ions

Ion Term Levelsa

La3+ f0 1S 1S0Ce3+ f1 2F 2F5/2

2F7/2Pr3+ f2 3H 3H4

3H53H6

Nd3+ f3 4I 4I9/24I11/2

4I13/24I15/2

Pm3+ f4 5I 5I45I5

5I65I7

5I8Sm3+ f5 6H 6H5/2

6H7/26H9/2

6H11/26H13/2

6H15/2

Eu3+ f6 7F 7F07F1

7F27F3

7F47F5

7F6Eu2+ f7 8S 8S7/2Gd3+ f7 8S 8S7/2Tb3+ f8 7F 7F6

7F57F4

7F37F2

7F17F0

Dy3+ f9 6H 6H15/26H13/2

6H11/26H9/2

6H7/26H5/2

Ho3+ f10 5I 5I85I7

5I65I5

5I4Er3+ f11 4I 4I15/2

4I13/24I11/2

4I9/2Tm3+ f12 3H 3H6

3H53H4

Yb3+ f13 2F 2F7/22F5/2

Lu3+ f14 1S 1S0aIn ascending energy order, ground state level bold

828 Lanthanoid Ion Color

colors arise from electronic transitions betweenthe ionic ground state and energy levels derivedfrom 4f electron configurations lying between1.77 and 3.10 eVabove it, giving absorption max-ima in the visible wavelength range(700–400 nm). Of more practical importance iscolor produced when ions excited to higherenergy levels fall back to these 4f-derived levelsand thence to the ground state, giving rise tocharacteristic visible emission spectra, which areused in many applications including fluorescentprinting inks used as security markers onbanknotes.

The 4f electrons in the lanthanoids are wellshielded beneath an outer electron configuration,(5s2 5p6 6s2), and so are little influenced by thesurrounding solid matrix, and although crystal-field effects (see Transition-Metal Ion Colours)contribute to the fine structure of the electronicspectra of the lanthanoid ions, these do not have agross effect upon the color. This implies that themost important optical properties attributed to the4f electrons on any particular lanthanoid ion donot (usually) depend significantly upon the hoststructure, so that lanthanoid elements find use inphosphors, lasers, and other light-emittingdevices, where a host lattice can be chosen withrespect to processing conditions without signifi-cantly changing the desirable color properties ofthe ion.

Lanthanoid Free-Ion Energy LevelsThe energy levels of a free lanthanoid ion areusually labeled with atomic term symbols derivedby Russell-Saunders (LS) coupling (althoughother coupling schemes are also used in thisrespect). A term is a set of states which are verysimilar in energy, and the appropriate term symbolis written as 2S+1L where L is a many-electronquantum number describing the total orbital angu-lar momentum of all of the electrons surroundingthe atomic nucleus and S is a many-electron quan-tum number representing the total electron spin.The superscript (2S+1) is called the multiplicity ofthe term and is given a name: 1, singlet; 2, doublet;3, triplet; 4, quartet; and so on. The total angularmomentum quantum number L is given a lettersymbol: L= 0, S; L= 1, P; L= 2, D; L= 3, F; and

thereafter alphabetically, omitting J. The energiesof the terms must be determined by quantummechanical calculations, except for that of theground state, which is given by Hund’s secondrule: the ground state is the term with the highestmultiplicity and, if more than one term of the samemultiplicity is present, by that with the highestL value.

The term symbol does not account for the truecomplexity of the energy levels of the lanthanoidions. This arises from the interaction between thespin, S, and the orbital momentum, L, called spin-orbit coupling. For this the quantum number, J, isneeded. It is given by:

J ¼ L þ Sð Þ, L þ S� 1ð Þ . . . L� Sj j

where |L – S| is the modulus (absolute value) of thequantity L – S. Each value of J represents a dif-ferent energy level. The new quantum number isincorporated as a subscript to the term, now writ-ten 2S+1LJ and called a level. It is found that asinglet term always gives rise to 1 level, a dou-blet 2, a triplet 3, and so on (Table 2). The energiesof these levels can be sorted in terms of energy byHund’s third rule: the level with the lowest energy

Lanthanoid Ion Color 829

L

is that with the lowest J value if the valence shell isup to half full and that with the highest J value ifmore than half full. The separation between thecomponents of the spin-orbit energy levels is ofthe order of 0.1–0.25 eV. (For comparison,crystal-field splitting of these energy levels,which is due to the interaction of the f orbitalswith the surrounding atoms in a solid or liquid, isabout 0.01 eV.)

In the presence of a magnetic field, the spin-orbit levels are split further due to the Zeemaneffect. The same is true of static electric fields,where it is called the Stark effect. In both cases,atoms or ions in a gas or free space will show anaverage effect because of the motion of the parti-cles. However, in a crystal the atom and ion posi-tions are more or less fixed, and the application ofeither magnetic or electric fields along certainsymmetry directions will, in general, cause differ-ent degrees of splitting of the levels than the samefields applied along other symmetry directions.Zeeman splitting has been used to change theoptical properties of lanthanoid ions in optical-magnetic materials.

Selection RulesElectron transitions are governed by selectionrules that give the probability that the transitionwill occur. Transitions between energy levelsderived purely from f orbitals are forbidden bythe Laporte selection rule. However, this rule maybreak down for ions in compounds. The mainreason for this is that a degree of mixing betweens, p, d, and f orbitals can occur when an ion is notlocated at a center of symmetry. As s, p, or d to ftransitions are allowed, transitions giving rise tocolor are also allowed, to that a degreecorresponding to the amount of orbital mixingachieved. In addition, transitions are only allowedbetween states of the same multiplicity, calledspin-allowed transitions. Transitions betweenstates of differing spin can be weakly allowedand in some circumstances can also contribute toobserved lanthanoid color. The weak colorsexhibited by Ln3+ ions are primarily due to theserestrictions, especially when compared to typicalcrystal-field colors of the 3d transition-metal ions(see Transition-Metal Ion Colours).

Some Lanthanoid Absorption Colors

The 4f0 ions La3+ and Ce4+ and f14 ions Yb4+ andLu3+ have no f – f energy levels and are colorless.The colorless ions Gd3+ and Tb4+ have a stable 4f7

configuration, and there are no energy levels in theappropriate energy range to give rise to color. Thesame is true for the 4f13 ion Yb3+.

Ce3+ and Eu2+

The lowest energy levels of Ce3+, arising from thesingle f electron, are 2F7/2 and 2F5/2. The nexthigher energy state for Ce3+ is the 5d level. Dueto interaction of the more exposed 5d electronswith the surrounding crystal structure, this isbroadened into a band of energies, which mayalso overlap with another broadened band of ener-gies derived from the 6s energy level (Fig. 1a).Transitions between the 5d band and 4f levels areallowed, and the colors produced by transitions ofthis type are intense. This transition absorbs atviolet end of the spectrum, and the absorptionband often encroaches into the visible with aconsequence that to the eye compounds are per-ceived as pale yellow.

Eu2+, with a configuration 4f7, also has a sim-ple energy level diagram because the energy of thestate obtained by transferring an f electron to theouter 5d orbitals is lower than the other 4f energylevels (Fig. 1b). As in the case of Ce3+, transitionsfrom the ground state to the upper energy band areallowed. The energy gap is slightly smaller than inthe case of Ce3+, and so the absorption movesslightly deeper into the visible spectrum. Becauseof this, the color of compounds is described as redbrown.

Note that as the d orbitals interact strongly withthe surrounding anions, the exact position of theband depends upon the host crystal. Thus, thecolors of Ce3+ and Eu2+ compounds, unusuallyfor lanthanoid ions, vary with host structure.

Pr3+, Tm3+, and Nd3+

The presence of Pr3+ ions generally colors the hostmatrix green. The principal transitions that con-tribute to the absorption spectrum are from theground state 3H4 to 3P0, at approximately485 nm (blue green), to 3P1 at approximately

Lanthanoid Ion Color,Fig. 1 Energy leveldiagrams (schematic) forCe3+ and Eu2+ ions

830 Lanthanoid Ion Color

470 nm (blue), and to 3P2 at approximately450 nm (blue). A weak spin-forbidden transitionto 1D2 at approximately 590 nm (orange) is alsopresent. Between these absorption peaks is a win-dow of low absorption in the green region of thespectrum (Fig. 2a). The other “green” ion is Tm3+.Here the main transitions are from the groundstate 3H6 to 3F3 at approximately 690 nm (red)and to 3F2 at approximately 665 nm (red) and aweak spin-forbidden transition to 1G4 at approxi-mately 465 nm (blue). Between these absorptionbands is a similar green transmission window toPr3+ (Fig. 2b). The Nd3+ ion tends to impart a lilachue to materials. The main transition here is fromthe ground state 4I9/2 to 4G5/2, which absorbsstrongly at approximately 585 nm (yellow),together with a transition to 4G7/2 at approxi-mately 525 nm (green) and 2G9/2 at 510 nm(blue green). These absorption peaks remove themiddle part of the optical spectrum, leaving boththe violet and red extremes, so imparting a lilachue to compounds.

Some Applications of LanthanoidFluorescence Colors

Trichromatic Fluorescent LampsTrichromatic (color 80) fluorescent lamps usephosphors with active lanthanoid ions. Generallythe lanthanoid ions can absorb a wide range ofultraviolet radiation efficiently, exciting the ionsfrom the 4f-derived ground state to a broad bandof energies formed from interaction of 5d and 6sband. Energy is then lost internally, in effect caus-ing the phosphor matrix to warm slightly, until thesharp 4f-derived energy levels are reached. Pho-ton emission between these energy levels thenoccurs, giving color output. The favored red emit-ter is Eu3+ doped into Y2O3, (Y2O3:Eu), with theEu3+ ions occupying the Y3+ sites. The groundstate of Eu3+ is 7F0. A transition from this state tothe higher energy band 5d band absorbs the ultra-violet radiation given off by excited mercuryatoms at 254 nm. Subsequent internal energyloss leaves the ion in the 5D0 level. The main

Lanthanoid Ion Color, Fig. 2 Energy level diagrams (schematic) for (a) Pr3+, (b) Tm3+, and (c) Nd3+

Lanthanoid Ion Color 831

L

optical transition is between this level and 7F2,producing emission at 611 nm (Fig. 3a). Thegreen emission is from Tb3+ in host matrices La(Ce)PO4, LaMg(Ce)Al11O19, or La(Ce)MgB5O10,in which the Tb3+ ions replace La3+. Tb3+ absorbsthe mercury emission poorly so is coupled with asensitizer, usually Ce3+, which is able to absorbthe 254 nm wavelength mercury radiation effi-ciently by way of the 5d band. This absorbedenergy is then transferred to the Tb3+ ions. Thegreen emission, at wavelength close to 540 nm, ismainly from a 5D4!7F5 transition (Fig. 3b).Three other peaks of lesser intensityoccur: 5D4!7F6, 489 nm; 5D4!7F4, 589 nm;and 5D4!7F3, 623 nm. The blue emission is pro-duced by Eu2+ ions. The excitation and emissionis directly to and from the 5d-derived band. Theposition of this band is strongly influenced by thehost structure (see above) and the usual tricolorlamp phosphor, BaMgAl10O17: Eu is chosen so asto have a suitable blue emission, with a maximumat 450 nm (Fig. 3c).

Plasma DisplaysPlasma display panels are made up of a pair ofglass plates containing a series of cells each ofwhich acts as a miniature fluorescent lamp asdescribed above. Each lamp is several hundredmicrons in size, and there are several millionsuch lamps in a display. Each pixel consists ofthree lamps, giving off red, blue, and green light.The working gas in the cells is a mixture of heliumand xenon. When a high voltage is applied acrosstwo electrodes above and below a well, the gas isexcited into a state that emits ultraviolet radiation,with principal wavelengths of 147 nm and172 nm. Each well is coated internally with ared, green, or blue phosphor. The main lanthanoidphosphors used at present are a yttrium gadolin-ium borate doped with europium, (Y, Gd)BO3:Eu3+, which gives a red emission, barium magne-sium aluminate doped with europium,BaMgAl14O23:Eu

2+, for blue emission (seeFig. 3). The green emission utilizes Mn2+ ratherthan a lanthanoid ion.

Lanthanoid Ion Color,Fig. 3 Energy leveldiagrams (schematic) for(a) Eu3+, (b) Ce3+/Tb3+, and(c) Eu2+ in trichromaticlamp phosphors. ET energytransfer

Lanthanoid Ion Color,Fig. 4 Energy leveldiagrams (schematic) for(a) Eu2+, (b) Tb3+, and (c)Ce3+ in ac powered ThinFilm ElectroluminescentDisplay (ACTFEL)displays

832 Lanthanoid Ion Color

Phosphor Electroluminescent DisplaysElectroluminescent displays containing a thin filmof a phosphor, called thin film electroluminescent(TFEL) displays, find use as flat panel color dis-plays and backlighting in products such as instru-ment panels. The most promising devices use acsupplies in a thin film electroluminescent(ACTFEL) display. Under the influence of anapplied electric field, electrons enter the phosphorat the junction with a surface insulating coating.These are accelerated under the influence of thefield until they collide with the luminescent cen-ters in the phosphor, transferring energy in theprocess. The excited luminescent centers then

fall back to the ground state and release energyby light emission.

Red emission is from calcium sulfide dopedwith europium (CaS:Eu2+), the color being gener-ated by the transition from the 5d band to theground state 8S7/2 (Fig. 4a). At first sight this issurprising as the Eu2+ derived tricolor lamp phos-phor has a blue output. However the position ofthe upper energy band depends upon the interac-tion of the d orbitals with the surrounding crystal,and in ZnS the softer bonding gives a broad emis-sion centered close to 640 nm. Green emission isproduced by zinc sulfide doped with terbium(ZnS:Tb2+), with an output at a wavelength close

Lanthanoid Ion Color,Fig. 5 Upconversion usingEr3+ doped into CeO2,schematic: (a) GSA +relaxation; (b) ESA +relaxation; (c) ESA +relaxation; (d) lightemission

Lanthanoid Ion Color 833

L

to 545 nm, mainly from a 5D4!7F5 transition(Fig. 4b). Three other peaks of lesser intensityoccur: 5D4!7F6, 489 nm; 5D4!7F4, 589 nm;and 5D4!7F3, 623 nm. Blue emission still posesa problem for these displays, but the thiogallatesCaSr2S4, SrGa2S4, and BaGa2S4 doped with the4f1 ion Ce3+ are currently favored. The transitionsbetween the 5d band and the 4f1 ground statedoublet 2F5/2 and

2F7/2 are both in the blue regionof the spectrum centered at 459 nm for the Cacompound and 445 nm for the Sr and Ba phases(Fig. 4c).

UpconversionThe conversion of infrared radiation to visible isvariously known as upconversion, frequencyupconversion, anti-Stokes fluorescence, or coop-erative luminescence. The majority of studies ofupconversion have involved the lanthanoid ionsEr3+, Tm3+, Ho3+, and Yb3+. At present the mostefficient upconversion materials for infrared tovisible conversion are lanthanoid fluorides suchas NaLnF4 doped with Yb3+/Er3+ or Yb3+/Tm3+

couples. The energy for upconversion can begained by the active ion via several competingenergy transfer processes. In principle, the sim-plest is for the active ion to pick up photons in twodistinct steps. The first photon excites the ion fromthe ground state to an excited energy level, aprocess referred to as ground state absorption(GSA). A subsequent photon is then absorbed tofurther promote the excited ion to a higher energylevel again – excited state absorption (ESA). The

oxide CeO2 doped with approximately 1 % Er3+

exhibits upconversion in this way. Irradiation withnear-infrared photons with a wavelength close to785 nm excites the Er3+ ions from the 4I15/2ground state to the 4I9/2 level:

4I15=2 þ hn 785 nmð Þ4 ! I9=2

Subsequent internal energy loss drops the energyto the 4I11/2 and

4I13/2 levels (Fig. 5a). The ions arethen excited by ESA. Those in the 4I11/2 energylevel are excited to the 4F3/2, 5/2 doublet:

4I11=2 þ hn 785nmð Þ4 ! F3=2, 5=2

These states subsequently relax to the 2H11/12,4S3/2 and

4F9/2 levels:

4F3=2, 5=2!2H11=2 þ 4 S3=2 þ 4F9=2 þ phonons

The ions in the 4I13/2 energy level follow a similarpath, being excited to the 2H11/2 energy level:

4I13=2 þ hn 785 nmð Þ2 ! H11=2

then subsequently relax to the 4S3/2 and 4F9/2levels:

2H11=2!4S3=2 þ 4F9=2 þ phonons

The result of this is that the levels 2H11/2,4S3/2 and

4F9/2 are populated to varying degrees, depending

834 Lanthanoid Ion Color

upon the precise details of the excitation andrelaxation steps. Subsequent loss of energy fromthese levels gives rise to green and red emission:

2H11=2!4I15=2 þ hn �525 nm, greenð Þ

4S3=2!4I15=2 þ hn �550 nm, greenð Þ

4F9=2!4I15=2 þ hn �655 nm, redð Þ

All upconversion spectra from Er3+, including thoseusing different mechanisms, are similar, but the rela-tive intensities of the three peaks vary with concen-tration of the ions and the nature of the host matrix.

Energy transfer, in which input radiation ispicked up by a sensitizer and is then passed to theemitter, is the mechanism of operation of host struc-tures containing the co-dopants Yb3+/Er3+ whichgives a strong green emission and Yb3+/Tm3+ thatgives a blue emission. The ion that absorbs theincoming infrared radiation is the Yb3+ ion, whichthen transfers energy to Er3+ or Tm3+ centers. Otherupconversion mechanisms are also known.

Lanthanoid Ion Color, Fig, 6 Visible light emission(quantum cutting) via photon cascade emission from Pr3+

ions irradiated by ultraviolet light

Quantum CuttingQuantum cutting is the reverse of upconversion, asone high-energy photon is processed (i.e., cut) togive out several lower energy photons, typicallytransforming ultraviolet to visible. There are severalmechanisms for quantum cutting; here photon cas-cade emission, exhibited by Pr3+ ions, is described.Initial absorption of high-energy 185 nm ultravioletphotons results in excitation to the 5d-6s energyband (Fig. 6). Subsequent relaxation takes the ionto the 1S0 level. Thereafter, the transitions giving riseto visible output are the following:

1S0!3P3 at �400 nm, then 3P0!3H4

ground state at �480 nm

1S0!1D2 at �330 nm, then 1D2!3H4

ground state at �605 nm

Other ions such as Tb3+ are also candidates forquantum cutting devices, but the mechanismsinvolved are more complex than that with Pr3+.

Cross-References

▶Transition-Metal Ion Colors

References

1. Tilley, R.J.D.: Chapter 7. In: Colour and the OpticalProperties of Materials, 2nd edn. Wiley, Chichester(2011)

2. Nassau, K.: Chapter 4. In: The Physics and Chemistryof Colour, 2nd edn. Wiley, New York (2001)

3. Huang, C.-H. (ed.): Rare Earth Coordination Chemis-try. Wiley, Singapore (2010)

4. Linganna, K., Jayasankar, C.K.: Luminescence Spec-troscopy of the Lanthanides, Scholars Press (2013)

5. H€aninen, P., H€arm€a, H. (eds.): Lanthanide Lumines-cence. Springer, Heidelberg (2011)

6. Cotton, S.: Lanthanide and Actinide Chemistry. Wiley,Chichester (2006)

Le Blon, Jacob Christoph 835

Layering

▶Compositing and Chroma Keying

Le Blon, Jacob Christoph

Rolf G. KuehniCharlotte, NC, USA

L

Biography

Le Blon was born onMay 2, 1667 in Frankfurt amMain, Germany, a descendant of Huguenots flee-ing France in 1576, having settled there. Hisgrandmother was a daughter of the artist andengraver Matthaus Merian the Elder(1593–1650). Showing an early interest inengraving and painting, he had, sometimebetween 1696 and 1702, an extended stay inRome where he is reported to have studied artunder the painter and engraver Carlo Maratta

J. C. Le Blon, Head of a woman, ca. 1720 (three-colorprinting process)

(1625–1713) [1]. Around 1702 Le Blon movedto Amsterdam, where he worked as a miniaturepainter and engraver. In 1708/1709 he is known tohave made colorant mixing experiments inAmsterdam, and in 1710 he made his first colorprints with yellow, red, and blue printing plates.In 1717 he moved to London where he received aroyal patent for the three-color printing and arelated textile weaving process [2]. In 1722 hepublished a small book on painting, Coloritto, inFrench and English [3]. There he stated that“Painting can represent all visible objects withthree colors, yellow, red, and blue.” During hisstay in England, he produced several dozenimages printed from three or four plates (thefourth for black ink) in multiple copies that ini-tially sold well in England and on the continent. Inthe long run his enterprise did not succeed, how-ever. Le Blon left England in 1735, moving toParis where he continued producing prints by hismethod. In 1740 he began work on a collection ofanatomical prints for which he had a solid list ofsubscribers. He died on May 16, 1741 in Paris.A detailed technical description of Le Blon’smethod was published in 1756 by Antoine Gautierde Montdorge who supported him during his finalyears in Paris [4].

Major Accomplishments/Contributions

The idea of three chromatic primaries, yellow, red,and blue, was quite well established in Le Blon’stime among painters, graphically represented byAguilonius in 1613 [5] and described by R. Boylein 1664 (p. 220) [6]. What was new in Le Blon’swork is that he applied this concept to color print-ing of images in an entirely new fashion makinggreater and much subtler detailing and colorationpossible. It required experience in deconstructingan image in terms of color so that printingmultiplecopies, based on only three or four plates, pro-duced good quality coloration. It required theability to mentally resolve the image into its pre-sumed primary chromatic components and under-standing and predicting the effects ofsuperimposed printing inks in certain areas, forwhich extensive trial and error work was required.

836 Light Bounces

Le Blon manually engraved copperplates, usingthe mezzotint process, with the relative compo-nents of the three primary colors printed succes-sively in registration in the sequence blue, yellow,and red onto the paper substrate. As he gainedexperience, he at times used a fourth plate printingin black to achieve greater tonality and contrast,thus employing an early version of the CMYKprocess. Le Blon used the pigments Prussian blue,Stil de grain (yellow lake), a mixture of red lakeand carmine for red, and a common printer’s blackink [4]. The pigments were dispersed in copal treeresin dissolved in copal oil to make the inks. Thetechnical problems associated with the processprevented it from becoming a standard methodand lithographic printing of color images fromup to a dozen wood engravings or stones perimage continued until H. E. Ives’ invention ofthe chromatic halftone printing process ca. 1890.

References

1. Lilien, O.M.: Jacob Christoph Le Blon, 1667–1741:Inventor of three- and four colour printing. Hiersemann,Stuttgart (1985)

2. Lowengard, S.: Jacob Christoph Le Blon’s system ofthree-color printing and weaving. In: The Creation ofColor in the 18th century Europe. Columbia UniversityPress, New York (2006). http:www.gutenberg-e.org/lowengard/C_Chap14.html. Accessed 11 May 2015

3. Le Blon, J. C.: Coloritto, or the Harmony of Colouringin Painting: Reduced to Mechanical Practice (with par-allel French text). London (ca.1725)

4. Gautier deMontdorge, A.: L’art d’imprimer les tableaux.Traité d’apres les écrits, les opérations et les instructionsverbales de J.-C. Le Blon. Mercier, Paris (1756)

5. Aguilonius, F.: Opticorum Libri Sex. Plantin, Antwerp(1613)

6. Boyle, R. Experiments and Considerations TouchingColours. Herringman, London (1664)

Light Bounces

▶Global Illumination

Light Depreciation

▶Lumen Depreciation

Light Distribution

Wout van BommelNuenen, The Netherlands

Synonyms

Candela distribution; Luminous intensitydistribution

Definition

A luminaire property that indicates the values ofthe luminous intensities radiated in all relevantdirections by the luminaire. Usually the luminousintensities are given as intensities per 1,000 lamplumen when the lamp or lamps are operated understandard test conditions: I/1,000 lm.

Luminaire Performance Characteristic

The light distribution of a luminaire represents themost important performance characteristic of aluminaire. It is the basis for the determination ofphotometric characteristics of luminaires such asupward and downward light output ratio, utiliza-tion factors for defined areas or zones, beamangles, and glare figures. Luminaire manufac-turers produce for their luminaires photometricdatasheets which show these photometric charac-teristics together with a graphical representationof the light distribution itself.

Measurement

Strictly speaking, the light distribution can onlybe measured for a point source. In practice themeasurement inaccuracy will be negligible pro-vided the optical path length of the measuringsetup is at least ten times the length of the lightemitting surface of the luminaire. For narrowbeam projector type of luminaires, greater opticalpath lengths are required. The instrument used for

Light Pollution 837

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measuring light distributions is called agoniophotometer. It exists of a mounting supportfor the luminaire, a photocell, and a, usually com-plex, system that rotates the combination of pho-tocell and luminaire such that the intensities fromall different light directions can be measured.Mirrors are used to keep the actual dimensionsas small as possible while keeping the requiredoptical path length. It is important that the lumi-naire rotates in such a way that its normal oper-ating position is at all times maintained. Themeasurements should be carried out under stan-dard test conditions. These conditions refer toambient temperature, air movement, mountingposition, and lamp ballast type. The coordinatingsystems used for indicating the directions areeither the so-called B-beta (usually used for pro-jector type of luminaires) or the C-gamma sys-tem. The B angles of the B-beta system can beunderstood as the angles the pages of a bookmake when the book axis is held horizontally.The beta angles are then angles in each pagemeasured from the middle of the book axis. TheC-gamma system in the same analogy can beunderstood as a book with its axis held vertically.C angles are the angles the pages make, andgamma angles are angles in the pages from themiddle of the book axis.

Intensity Table

All lighting calculations of lighting installationsare based on the light distribution of the lumi-naires used. For this purpose the light distributionis given as a two-dimensional table, the so-calledI-table. There are some different formats for theseI-tables such as CIE, IESNA, Eulumdat, CIBSE,and some luminaire manufacturer’s specific for-mats. Eulumdat is the most popular format inEurope, whereas the IESNA format is more pop-ular in the USA. The CIE format is an attempt todefine a “recommended file format for electronictransfer of luminaire photometric data” on aglobal level, in order to overcome the regionallydifferent formats currently in use. Many light cal-culation programs can use some different formatsas input. Moreover, free software for conversion

from one format to the other is available as well.The layout of the I-table is different for differenttype of luminaires. So have road lighting lumi-naire I-tables more values around the direction ofthe horizon than for other directions because thesevalues very much determine glare, general light-ing interior luminaire I-tables have a somewhatmore regular layout because of the more diffusecharacter of these luminaires and flood and accentlighting luminaires (projector type of luminaires)have I-tables with extra values around the direc-tion of the maximum beam intensity.

Cross-References

▶Luminaires

Light Pollution

Terry McGowanLighting Ideas, Inc, Cleveland Hts., OH, USA

Synonyms

Glare; Sky glow; Unwanted light especially refer-ring to unwanted electric lighting at night

Definition

Outdoor electric lighting at night which isunwanted, unneeded, or wasteful and whichresults in glare, light trespass and sky glow orother harmful effects on people and theenvironment.

Light Pollution Recognized

Electric lighting makes travel, working, playing,and other normal living activities possible atnight. Indeed, the view of a lighted city from anairplane window or tall building is a hallmark oftwenty-first-century civilization, yet little more

838 Light Pollution

than a century and a half ago, there were no suchviews and human activities outdoors at night werelimited to those considered essential. Even then,they were carried out only with difficulty espe-cially tasks involving work or travel. Limitationsremained as candles and then gas flames andmantles began to generate more powerful andreliable night lighting, but the benefits were notwidespread because of the relatively high cost ofproviding light especially over large outdoor areasat night.

When electric lighting was invented, one of itsfirst uses was for outdoor illumination. Carbon arclamps were used initially and then incandescentlight sources, which were developed during the1870s. These became the worldwide standard forall types of lighting until the commercial intro-duction of the fluorescent lamp in the late 1930s.Outdoor lighting, however, did not dramaticallychange until mercury lamps, the first of theso-called high-intensity discharge (HID) lampsbecame widely used for street, parking, and arealighting during the 1960s and then other types ofHID lamps – high-pressure sodium (HPS) andmetal halide (MH), due to their relativelyhigher efficacies, transformed the cost and use ofoutdoor lighting leading to rapid growth and thenight time appearance of cities that we see today.Now, however, a new transformation is underwaycharacterized by the growing use of solid statelighting in the form of Light Emitting Diodes(LEDs) which are increasingly being used foroutdoor lighting.

Electric lighting has changed the night. Andthere are negative aspects that were recognizedearly on by critics who pointed out that brightlights frightened horses, annoyed pedestrians,and produced glare. There were those who alsonoticed that electric light changed the egg-layinghabits of chickens, attracted insects, and modifiedthe leaf and flowering characteristics of certainplants – early examples that hinted at more signif-icant problems that later appeared due tounneeded or unwanted light. But the term “lightpollution” did not appear until much later – in the1980s. That’s when the large, powerful HIDlamps began to be used in large quantities formajor outdoor lighting systems such as street

lighting, parking areas, signs, sports stadiums,and floodlighting. The stray, uncontrolled lightfrom these new sources as typically used outdoorsplus the light from the headlights of cars and lightescaping through windows from the interior light-ing systems of buildings at night has nowtransformed the night environment in conspicu-ous ways including making the night sky lessvisible. Instead, light into the sky blankets citieswith “sky glow,” a fog, or haze of light.

Astronomers, both professional and amateur,were among the first to recognize and talk aboutsky glow as light pollution as they tried to observethe stars from urban locations. As a first response,professional astronomers moved telescopes andobservatories from cities to new locations inremote areas. Then, it was recognized that suchplaces were increasingly rare and costly todevelop. Further, most amateur astronomerswere not able to move or take advantage of suchsites. Members of the public who simply wantedto catch a glimpse of the Milky Way from theirbackyard or to show their children a comet or starconstellation were not able to do so. The result is agrowing and widespread reaction to uncontrolledelectric lighting outdoors at night.

Efforts began, usually via zoning, planning, orthe so-called “nuisance” regulations to limit andcontrol unwanted light especially around astro-nomical observatories. It was found that lightfrom urban areas could significantly affect thesky brightness 100–200 km away and that mini-mizing light directed into the sky was a simple andeffective way to control such light. The term “lightpollution” became familiar and efforts began tounderstand the various associated problems and tofind ways to limit the detrimental effects. A globalorganization, the International Dark Sky Associa-tion (IDA), founded in 1988, focused on the taskand continues to work “to preserve and protect thenighttime environment . . . through environmen-tally responsible outdoor lighting” [1]. Otherregional and local organizations also appearedworldwide to raise awareness of the problems,write laws, provide educational materials, andorganize local activities such as promoting theuse of shielded luminaires. Lighting technicaland professional organizations have also

Light Pollution 839

recognized light pollution problems and havesponsored research, issued reports, producedrecommended lighting practices, and writtenmodel legislation [2–4] in order to control it.

L

Types of Light Pollution

Light pollution was initially understood asprimarily a visual problem caused by unnecessaryor uncontrolled outdoor lighting that interferedwith the viewing of the sky. But, there arenow other considerations, and light pollution hasbeen more broadly defined and classified asfollows:

Light trespass – Light generated in one locationwhich, when poorly controlled, travels toanother location where it is not wanted. Anexample would be a streetlight which isintended to light the street surface but alsodirects light into front yards or bedroom win-dows along the street.

Glare – Excessive light directed into the eyewhere it causes visual discomfort or reducesthe ability to see. Overly bright unshielded ormisaimed luminaires are usually the mainsources of glare.

Sky glow – Light directed or reflected into the skywhere it is scattered by air molecules, water,and other particles in the atmosphere andreflected diffusely to produce a fog or haze ofbrightness that reduces the visibility of thenight sky.

Light pollution has energy implications sincestray or misdirected light is luminous energywhich has been converted from electric energyby the light source. That electric energy, the fuel,and other resources which produce it have there-fore been wasted, delivering no value to thosewho pay for those resources.

The IDA estimates that some 30 % of theelectric energy used for outdoor lighting goesinto the sky. Considering the hours of operationand an average electrical energy rate of $0.10/kWh (USA), the cost of that wasted energy iswell over $2 billion/year.

Spectral Effects

Electric lighting is the cause of light pollution.However, all light sources are not equally effec-tive at generating light pollution of the typesdescribed. The scattering of light in the atmo-sphere that affects sky glow, for example, is afunction of the wavelength of the light (Rayleighscattering). Such scattering affects how far lightpollution will travel. Similarly, the intensity(candlepower) of the luminaire as well as theway the light is aimed and directed matters too.

Astronomers who first sought to alleviate skyglow during the 1970s and 1980s found that thelight from low-pressure sodium (LPS) lamps washelpful because all of the light from such lamps isemitted in a narrow range of wavelengths in theyellow part of the spectrum. A telescope equippedwith a filter that absorbed those wavelengths andtransmitted the rest of the spectrum would there-fore see the sky as “dark.” US cities such asTucson, AZ, and San Jose, CA, installed LPSstreet and area lighting as a way to maintain darkskies for area observatories while still providingessential lighting for drivers and pedestrians. LPSlamps, however, have poor color rendering and sothere were objections to the light. Provisions weremade for exceptions so that white light could beused in such applications as outdoor automobilesales lighting and sports facilities. Installationspermitted as exceptions usually had other condi-tions attached that limited total lighting wattage,mandated certain operating hours, or requiredshielding – often in combinations. Of course, forthe amateur astronomer or the casual viewer of thesky, outdoor lighting with LPS lamps and lumi-naires, which are typically unshielded, offers noadvantage since the yellow light is visible to theeye and continues to produce light pollution viaglare, light trespass, and sky glow (Fig. 1).

Note that while the spectral characteristics ofthe light source matter, overall light pollution,including sky glow, is the result of not only howthe light is generated but also how it is modifiedby outdoor surfaces via reflection, refraction, andabsorption. However, light pollution which inter-feres with a view of the sky in any given situationis always directly proportional to one quantity:

Light Pollution, Fig. 1 Example of the effect of lightpollution on the visibility of the sky in a residential areanear Toronto, Ontario, Canada. Left: during an area-widepower failure stars and constellations can be clearly seen.

Right: the same view after the power had been restoredwith streetlights, floodlights, and other outdoor lightingcontributing to sky glow

840 Light Pollution

total luminous power. Measured in lumens, lumi-nous power is radiant power modified by thespectral sensitivity of the human eye. The key tocontrolling light pollution is to limit or reduce thetotal lumens being generated from light sourcesand sent into the night environment. Turninglights off when not needed, shielding luminaires,and reducing illumination levels to the minimumvalues required are all techniques that can be usedto minimize light pollution problems. These arecommon-sense ideas, and there are many more[5, 6] although what should happen, of course, isthat lighting should be designed to incorporate thelong-established principles of proper lightingdesign which include the requirements to controlstray light, minimize glare, and not use excessivelight for the intended application. The problem isthat much outdoor lighting is not “designed” inthe sense that major buildings are designed withcareful thought given to materials, appearance,function, and purpose. Rather, outdoor lighting

is often a make-do or do-it-yourself affair. It “hap-pens” or factors of cost, expediency, or lack ofinformation drive the installation (Fig. 2).

Other Light Pollution Effects

Research reported during the 1990s has shownthat light pollution seriously affects the healthand behavior of animals, including mammals,birds, amphibians, and insects. Changes in migra-tion patterns, food gathering and feeding habits,mating, and social behavior have all beendocumented [7, 8]. However, bird and sea turtleproblems have received the most attention.

Birds may become disorientated by lighting tothe extent that they die from exhaustion whilecontinuously circling lighted buildings or, con-fused, they collide with the buildings. Accordingto a Chicago Audubon Society study [9]documented by actual counts of dead birds

1.0

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Light Pollution, Fig. 2 Acomparison of the spectra oflight sources commonlyused for outdoor lighting.Top: high-pressure sodiumis the most widely used andis characterized by itsfamiliar orange color.Middle: metal halide lampsemit white light but canhave a visually “warm” or“cool” color dependingupon the chromaticityrating. Emissions, however,are relatively balanced overthe visible spectrum.Bottom: white-light LEDlight sources use blue-emitting LEDs to activatephosphors which emit thelight shown in the 500 nmand above range. Theseso-called “blue-rich white-light” LEDs are of concernbecause light and humanhealth research indicatesthat eye receptors whichcontrol circadian rhythmshave a peak sensitivity near460 nm, similar to theemission curve of the blueLEDs

Light Pollution 841

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found at the base of tall buildings, bird deaths dueto building collisions were reduced significantlywhen just the decorative floodlighting of build-ings was switched off for most of the night hoursduring migration seasons.

Baby sea turtles hatch on beaches and have ashort time to dig out of the sand and to find theirway to the sea before they die by dehydration orare eaten by predators. The turtles appear to usethe brightness of the horizon as a visual clue tofind the water. Area and streetlighting alongbeachfront areas confuses the turtles, so effortshave been made to remove, turn off, or modifysuch systems including changing to amber-colored light sources which, because of their spec-tral sensitivity, appear less bright to turtleeyes [10].

Human Health Effects

A controversial issue involving light pollution iswhether or not outdoor lighting negatively affectshuman health. Research beginning in the 1970shas indicated that certain uses and kinds of electriclight and, particularly, light at night (LAN) whenthe human body expects and needs darkness forsleep, can be detrimental to health because itinterferes with the body’s normal circadianrhythms.

These rhythms mark the periods of alertnessand sleep due to the ebb and flow of hormones,such as melatonin, in the body. According to theresearch, strong circadian rhythms are essentialfor overall health and mental functioning andparticularly for cell repair which helps protect

842 Light Sensors

against major diseases including cancer. The gen-eral rule is that people need bright days and darknights so current research is exploring questionsabout light intensity, spectrum, timing, and expo-sure duration to determine the “dose” of light thatmight result in the interruption of the melatonincycle and, further, what kind of interruptions leadto an increased risk of disease. The research liter-ature on the subject is already extensive – onecompilation now lists over 1,000 citations [11],and how outdoor lighting impacts human healthwill remain an important subject of research anddiscussion.

The American Medical Association adopted aresolution in 2009 entitled “Advocating and Sup-port for Light Pollution Control Efforts and GlareReduction for Both Public Safety and EnergySavings” [12]. Significantly that AMA view wasbroadened in 2012 when they said, “The (AMA)policy also supports the need for developing light-ing technologies that minimize circadian disrup-tion and encourages further research on the risksand benefits of occupational and environmentalexposure to light at night” [13].

Outdoor lighting, because such lightingimpacts the natural environment and so manypeople see it, use it, and experience it, has becomecomplicated. It is now a social, technical, medical,and environmental problem that has to be care-fully analyzed rather than a traditional exercise inilluminating engineering so that people can see.Light pollution issues contribute to outdoor light-ing’s complexity, but light pollution also callsattention to important and growing outdoor light-ing problems that have to be addressed andresolved.

References

1. Mission Statement. The International Dark Sky Asso-ciation (IDA). www.darksky.org. Accessed 13December 2014

2. Model Lighting Ordinance (MLO): A joint effort ofthe International Dark Sky Association and the Illumi-nating Engineering Society of North America. www.darksky.org or www.ies.org (2011). Accessed 13December 2014

3. International Commission on Illumination: TechnicalReport – Guide on the Limitation of the Effects of

Obtrusive Light from Outdoor Lighting Installations.CIE 150. CIE Publications. http://www.cie.co.at/index.php/Publications (2003)

4. International Commission on Illumination: TechnicalReport – Guidelines for Minimizing Sky Glow. CIE126. CIE Publications. http://www.cie.co.at/index.php/Publications (1997)

5. Mizon, B.: Light Pollution: Responses and Remedies.Springer, New York (2012, 2nd. Edition) ISBN-13978-1461438212

6. Rich, C., Longcore, T. (eds.): Ecological Conse-quences of Artificial Night Lighting. Island Press,Washington, DC (2005)

7. The Royal Commission on Environmental Pollution:Artificial light in the environment. The StationeryOffice, United Kingdom (2009)

8. Chicago Audubon Society: Lights out after 11:00p.m. During migration. http://www.chicagoaudubon.org/lightsout.shtml Accessed 20 June 2012

9. National Geographic News: Saving sea turtles with alights out policy in Florida. http://news.nationalgeographic.com/news/2003/03/0310_030310_turtlelight.html Accessed 20 June 2012

10. International Dark Sky Association: Visibility, envi-ronmental and astronomical issues associated withBlue-Rich White Outdoor Lighting, Tucson, IDA,2010. Available at: http://darksky.org/assets/documents/Reports/IDA-Blue-Rich-Light-White-Paper.pdf

11. Wagner, R.: Light at night, human health – referenceswith abstracts. http://www.trianglealumni.org/mcrol/References-With_Abstracts.pdf (2011). Accessed12 Jun 2012

12. American Medical Association House of Delegates:Resolution: 516 (A-09) http://www.eficienciaenergetica.gub.uy/con_luminica/American%20Medical%20Association%20-%20Resolution%20516.pdf (2009). Accessed 10 June 2012

13. American Medical Association: AMA Adopts NewPolicies at Annual Meeting. http://www.ama-assn.org/ama/pub/news/news/2012-06-19-ama-adopts-new-policies.page (2012). Accessed 2 July 2012

14. Bogard, P.: The End of Night:Searching for NaturalDarkness in an Age of Artificial Light. Little, Brownand Co. New York. First Edition July, 2013

Light Sensors

▶ Photodetector

Light Stimulus

▶Light, Electromagnetic Spectrum

Light, Electromagnetic Spectrum 843

Light, Electromagnetic Spectrum

Joanne ZwinkelsNational Research Council Canada, Ottawa, ON,Canada

Synonyms

Light – Light stimulus, Optical radiation, Electro-magnetic radiation; Electromagneticspectrum – Optical spectrum

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Definitions

Light is so much a part of our everyday existencecontributing to our quality of life, as well as a keyenabler in photonics, solar power, and new light-ing technologies, that the year 2015 has beendeclared by the UNESCO as the InternationalYear of Light and Lighting Technologies.

Light has many different meanings dependingupon the application.

When it is used to broadly describe opticalradiation, it is defined as electromagnetic radia-tion with wavelengths between approximately10 nm and 1 mm, i.e., a term that is used todescribe the ultraviolet, visible, and infraredregions of the electromagnetic spectrum.

When it is used to describe a light stimulus, theInternational Lighting Vocabulary [1] gives thefollowing two definitions: (1) It is a characteristicof all sensations and perceptions that is specific tovision, and (2) it is radiation that is considered fromthe point of view of its ability to excite the humanvisual system, i.e., it is limited to electromagneticradiation with wavelengths in the visible spectralregion between approximately 380 and 780 nm.

The term light is also sometimes used as asynonym for electromagnetic radiation whichencompasses not only the ultraviolet, visible,and infrared range but also the X-ray andgamma-ray ranges and the radio range.

Electromagnetic radiation is the emission ortransfer of energy in the form of electromagneticwaves and in the form of photons.

Photon is a quantum of electromagnetic radia-tion that is usually associated with radiation that ischaracterized by one wavelength or frequency(monochromatic radiation).

Electromagnetic wave is a transverse oscilla-tion of inextricably linked electric and magneticfields traveling through space; through emptyspace, it travels at the speed of light in vacuum;through other media, its speed is reduced by thevalue of the refractive index of the medium.

Electromagnetic spectrum is defined as thegraphical representation of electromagneticwaves arranged according to their wavelength.

Overview

Early Views of LightThe nature of light has intrigued philosophers andscientists for thousands of years. Because lighttravels in straight lines (rectilinear propagation),it was long believed that it was made of particles(corpuscles) that emanated from the source. How-ever, some properties of light, such as the fussi-ness produced when light passes around obstaclesor through openings (diffraction), needed a differ-ent model of light where it behaved like a type ofwave or propagating disturbance. In the early1700s, both the particle and wave models oflight prevailed with Sir Isaac Newton being astaunch supporter of the particle theory andothers, such as the Dutch physicist, ChristiaanHuygens, being proponents of the wave theory[2]. But, in the early 1800s, when the Englishphysician Thomas Young showed that lightdiffracted and interfered to produce fringes whenit passed through a double slit, the wave model oflight became the accepted theory. At the end of thenineteenth century, the particle theory of light wasrevived by Einstein, who used particle behaviorand a new type of theory – quantum physics – toexplain the photoelectric effect. It is now widelyaccepted that light exhibits all three of thesebehaviors: particle, wave, and quantum, whichare used to explain all known properties oflight [2].

The history of light has also included interest-ing debates on how it is propagated. Ancient

WavelengthElectric field

Magnetic field

Direction of energy flow

Light, ElectromagneticSpectrum,Fig. 1 Simplified pictureof an electromagnetic wave.The oscillations areperpendicular to each otherand to the direction ofenergy flow

844 Light, Electromagnetic Spectrum

astronomers thought that light traveled at infinitespeed. The Greek philosopher Aristotle proposedthat light propagated through a hypothetical sub-stance, luminiferous ether or light-bearing ether,because it was believed that all types of wavesrequired a medium for propagation such as soundwaves in air and water waves in water. However,light is very different from other types of waves.In 1881, in one of the most famous experimentswith light, Michelson andMorley tried to measurethe rotational motion of the earth in this luminif-erous ether [2]. However, they found that the lightdid not travel at different speeds for differentpaths. This failed experiment indirectly showedthat light does not need amedium and can travel infree space (vacuum) and at constant speed.Michelson also carried out experiments to mea-sure the speed of light by improving on Galileo’secho technique by replacing men carrying lan-terns with a series of mirrors and a telescope. Byadjusting the rotation speed of the mirrors, hecould observe the light in the telescope, andfrom the rotation of the mirror and the knowndistance traveled by the light, he was able todetermine the speed of light to about six signifi-cant figures [2]. Presently, the speed of light is themost accurately known fundamental constant ofnature.

Description of ElectromagneticRadiation and Light

Electromagnetic radiation (emr) can also bedescribed as the emission or transfer of energyfrom vibrating charged particles that creates a

disturbance in the form of oscillating electric andmagnetic fields (Fig. 1). The British physicistJames Clerk Maxwell [3] was the first to discoverthat these electric and magnetic waves alwaysoccurred together, i.e., were coupled, giving riseto the name, electromagnetic waves. It was alsofound that the lines of force of these electric andmagnetic waves were always perpendicular toeach other and perpendicular to the direction ofwave propagation, i.e., transverse, never longitu-dinal. This transverse nature of light gives rise tovarious phenomena associated with polarization.

Polarized electromagnetic radiation is definedas radiation whose electromagnetic field, which istransversal, is oriented in defined directions [1, 4]where this polarization can be linear, elliptic, orcircular. Light is totally linearly polarized(or plane polarized) if the electric field vectorsare all oriented in the same plane, parallel to afixed direction which is referred to as the polari-zation direction. Light is unpolarized when itselectric field vectors vibrate randomly in alldirections; this can also be considered to beequal amounts of plane-polarized radiationwhose vibration directions are perpendicular toeach another. The state of polarization of theradiation (linear, circular, elliptic) is described bythe phase relationship between these twoorthogonal components. If this phase differenceis zero or 180�, the radiation is linearly polarized;if the phase difference is 90� or 270� and bothcomponents have the same amplitude, the radia-tion is circularly polarized; and if the phase dif-ference is not 0�,180�, 90�, or 270� and/or theamplitudes are different, the radiation is ellipti-cally polarized.

Light, Electromagnetic Spectrum 845

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Light is partially polarized if the electric vec-tors have a preferred direction. If the electric vectoris polarized vertical to the plane of incidence, this isreferred to as s-polarized (or TE), and if the electricvector is polarized parallel to the plane of inci-dence, this is referred to as p-polarized or (TM).

In addition to its state of polarization, theseelectromagnetic waves are characterized by sev-eral other measurable properties, such as the num-ber of oscillations per second, the separationbetween successive peaks or troughs, and thesize or amount of oscillations. These quantitiesare described by the frequency, n; wavelength, l:and amplitude or intensity, I, respectively. Theintensity describes the amount of energy flowingin the electromagnetic wave and is proportional tothe square of the amplitude. It is also necessary tospecify the direction that the wave is traveling,which is indicated in Fig. 1 by using an arrow. Itcan be seen that these waves crest up and down ina sinusoidal fashion, which is described as thewavefront moving up and down. When more than onewave is traveling, it is important to describe howthey differ in phase. Two waves are in-phase, iftheir crests coincide and their troughs coincide.Two waves are out-of-phase, if the crest of onewave coincides with the trough of the secondwave and vice versa. A plane wave is a wavewhose surfaces of constant phase are infinite par-allel lines transverse to the direction of motion. Ifall of the waves vibrate in-phase, the emr is referredto as coherent. The laser is an example of a coher-ent light source. If there is no fixed phase patternbetween the waves emitted by a light source, theemr is referred to as incoherent. Most ordinary lightsources, such as an incandescent lamp, are exam-ples of incoherent light sources.

Properties of Electromagnetic Waves

As mentioned above, generally a wave, such as awater wave, is a propagating disturbance of someequilibrium state in a continuous medium. How-ever, no medium is required for propagation ofelectromagnetic waves. This property of electro-magnetic waves makes them unique from othertypes of waves that can be considered mechanical

in nature and was first demonstrated by Einstein in1905. He also proposed the idea that light couldbehave both as a wave and as a particle in order toexplain the spectral distribution of the radiationemitted from a hot (incandescing) object. Thistype of electromagnetic radiation is referred to asblackbody radiation (give link to encyclopediaarticle on blackbody radiation).

Electromagnetic waves of all wavelengthstravel exactly at the same speed in vacuum. Thisfundamental constant of nature is known as thespeed of light in vacuum, denoted by the symbol,c. It has a value of 299,792,458-metre per second.The frequency, v, is related to wavelength, l, byn = cl�1. Waves are also sometimes described bytheir wave number, s, which is the inverse ofwavelength, defined by s = l�1.

When an electromagnetic wave travels throughother media such as air, glass, or water, its fre-quency remains constant, but its wavelength andspeed are both reduced by a factor, n, the refractiveindex of the material. For commonly used opticalglasses, the value of n ranges from 1.52 to 1.72 andis a function of frequency. For standard air, n has avalue of 1.00028 [5] and can be ignored for mostpractical applications.

The behavior of electromagnetic waves at theboundary from one medium to another giving riseto reflection, transmission, and refraction phe-nomena is described by a set of wave equationsknown as Maxwell’s equations [6].

These four equationshavebecome the fundamen-tal laws inelectromagneticsconnecting theprinciplesofelectricityandmagnetism.Solvingtheseequationsreveals the wave equation and the form of the elec-tromagneticwaves. TheMaxwell’s equations are notgiven here because this is outside the scope of thisentry; the interested reader can find a good descrip-tion in the following references [2, 6].

The smallest unit or quantity of electromag-netic radiation is called a photon. The energy ofa photon is given by E = hn where h is Planck’sconstant (6.62606 � 10�34 J s). Light can then bedescribed as a stream of individual photons, eachwith a definite energy and that can interfere witheach other like waves and diffract around corners[3]. The motion of these photons is controlled bythese same set of Maxwell wave equations [2, 3].

846 Light, Electromagnetic Spectrum

The electromagnetic spectrum spans the totalrange of wavelengths of electromagnetic radiationfrom the shortest to the longest wavelength thatcan be generated physically. This range of wave-lengths spans practically from zero to near infinityand can be broadly divided into regions as shownin Table 1 [7], which includes radio waves, infra-red, visible, ultraviolet, X-rays, and gamma rays.This division is not exact since there is a gradualtransition from one region to the next, which isshown schematically in Fig. 2 where the visibleand ultraviolet regions are highlighted. This classi-fication of light into regions is also shown as afunction of frequency. It is important to note thatthe only difference between electromagnetic

Light, Electromagnetic Spectrum, Table 1 Regions ofthe electromagnetic spectrum

Wavelength range(nm)

Frequency range(s�1) Description

<0.1 nm 1020–1023 Gamma rays

0.1–10 nm 1017–1020 X-rays

10–400 nm 1015–1017 Ultraviolet

400–700 nm 1014–1015 Visible

700 nm to 1 mm 1011–1014 Infrared

1 mm to 1 cm 1010–1011 Microwaves

1 cm to 100 km 103–1010 Radio waves

100–1,000 km 102–103 Audiofrequency

X-rays Ultraviolet

VACUUM-UV

100 200 280 315 400

Hg-Low PressureLamp 254nm

UV-C UV-B UV-A

Vis

Light, Electromagnetic Spectrum, Fig. 2 The regionsspectrum which includes the visible and ultraviolet regions.

radiations in all these regions is its wavelength(and frequency). It has different descriptionsbecause of the relationship between its frequenciesand those that are excited in the various materialsthat the electromagnetic radiation can interact with.For example, the visual receptors in the human eyeare only sensitive to electromagnetic radiation in avery narrow frequency range from 1014 to 1015 s�1

(or 400–700 nm),whereas X-rays are used to excitefeatures in the body that are of the size of an atom(0.1 nm). These different applications of emr aredescribed in more detail below.

Interaction of Light with Matter

When light interacts with matter, different phe-nomena can occur depending upon the relation-ship of the wavelength (frequency) of the lightwith the physical size (resonant frequencies) ofthe interfering matter. This matter consists ofatoms, ions, and molecules. As mentionedabove, only light in the visible portion of thespectrum with frequencies from 1014 to 1015 s�1

can stimulate the visual receptors in the humaneye. Light can also exhibit wave or particle prop-erties depending upon this relationship betweensize (or frequency). For light waves that areclosely spaced in relation to the spacing of the

780

Wavelength (nm)

ible Light Infrared

of the electromagnetic spectrum, highlighting the optical

Light, Electromagnetic Spectrum,Table 2 Dependence of color on wavelength

Wavelength range (nm) Color

400–430 nm Violet

430–480 nm Blue

480–560 nm Green

560–590 nm Yellow

590–620 nm Orange

620–700 nm Red

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interfering matter, for example, a wave front pass-ing through a slit or an opaque edge, secondarywave fronts are generated. These will interferewith the primary wave front, as well as witheach other, and produce diffraction patterns.Since these secondary wave fronts were producedfrom the same primary wave, their phase willchange in step. Thus, even for a normally inco-herent light source, wave-like behavior such asdiffraction and interference can be seen undercertain conditions [8].

Many colorful effects are produced by thiscombination of light diffraction and interference.In nature, this can be seen, for example, in thebeautiful array of colors displayed by mother ofpearl (opalescence) or by thin film iridescence(add link to iridescence article). For light wavesthat are very small in relation to the interferingparticles, such as the interaction of visible lightwith dust particles or gas molecules in the atmo-sphere, Rayleigh scattering occurs which pro-duces the blue color of the sky (add link toRayleigh and Mie scattering article).

However, when light interacts with objects thatare large in relationship to their wavelength, suchas a brick wall or pane of glass, the light behavesmore like a ray (particle) and the laws of geomet-rical optics can be applied.

Depending upon the transparency or translu-cency of the material and its surface quality, thelight will be reflected, refracted, transmitted,absorbed, or scattered. The type of light interactionthat dominates depends upon the precise nature ofthe matter, e.g., for a smooth opaque colorlesssolid, specular (mirror-like reflection) will domi-nate, whereas for an opaque colored material, dif-fuse reflection or scattering will dominate. Theselective modification of the energy distributionof the incident light by the diffusely reflectingmaterial gives rise to different colors.

Light Intensity Distribution and Color

The different wavelengths in the visible spectrumcan also directly stimulate different colors in thehuman visual system. This dependence of coloron wavelength is shown in Table 2.

Radiation of a single wavelength is calledmonochromatic. Except for lasers and certain spe-cialized lamps, most sources of optical radiationemit energy over a broad wavelength region. Thecurve describing the power at each wavelength iscalled the spectral power distribution (SPD).A color stimulus generally has an SPD that varieswith wavelength across the visible spectrum pro-ducing a color or sensation that is a shade ormixture of the colors listed in Table 2. Stimulicontaining all the visible wavelengths in roughlyequal proportions appear white.

This is demonstrated by passing white lightthrough a glass prism, which spreads the lightout into a spectrum of colors (see Fig. 3).

However, white light can also be produced bysuperimposing discrete monochromatic lights.For example, white light can be produced bysuperimposing red, blue, and green lights in cer-tain proportions. This was first demonstrated byThomas Young [3] who used this experimentalfinding to theorize that the human eye containedthree different types of color receptors, which isknown as the trichromatic theory of human colorvision.

Because of this trichromacy, white light can beproduced by many different SPDs. The variationin these SPDs is illustrated in Fig. 4, which showsthe SPD of a typical tungsten lamp, a fluorescentlamp, a white LED source produced by a blueLED and yellow phosphor, and noon daylight.The most prevalent source of light in our naturalenvironment - the sun, has an SPD that can changedramatically during the course of the day, from abluish cast at noon to a reddish-orange cast atsunset. These variations in SPD of the light sourceare very important for visual evaluations of

White light

Slit

Light, ElectromagneticSpectrum, Fig. 3 Use of aprism to separate whitelights into a spectrum ofcolors

Light, Electromagnetic Spectrum, Fig. 4 Relativespectral power distributions (SPDs) of four differentsources of white light; CIE illuminant (if applicable)

given in parentheses: Top Left: tungsten lamp (CIE A);Bottom Left: noon daylight (CIE D65); Top Right: fluo-rescent lamp (FL12); blue LED + yellow phosphor

848 Light, Electromagnetic Spectrum

colored goods. For this reason, the SPDs of dif-ferent phases of daylight have been standardizedby the International Commission on Illumination(CIE) as a series of D illuminants. Shown in Fig. 4is the SPD of CIE standard illuminant D65, whichis average daylight with an approximate corre-lated color temperature of 6500 K.

Applications of ElectromagneticRadiation

The applications of electromagnetic radiation(emr) cover almost all aspects of our daily lives.The importance of visible light for human visionhas already been discussed. Light emanating from

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the sun spans the solar wavelength region fromabout 250 nm in the UV to 2,500 nm in the near-infrared region. This solar radiation is extremelyimportant to the energy balance of the earth andfor various building design applications, such asthe design of glazing materials. It is also exploitedin light-based technologies, such as photovoltaics.

Light in the infrared region of the spectrum isfelt as heat and is used in the design of radiantheating, solar panels, and collectors. The frequen-cies in the infrared region are close to the resonantvibrational frequencies in molecules and can beused for molecular identification by spectroscopicmeans. Similarly, the frequencies in the micro-wave region are close to the resonant rotationalfrequencies in molecules, in particular the watermolecule. Since most foods contain a significantamount of water, the microwave radiation heatsthe food by the increased rotational motion of theinterspersed water molecules. These microwavespass through glass, ceramic, and plastic but reflectfrommetal which can also be used to advantage inthe selection of the food container and walls of themicrowave, respectively.

Light in the ultraviolet (UV) region has manyimportant uses. In the early nineteenth century, itwas called “chemical rays” because it was foundthat UV radiation could cause chemical changes.This was used to advantage to harden specialglues in inks, coatings, and adhesives morequickly, in a process called UV curing throughpolymerization. The skin of our bodies needsexposure to UV with wavelengths in the region280–315 nm (UVB) in order to stimulate theproduction of Vitamin D. However, too muchexposure to UVB can cause skin cancers, so it isimportant to find the optimum exposure level. UVlight with wavelengths in the region 200–280 nm(UVC) is called “germicidal UV” because it hassufficient energy to inactivate bacteria and virusesby disabling their DNA strands. For this reason, itis also used for sterilization of surfaces such asmedical equipment. This type of UVC radiation isoften produced by a low-pressure mercury vapourlamp (see Fig. 2). UV radiation is also needed toexcite a class of molecules referred to asfluorescent-whitening agents (FWAs) or opticalbrighteners in manufactured white goods, such

as paper, fabrics, detergents, soaps, and cosmetics,which emit in the blue portion of the visible spec-trum and produce an enhanced whiteness effect.UV radiation is also of the appropriate frequenciesto absorb ozone molecules in the atmosphere sovery little UVradiation reaches the earth’s surface.

X-rays have a higher frequency than UV radi-ation and can pass through the skin and soft tissue,but they do not easily pass through the bone ormetal. This type of emr is used to produce photo-graphs of bones in medical diagnostics to checkfor damage such as fractures. The frequencies ofgamma radiation, on the other hand, are morepenetrating and can destroy chemical bonds byinteracting with the electrons of the constituentatoms or disrupt DNA bonds, resulting in preven-tion of cellular division. This can be used toadvantage to kill microorganisms throughout thematerial. Gamma sterilization is used, for exam-ple, in sterilization of water and food products.

Other important applications of emr are radio,television, and electric current.

Cross-References

▶Blackbody and Blackbody Radiation▶ Iridescence (Goniochromism)▶Rayleigh and Mie Scattering▶ Spectral Power Distribution

References

1. CIE S 017/E: ILV – International Lighting Vocabulary.CIE Central Bureau, Vienna (2011)

2. Jenkins, F.A., White, H.E.: Fundamentals of Optics,4th edn. McGraw-Hill, New York (1976)

3. Overheim, R.D., Wagner, D.L.: Light and Color. Wiley,New York (1982)

4. Shurcliff, W.E.: Polarized Light: Production and Use.Harvard University Press, Cambridge, MA (1962)

5. Ciddor, P.E.: Refractive index of air: new equations forthe visible and near infrared. Appl. Optics 35,1566–1573 (1996)

6. Stern, F.: In: Seitz F., TurnbullD. (eds.): Elementary theoryof the optical properties of solids. Solid State Physics, vol.15. Academic, New York, pp 301–304 (1963)

7. Williamson, S.J., Cummins, H.: Light and Color inNature and Art. Wiley, New York (1983)

8. Falk, D., Brill, D., Stork, D.: Seeing the Light: Optics inNature, Photography, Color, Vision, and Holography.Harper & Rowe Publishers, New York (1986)

850 Light-Emitting Diode, LED

Light-Emitting Diode, LED

Wout van BommelNuenen, The Netherlands

Synonyms

Solid-state light source; Optoelectronic lightsource

Definition

Light source that produces light as a result ofrecombination of positively charged holes andnegatively charged electrons at the junction ofinorganic, solid-state, p- and n-type semiconduc-tor material.

Solid-State LED Light Sources

LEDs are solid-state radiators where the light iscreated inside solid-state material [2–4]. Commonsemiconductor diode chips, used today in so manyelectrical circuits, all use much the sametechnology. The light-radiating diode versionsare called light-emitting diodes or LEDs. Thename LED is commonly used for light-emittingdiodes made of inorganic semiconductor material;they are point light sources. Light-emitting diodesmade of organic semiconductor material arereferred to as OLEDs; they are planar lightsources. A separate chapter deals with OLEDs.

Until the mid-1990s of the last century, LEDshad a low lumen output and low efficiency, mak-ing them only suitable as small indicator lamps(e.g., in electrical appliances). Today, the efficacyof LEDs is comparable to that of gas dischargelamps. The lumen output of a single LED can bemore than 1,000 lm. To distinguish these LEDS

Modified and reproduced from [1] by permission of PhilipsLighting, Eindhoven, The Netherlands. # 2012Koninklijke Philips Electronics N.V.

from the indicator type of LEDs, they are referredto as high-brightness or high-power LEDs(Fig. 1). Further improvements in high-brightnessLEDs are expected, ultimately leading to effica-cies of probably slightly more than 200 lm/W (forwhite-light LEDs). This is approximately twicethe efficacy of today’s most efficient white-lightgas discharge lamps. With its light-emitting sur-face of some 0.5–5 mm2, an individual LED chiprepresents the smallest artificial light source.LEDs have a long lifetime and are, given thesolid-state material, extremely sturdy. They areavailable in white and in colored-light versions.The colored versions are extensively used in traf-fic signs. Colored versions were also the first onesto be used on a large scale for lighting: specifi-cally, the exterior floodlighting of buildings andmonuments. Both the efficacy and the color qual-ity of white LEDs have been improved so muchthat they are now used in many different lightingapplications, including road lighting, indooraccent lighting, domestic lighting, and automobilelighting. Examples of the use of LEDs for officelighting and outdoor sports lighting can also befound. Given the potential for a further increase intheir efficacy, the number of LED applications isbound to increase. Their small size and their avail-ability in many different colors and the easiness oflighting control, both in terms of dimming andcolor changing, are properties that permit ofcompletely new applications.

Working Principle

Principle of Solid-State RadiationLike any diode, an LED consists of layers ofp-type and n-type of semiconductor material.The n-type of material has an excess of negativelycharged electrons, whereas the p-type material hasa deficiency of electrons, viz., positively chargedholes. Applying a voltage across the p-n semicon-ductor layer pushes the n- and p-type atomstowards the junction of the two materials(Fig. 2). Here the n-type of atoms “donate” theirexcess electrons to a p-type of atom that is defi-cient in electrons. This process is called recombi-nation. In doing so, the electrons move from ahigh level of energy to a lower one, the energy

Light-Emitting Diode,LED, Fig. 1 Indicator lampLEDs (left) and a high-power LED (right)

Light-Emitting Diode,LED, Fig. 2 Principle ofoperation of solid-stateradiators

Light-Emitting Diode, LED 851

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difference being emitted as light. The wavelengthof the light is dependent on the energy-level dif-ference between the p and n materials, which inturn depends on the semiconductor material used:different semiconductor materials emit differentwavelengths, and thus different colors, of light.The process is very much like the process ofexcited electrons in a gas discharge falling backto their original orbit with a lower energy levelwhile emitting light. In this sense it is surprisingthat the expression “solid-state discharge” and“solid-state discharge lamp” is hardly ever used.Not all recombinations result in light emission.Some recombinations are non-radiative and justheat up the solid material. This limits the effi-ciency of light creation. A further limitation ofthe efficiency is caused by absorption of light inthe solid material of the chip itself. Improvementsin radiative-recombination efficiency and light-extraction efficiency have been the most impor-tant reasons for the dramatic improvements ofefficacy of LEDs during the past decade. Furtherimprovements will be sure to further greatlyincrease the efficacy of LEDs over the comingdecade.

Like gas discharge lamps, solid-state lampscannot function when they are operated directfrom the mains supply voltage. Solid-state lightsources are low-voltage rectifiers that allow cur-rent to pass in one direction only. This means thatthe AC mains supply has to be transformed to lowvoltage and then rectified into a DC supply. Smallfluctuations in supply voltage cause large varia-tions in current that can damage the light source.High-power LEDs need an electronic driver toobtain a constant current characteristic.

Principle of White LEDsThe spectrum of a single LED is always narrow.Consequently, its light is colored. White LEDlight can nevertheless be obtained by combiningthree (or more) differently colored LED chips.A common method is to combine red, green, andblue LED chips into a single module to producewhite light (RGB LED, Fig. 3). Sometimes thethree chips are mounted in the same LED package(Fig. 4). The color rendering of an “RGB white-light” system is not good, since large areas of thefull color spectrum are not included in its light.Sometimes an amber color chip is added to the

852 Light-Emitting Diode, LED

RGB combination to improve the color quality ofthe light (RGBA LED). Research is going on toproduce single, multilayer LED chips, each layerproducing a specific color of light. A single LEDproducing red, green, and blue light would there-fore result in white light.

Good-quality white light, which is especiallyimportant when it comes to providing good colorrendering, is obtained by using a blue LED chip incombination with fluorescent material that

Light-Emitting Diode, LED, Fig. 3 White light by com-bining red, green, and blue LED light [1]

Light-Emitting Diode, LED, Fig. 4 RGB chips mountedin a same LED package

Light-Emitting Diode,LED, Fig. 5 The maincomponents of a high-brightness LED [1]

converts much of the blue light into light of dif-ferent wavelengths spread over almost the wholevisible spectrum. In LED technology it is custom-ary to call such fluorescent materials, phosphors:hence white LEDs based on this principle arecalled “white-phosphor LEDs.” By mixing differ-ent phosphors in different proportions, whiteLEDs producing different tints of white lightwith different color-rendering capabilities canbe made.

Materials and ConstructionThe LED chip is embedded in a larger structurefor mechanical protection, for the electrical con-nections, for thermal management, and for effi-cient light out-coupling. The main parts of anLED are (Fig. 5):

• Semiconductor LED chip• Reflector cup• Supporting body• Electrodes and bond wires• Heat sink• Primary optics• Phosphors (for white LEDs)

Semiconductor Chip MaterialThe p-n semiconductor sandwich forms the heartof the LED and is called the LED chip or die. Thesemiconductor material used determines thewavelength and thus the color of the light emitted.For LEDs, compound semiconductor material isused, which is composed of different crystallinesolids. These are doped with very small quantitiesof other elements (impurities) to give their typicaln and p properties. For the colors blue, green, andcyan, the elements indium, gallium, and nitride(InGaN) are used in different compositions(Fig. 6, left). The elements aluminum, indium,

450 nmBlue

498-500 nmGreen-Blue

505 nmBlue-Green

525 nmGreen

590 nmAmber

AllnGaP ColoursInGaN Colours

605 nmOrange

626 nmRed

615 nmRed-Orange

Light-Emitting Diode, LED, Fig. 6 The main semiconductor material elements used in high-brightness LEDs, withexamples of the corresponding light colors [1]

Light-Emitting Diode, LED, Fig. 7 An example of aspecifically shaped LED chip that improves light-extraction efficiency. On top of the chip, the anode withits bond wire can be seen [1]

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gallium, and phosphide (ALInGaP) are used toproduce the colors amber, orange, and red(Fig. 6, right). It was the Japanese Nakamurawho, in 1993, succeeded for the first time inproducing a blue LED suitable for massproduction. This was the “missing link” thatenabled the production of white-phosphor LEDsand the production of white LED light on the basisof mixing the light of red, green, and blue LED-S. As can be seen from Fig. 6, today a very smallarea of the spectrum, in greenish yellow, is stillmissing.

Shape of the ChipUnfortunately, the LED chip is a “photon or lighttrap”: that is to say, much of the lightemitted within the chip is internally reflected byits surfaces (borders between the material andair) and, ultimately, after multi-reflections,absorbed in the material (heating up the material).Only light that hits the outer surface more orless perpendicularly (approximately 20�) canleave the material. By giving the chip a specificshape and by keeping it thin, the so-calledlight-extraction efficiency can be improved. Fig-ure 7 gives an example of such a specificallyshaped chip.

Reflector CupIn many cases, the chip is placed in a reflector cupwhich, because of its shape, helps to direct thelight in an upwards direction. Highly reflectivematerial is used: for example, metal or ceramicmaterial.

Primary OpticsThe silicon lens on top of the LED chip serves asprotection for the chip. More importantly, it helpsin increasing the light extraction from the chip andas such is essential for a high lumen efficacy of theLED. This is because by introducing a lensmedium between the chip and the air with arefractive index value between that of air andthat of the chip material, the angle over whichlight can escape from the chip increases.

Electrodes and Bond WiresIn order to be able to apply power to the chip, thep and n parts of the chip havemetal contacts calledelectrodes. Bond wires connect the electrodeswith the electrical connections. They are usuallygold wires. Since the electrodes intercept lightleaving the chip, the dimensioning of the

Light-Emitting Diode,LED, Fig. 8 Examples ofluminaires with cooling fins

Light-Emitting Diode,LED, Fig. 9 Principle ofcreating white light with ablue-light chip covered withphosphor

854 Light-Emitting Diode, LED

electrodes and bond wires, especially on the sideof the main light-escape route, is one of the factorsthat determines the light efficiency of the LED.

Heat SinkLEDs do not radiate infrared radiation and conse-quently give a cool beam of light. However, thisdoes not mean that they do not generate heat.Non-radiative recombinations of electrons andholes in the p-n sandwich, and light trapped inthe chip, do heat up the chip. The larger the powerof the chip and the lower its luminous efficacy, thehigher is this heating effect. The higher the tem-perature of the p-n junction in the chip, the lowerthe light output of the chip. A too-high chip tem-perature also seriously shortens LED life, and italso slightly shifts the emitted wavelength andthus the color of the LED. Effective thermal man-agement is therefore critical for a proper function-ing of LEDs. All high-power high-brightnessLEDs therefore have a heat sink of high-thermal-conductivity material (like aluminum or copper)on their rear side to conduct the heat away fromthe chip towards the outside world through theluminaire housing. LED luminaires must there-fore incorporate in their design thermal conduc-tion and convection features (such as cooling fins

Fig. 8) to dissipate the heat to the immediatesurroundings. For retrofit LED lamps (LEDbulbs), the size of the heat sink is limited by thesize of the bulb. The heat sink therefore has alimited capacity, thus limiting the power of theretrofit LED bulbs.

PhosphorsAs mentioned above, the most important methodfor producing white light with LEDs is by apply-ing a phosphor coating to a blue-light LED thatconverts part of the blue light into longer-wavelength, green, yellow, and red light. Differentcompositions of different phosphors are used toproduce white light of different color tints. Sincethe basis of the process is the blue light of the chip,the final efficacy will become higher as more blueis kept in the light. However, this implies a highcolor temperature (cool-white light) and relativelypoor color rendering. If, with a different phosphorcomposition, more blue light is converted, thecolor quality will improve at the expense of asomewhat-lower efficacy.

Phosphors Applied Direct to the Chip Thephosphor is often applied on or very near to theblue LED chip (Fig. 9). The thickness of the layer

Light-Emitting Diode,LED, Fig. 10 Principle of aremote-phosphor LEDmodule creating whitelight [1]

Light-Emitting Diode, LED, Fig. 11 LED cluster mod-ule with multiple LEDs mounted on a printed circuit board

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has to be very uniform as a variation in thicknessof the phosphor layer causes a variation of thecolor temperature in the light beam.

Remote Phosphor In the case of multi-LEDunits, the phosphor is sometimes applied at agreater distance from the LEDs. Such modulesare called remote-phosphor LED modules(Fig. 10). Here, a number of blue LEDs are placedinside a mixing chamber of high and diffusereflective material. The phosphor layer, posi-tioned remotely from the LEDs on the bottom ofthe chamber, converts the blue light of the chipsinto white light. In this way, thanks to the mixingprocess, small differences in light output and orcolor of individual chips are not visible. The riskfor disturbing glare is also reduced because thelight intensity from the large-sized phosphor layeris much lower than the intensities of small, indi-vidual LEDs. In normal phosphor LEDs the hotLED heats up the phosphor that is applied on theLED itself, reducing its conversion efficiency.Heating of remote positioned phosphor is muchless and thus the conversion efficiency of remote-phosphor systems is higher. In normal phosphorLEDs a relatively large amount of light from theLED is reflected back from the phosphor towardsthe LED and absorbed there. In the remote-phosphor situation, most of the light reflectedback from the phosphor layer takes part in themixing process without reaching the LEDs, thusfurther increasing the final efficiency of the sys-tem. The phosphors used for the blue-light con-version appear yellow when they are notactivated, that is to say, when the LED is notswitched on (see Fig. 10 right). This sometimesmakes people think, erroneously, that the blue

light is filtered through a yellow filter. It is reallywavelength conversion and not light filtering thattakes place.

LED Cluster ModulesThe luminous flux of one individual LED is quitelow compared to that of most conventional lightsources. Multiple LEDs are therefore oftenmounted on a printed circuit board (PCB) toobtain an LED module emitting a high luminousflux (Fig. 11). The PCB establishes the electricalconnections between all components and theexternal electrical driver. The PCB must also con-duct the heat from the heat sinks of the LEDs tothe outside world.

856 Light-Emitting Diode, LED

Properties

Temperature of the Chip JunctionIt has already been mentioned that with risingtemperature of the p-n chip junction, the perfor-mance of LEDs decreases: particularly the lightoutput and lifetime. The performance data areusually specified for a junction temperature (Tj)of 25 �C. However, under normal operating con-ditions, a junction temperature of 60–90 �C iseasily obtained. Depending on LED type, thelumen output falls to 60–90 % when the junctiontemperature increases from 25 to 80 �C. Amberand red LEDs are the most sensitive to changes injunction temperature, and blue LEDs the least.

BinningThe mass production of LEDs results in LEDs ofthe same type varying in color, light output, andvoltage. In order to ensure that LEDs neverthelessconform to specification, LED manufacturers usea process called binning in their production pro-cess. At the end of the manufacturing process,LED properties are measured and LEDs are sub-sequently sorted into subclasses or “bins” ofdefined properties. As far as the color quality isconcerned, the tolerances in these definitions(based on MacAdam ellipses) are such thatvisible differences in color between LEDs fromthe same bin are minimized. As the definition of abin does not change with time, the same quality isalso assured from production run toproduction run.

With the advancement of knowledgeconcerning LED materials and the mass-production process, it may be expected that bin-ning will ultimately no longer be required(“binning-free LEDs”).

Energy BalanceThe energy balance of an LED is much easier tospecify than that of conventional light sources.This is because no energy is radiated in the UVand infrared region of the spectrum, which meansthat the energy balance comprises only visibleradiant energy and heat energy. Today, whiteLEDs transform 20–30 % of the input powerinto visible light and the remaining part into

heat. The light percentage comes close to that offluorescent lamps and will soon supersede it.

System Luminous EfficacySometimes, luminous efficacies are specified forthe bare chip. It is evident that the ancillarydevices described in the previous sections, whichare essential for a proper functioning of LEDs,absorb light. The only realistic thing to do, there-fore, is to specify luminous efficacy (and lightoutput as well) for the total LED package. Aswith most conventional lamps, the luminous effi-cacy of LEDs is dependent on the power of theLED and on the color quality of the light it pro-duces. Higher-power LEDs have higher effica-cies, while those with better color rendering havelower efficacies. Today cool-white LEDs are com-mercially available in efficacies up to some 150lm/Wand warm-white LEDs with color-renderingindices larger than 80 in efficacies of around 130lm/W (all lm/W values include driver losses).Retrofit LED bulbs with warm-white light(around 2,700 K) and color-rendering index betterthan 80 are available in efficacies up to80 lm/W. Here, too, cooler-white versions areslightly more efficient.

As discussed at the beginning of this chapter, incoming years it may be expected to see furtherimportant improvements in luminous efficaciesfor white LEDs – even up to slightly more than200 lm/W.

Lumen-Package RangeToday, single LEDs exist in lumen packages vary-ing from a few lumen (indicator lamps) to morethan 1,000 lm. In the latter case, severe screeningis called for to restrict glare, because so much lightcomes from such a very small light-emitting sur-face. Bymounting multi-LEDs on a printed circuitboard, LED modules can be achieved with muchlarger lumen packages.

Color CharacteristicsPrincipally, LEDs have a quasi-monochromatic,narrow-band spectrum. Figure 12 shows the spec-tra of blue, green, and red LEDs. The half-maximum width of the spectrum bands is smallerthan ca. 50 nm. It has been already been shown

Light-Emitting Diode,LED, Fig. 12 The relativespectral power distributionsof a typical blue, green, andred LED

Light-Emitting Diode,LED, Fig. 13 The relativespectral power distributionof a white-phosphor LEDwith color temperatureTk = 4,000 K and colorrendering Ra = 70

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what colors can be produced by using differentsemiconductor materials (see Fig. 6). With someLED modules, different colored LEDs can becontrolled (dimmed) individually. With suchLED modules (e.g., making use of RGB LEDs)the color of the light can be dynamically changedfrom white to all colors of the spectrum.

White phosphor LEDs can have a near-continuous spectrum (Figs. 13 and 14). By apply-ing different phosphors, white light within thecolor temperature range of 2,700–10,000 K canbe produced. Often, the higher-color-temperatureversions have only moderate color rendering (Ra

between 50 and 75). In the lower-color-temperature versions, LEDs are available with

good (Ra larger than 80) to excellent color render-ing (Ra larger than 90 or even 95).

Beam ControlFor lighting designers, one of the most interestingproperties of an LED is its small light-emittingsurface. This allows the creation of very accu-rately defined beams. As an illustration of this,Fig. 15 shows a near-parallel light beammade with an LED-line luminaire that is impossi-ble to create with conventional light sources.The other side of the coin is that smalllight-emitting surfaces often need professionalscreening in order to limit excessive glare.Multi-LED luminaires have also multi-light

Light-Emitting Diode,LED, Fig. 14 The relativespectral power distributionof a white-phosphor LEDwith color temperatureTk = 2,750 K and colorrendering Ra = 85

Light-Emitting Diode, LED, Fig. 15 Near-parallel lightbeam with an LED-line luminaire [1]

858 Light-Emitting Diode, LED

beams that may cause multiple shadowsbecause a lighted object is illuminated frommany slightly different directions. With RGBcolor mixing, this may lead to disturbing,multicolored shadows.

LifetimeIn the case of high-performance LEDs, it takes avery long time before they actually fail – usually

considerably more than 50,000 h. Before thattime, however, their lumen depreciation is sogreat that the LED is no longer giving sufficientlight for most applications. Therefore, for LEDlifetime specifications, the length of time that ittakes to reach a certain percentage of its initiallight value is used. Based on a depreciation valueof 70 %, lifetime values of between 35,000 and50,000 h are common for high-performanceLEDs. LED bulbs, with their limited space forhandling heat, have a lifetime of some25,000–35,000 h (25–35 times longer than anincandescent lamp).

Lumen DepreciationThe electric current passing through the chip’sjunction, and the heat generated in it, degradesthe chip material and is so responsible for lightdepreciation. Decoloration of the housing andyellowing of the primary lens may be furtherreasons for lumen depreciation. In white-phosphor LEDs, chemical degradation of thephosphor material also causes lumen deprecia-tion. As mentioned above in the section on LEDlifetime, for high-performance LEDs 30 % lumendepreciation is reached at around35,000–50,000 h.

Run-Up and ReignitionLEDs give their full light output immediately afterswitch on and after reignition.

Light-Emitting Diode,LED, Fig. 16 Single LEDs

Light-Emitting Diode,LED, Fig. 17 PCB-mounted LEDs

Light-Emitting Diode,LED, Fig. 18 LED engines(bottom: for use in roadlighting)

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DimmingLEDs can be dimmed by simple pulse-width mod-ulation down to 5 % of full light. Not all retrofitLED lamps can be dimmed on normal, commer-cially available dimmers. Special retrofit LEDlamps that are designed to be dimmed on suchdimmers are so specified on their packaging.

Ambient-Temperature SensitivityAs has already been mentioned several times,limitation of the junction temperature of theLED chip is essential for the proper functioningof LEDs in terms of lumen output, lumen efficacy,

lamp life, and even color properties. In high-temperature environments the products performworse, while at low temperature they performbetter. The actual influence of the junction tem-perature is different for the different types ofLEDs. In extreme temperature environments,therefore, relevant information for a particularproduct has to be obtained from the manufacturer.

Mains Voltage VariationsLED drivers are designed to drive LEDS on con-stant current. In this way the influence of mainsvoltage variations is not an issue.

Light-Emitting Diode, LED, Fig. 19 Foldable LED string

Light-Emitting Diode,LED, Fig. 20 Retrofit LEDlamps for incandescent,halogen, and fluorescentlamps, respectively

860 Light-Emitting Diode, LED

UV and IR ComponentLEDs radiate only visible radiation. There is noultraviolet or infrared radiation.

Product RangeLED products are available as:

• Single LEDs (Fig. 16).• Multiple LEDs on flat or three-dimensional

PCB boards (Fig. 17).

• LED modules (or LED engines) with second-ary optics and with or without built-in driverthat can be used in the same way as lamps.Interfaces are standardized for interchangeabil-ity (Fig. 18).

• Multiple LEDs on strings (Fig. 19).• Retrofit LED lamps with built-in driver and

conventional design (Fig. 20); new type ofdesigns are introduced for retrofit LED lampsas well (Fig. 21).

Light-Emitting Diode,LED, Fig. 21 Designertype of retrofit LED lamps

Light-Emitting Diode, OLED 861

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Cross-References

▶Light-Emitting Diode, OLED▶ Phosphors and Fluorescent Powders

References

1. Van Bommel, W.J.M., Rouhana, A.: Lighting Hard-ware: Lamps, Gear, Luminaires, Controls. CourseBook. Philips Lighting, Eindhoven (2012)

2. Schubert, E.F.: Light Emitting Diodes, 2nd edn. Cam-bridge University Press, Cambridge, UK (2006)

3. Mottier, P.: Led for Lighting Applications. Wiley,Hoboken, NJ (2010)

4. DiLaura, D.L., Houser, K., Mistrick, R., Steffy, G.: IESHandbook. 10th edn. Illuminating Engineering Societyof North America, IESNA, New York (2011)

Light-Emitting Diode, OLED

Wout van BommelNuenen, The Netherlands

Synonyms

Optoelectronic light source; Solid-state lightsource

Modified and reproduced from Ref. [1] by permission ofPhilips Lighting, Eindhoven, The Netherlands. # 2012Koninklijke Philips Electronics N.V.

Definition

Light source that produces light as a result ofrecombination of positively charged holes andnegatively charged electrons at the junction oforganic, solid-state, p- and n-type planar semicon-ductor materials.

Solid-State Light Sources

LEDs are solid-state radiators where the light iscreated inside solid-state material [2–4]. Thelight-radiating diode versions are called light-emitting diodes or LEDs. Light-emitting diodesmade of organic semiconductor material arereferred to as OLEDs; they are planar light sources(Fig. 1). Organic material is a semiconductorchemical compound whose molecules containcarbon and hydrogen (C – H bonds). The nameLED is commonly used for light-emitting diodesmade of inorganic semiconductor material; theyare quasi point light sources. A separate chapterdeals with LEDs. Serious development of OLEDsonly started in the mid-1990s of the last century.OLEDs can be produced to give most colors of thespectrum and white. The technology permits pro-ducing OLED windows that are transparent whennot switched on (Fig. 2). Currently, the mainapplication for OLEDs is displays for mobilephones and for other display screens including

Light-Emitting Diode,OLED, Fig. 1 Flat OLEDlight sources

Light-Emitting Diode,OLED, Fig. 2 OLEDwindow [1]

862 Light-Emitting Diode, OLED

small television screens. OLED lighting productsare now gradually also coming onto the market forilluminating purposes. Initially, the most interest-ing application for OLEDs, in this respect, isarchitectural, decorative lighting. For generallighting purposes the efficacies have to beimproved much further.

Working Principle

The process that is responsible for the emission oflight in OLEDs is very similar to that with LEDs:positively charged holes and negatively chargedelectrons are pushed through semiconductorlayers towards each other and recombine (seeentry “▶Light-Emitting Diode, LED”). Part of

these recombinations results in the emission oflight. The color of the light is dependent on thecomposition of the semiconductor material. Whitelight can be obtained by bringing phosphorescentmaterial in the emissive layers.

Materials and Construction

The semiconductor organic layers are placedbetween electrodes, the one on the light-escapeside being transparent (Fig. 3). The layers aresupported by a glass substrate and, for protectionof the organic materials against oxygen andwater, are sealed in glass. Development is goingon to substitute the glass seal by thin-filmencapsulation. This development not only

Light-Emitting Diode,OLED,Fig. 3 Composition of anOLED [1]

Lighting Control Systems 863

reduces the thickness and weight of OLEDs butalso opens up the possibility of bendable OLEDs.The first laboratory examples have already beenproduced.

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Properties

Commercial OLED products for lighting are justrecently coming onto the market. Data of theproperties of commercial products are onlyscarcely available. It is expected that they willchange relatively fast the coming years. Only arough indication will therefore be given here.

System Luminous Efficacy and BrightnessCommercially available white OLEDs now haveefficacies between 15 and 30 lm/W. Ultimately,white-light, large-sized OLEDs with efficacies upto 150 lm/W would seem to be possible. Productswith luminance values of up to 2,000 cd/m2 havebeen shown. Compare this with a luminance valueof some 6,000–10,000 cd/m2 of fluorescent tubes.

DimensionsToday sizes go up to some 30 � 30 cm. Theexpectation is that that sizes of more than 100 �100 cm will be available in just a few years.

LifetimeLifetimes of some 10,000 h have been reported.

Color PropertiesAs has been mentioned, by choosing differentmaterials of semiconductor material, different

colors are obtained. The quality of white lightdepends upon the combination of the semicon-ductor and phosphorescent material used.Spectra similar to those of inorganic LEDs areobtained (see entry “▶Light-Emitting Diode,LEDD”).

Cross-References

▶Light-Emitting Diode, LED▶ Phosphors and Fluorescent Powders

References

1. Van Bommel, W.J.M., Rouhana, A.: Lighting Hard-ware: Lamps, Gear, Luminaires, Controls. Coursebook, Philips Lighting, Eindhoven (2012)

2. Schubert, E.F.: Light Emitting Diodes, 2nd edn. Cam-bridge University Press, Cambridge (2006)

3. Mottier, P.: Led for Lighting Applications. Wiley,Hoboken, NJ (2010)

4. DiLaura, D.L., Houser, K., Mistrick, R., Steffy, G.:Illuminating Engineering Society of North America,New York. 10th edn. (2011)

Lighting Control Systems

▶Lighting Controls

864 Lighting Controls

Lighting Controls

Peter DehoffZumtobel Lighting, Dornbirn, Austria

Synonyms

Lighting control systems; Lighting management

Definition

Lighting controls are intelligent network-basedlighting solutions. They incorporate the commu-nication between various system inputs and out-puts related to lighting control. Input can be givenmanually by humans and/or by sensor signals orparameter settings. Part of the control systems areunusually one or more local or central computingdevices.

Basics

Lighting control is understood tomean the manualor automatic tuning of a lighting situation inside abuilding, fixed to a building or in an outdoor area.In its simplest form, the tuning is performed usinga light switch or dimmer; in more complex con-figurations, sensors are used as well as controlcurves (time lines) and/or control inputs. As abasic requirement, the lighting installation mustallow the light sources it incorporates to be variedindividually, in groups or all together. As a mini-mum, this means the ability to alter the brightnessof the light sources but can also include light colorand/or spectral composition or even colors. Thelighting installation incorporates not only the lightfittings (luminaires), operator control units, andcontrol devices, but also the blinds or other shad-ing devices which regulate the amount of daylightentering the building. The emergency lightingsystem can also be integrated. In the case of build-ing management systems, the lighting installationis linked up with other automated services such asheating, ventilation, and air-conditioning.

This type of integrated lighting control is oftenreferred to as lighting management. The purposeof lighting management is to provide the rightlight in the right place at the right time [1–3].

Overview

The usefulness of lighting control is easy to under-stand as tuning the light for a particular purpose isnot an invention of modern times. Human beingsintuitively seek the ideal lighting situation. Theyspend their lives alternating between periods ofactivity and relaxation in keeping with the rhythmof day and night. Over the course of the day andthe year, they seek and experience the change andvariation of natural light.

The invention of electric light brought with it theadvantage of having “brightness at the push of aswitch,” available at all times. However, there wasalso an inherent technical limitation: the ability totune the electric light was initially restricted toswitching it on and off. Lighting was a staticarrangement. Despite the fact that the incandescentlamp was dimmable from the outset, this featureproved difficult in the case of discharge lamps.The first widely used fluorescent lamps as well ashigh-intensity discharge lamps proved reluctantwhen it came to dimming. Both the physics ofthese lamps and the ballasts required to operatethem made it difficult to regulate their light outputwithout considerable effort. Merely switching themon and off therefore became the common practice.Switching different groups of luminaires was theonly approach that came anywhere near “lightingcontrol.”

In the meantime, the available technology hasvastly improved. Today’s fluorescent lamps canbe cost-effectively dimmed using electronic bal-lasts, and in the case of LED light sources, tuningthe brightness was never a problem. High-intensity discharge lamps, on the other hand, con-tinue to hold out against straightforward dimming.

Lighting control is attracting a lot of attentionand is considered a necessary component of alighting solution.

The main reasons for using lighting control areas follows:

Lighting Controls 865

To increase energy efficiency: Dimming andswitching reduce the amount of energy con-sumed by the artificial lighting. When userswant less or no light, the artificial lighting canbe dimmed or switched off. And if sufficientdaylight is available, this can partially orentirely replace the artificial lighting.

To improve the light quality: The ability to adjustthe lighting environment to suit the activity orindividual preference ensures optimum lightquality. Light can be used to set the scene ininteriors and in outdoor areas.

To enhance safety: Emergency lighting can beintegrated.

To ensure flexibility: By readdressing the individ-ual luminaires, it is possible to provide a fastand flexible response to changes in organiza-tion and work layouts. Individual lightingscenes (moods) can also be programmed andchanged.

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Applications

Where should lighting control be used? Theemphasis will vary depending on the application:

ShopsIntuitive control points invite users to adapt thelighting scenes to suit the activity – ranging fromlight for working at the cash desk to attractivecolor changes in the lounge.

Growing awareness of the need to save energyhas paved the way for dimmable luminaires inretail spaces. It is possible to enhance the visualimpression of merchandise and architecture bycoordinating changes in color temperature andluminance. High comfort, great flexibility, andlow maintenance are the characteristic features ofthese controlled lighting solutions. The light spec-trum, for instance, can be optimally adjusted to suitthe illuminated object without the complication ofhaving to change filters. Using control points, time-lines, or daylight-linked management of the artifi-cial lighting, the general illumination can also begradually changed in linewith expected light levelsover the course of the day. The staging of merchan-dise to create strong emotional appeal and surprise

effects is achieved with the appropriate static anddynamic lighting scenes. Making use of availabledaylight not only saves energy but also helps tomake zones within an interior particularlyattractive.

FaçadesThe use of LED technology in conjunction withlighting control systems has brought about a rev-olution in façade illumination. The subtlest ofmessages can be communicated using mediafaçades. Dynamic façade design in particular isaimed at directing the gaze and conveying infor-mation. While ecological discussion focuses pri-marily on the amount of scattered light in the nightsky, avoiding the unnecessary use of light bydefining sensible operating hours would seemmore important. The ultimate purpose of stagingfaçades with light is to attract attention. It there-fore makes sense for façades to be lit up exclu-sively in the evening and morning hours whengreater numbers of people are circulating. Thisapproach enables the identity of companies andcommunities to be highlighted, structures outdoorspaces at night, and assists nighttime perceptionwhile simultaneously addressing ecologicalobjectives.

MuseumsLighting control ensures that sensitive exhibits areonly exposed to light which is absolutely neces-sary, i.e., the level of luminance or the light colorrequired for good perception or when visitors arepresent, e.g., using occupancy sensing. On and offtimes can be defined for specific hours of the day.Blinds control and daylight sensors ensure thatexactly the right amount of daylight is admittedto achieve a balance between the needs of archi-tecture, human well-being, preventing harm toexhibits, and saving energy. The emergency light-ing is discreetly and safely integrated into thelighting management system which provides cen-tral monitoring and ensures reliable visibility in anemergency situation.

HotelsIn hotel rooms, intuitive control points whichallow the guest to make selections according to

866 Lighting Controls

individual preference are the first priority. Theguest sets the lighting environment. With conve-nient control of the blinds, the levels of artificiallight and daylight are adjusted to suit differentactivities and visual needs such as watching TV,putting on makeup, or reading. Dynamic lightingmoods, with flexible definition based on time-lines, or controlled according to weather situationor time of the day, significantly affect the well-being of the guests, particularly in hospitality orwellness areas. In entrance zones, lighting moodsbased on the outdoor light can also optimizevisual adaptation for arriving guests, giving themsecurity and orientation. Defined lighting scenesin conference areas support the use of differentmedia and enable the appropriate light to be pro-vided at the press of a button.

Health and CareA homely atmosphere and care activities call forentirely different lighting scenes which are avail-able at the press of a button. Intuitive operatingfeatures to suit the age and physical abilities of theoccupants as well as the accessibility of the con-trol units are the key to successful lighting solu-tions in hospitals and care homes. The amount oflight required by the eye increases with age. Forprecise visual tasks, the artificial lighting can beindividually adjusted by care staff and patients. Inaddition, the aging eye filters out blue light, affect-ing biological processes such as the internal clockand cycles of rest and repair. This must be com-pensated at specific times of the day by spendingtime outdoors or through biologically effectiveartificial light with high intensity and a high bluelight component. The use of timelines in lightingmanagement systems permits this interplay ofartificial light and daylight at appropriate timesof the day.

OfficeLighting control optimally addresses individuallighting needs depending on the age and visualtask. Concentrated work and increasing commu-nication call for entirely different requirements tobe met by the lighting concept. Biologically effec-tive artificial lighting components at the right timeof day in addition to daylight help the internal

clock and raise alertness. Lighting managementsystems with a high level of automation canachieve maximum energy savings as well as flex-ibility in the case of office moves, thanks to timemanagement, daylight-linked control, and occu-pancy sensing. Employees readily accept the tech-nology when it gives them the freedom to altertheir lighting environment. This means providingadequate means of control and small groups ofluminaires with assigned access.

IndustryIndustry offers particularly high potential for sav-ing energy. Long periods of lighting use due toshift and night work as well as a lack of daylight inmany work areas and rooms mean that invest-ments in lighting management have short paybacktimes. Lighting management ensures the requiredflexibility in production areas. New lightinginstallations have to be overdimensioned inorder to take into account the effects of deteriora-tion of the installation over time. This additionalenergy consumption can be counteracted withdaylight-linked or constant light control whichcontinuously adjusts lamp output in line with theavailable daylight or length of service. Integratedlighting solutions allow the interaction betweendifferent services. Maintenance and monitoringoperations are also optimized through the incor-poration of emergency lighting. Interfaces to otherservices ensure the convenient and cost-effectiveoperation of buildings.

EducationNew forms of teaching and new media technolo-gies call for flexible room use and frequent adap-tation of the lighting situation. Intuitive controlunits are used to select defined lighting scenes atthe press of a button, i.e., for working in smallgroups or traditional teaching, reduced lightinglevels for presentations using LCD projectors, orhigher vertical illuminance for blackboards andflip charts. Comfortable control units enable theuser to rapidly adapt the lighting situation.

Daylight activates human beings and enhancestheir feeling of well-being as well as their perfor-mance. With daylight-linked control of the artifi-cial lighting or occupancy sensing, it is possible to

Lighting Controls 867

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achieve maximum energy savings withoutrestricting the quality of light. Blinds control notonly improves the contrasts of presentation mediabut also increases room comfort because glare andheat gain can be minimized. Conveniently placedcontrol units near the door and on the teacher’sdesk enable intuitive selection of the appropriatelighting situation.

Corridors and StairwaysCirculation zones are predestined for lightingmanagement. Ideally, the lighting should only beactivated if a person approaches. The light shouldbe on once the person has entered the circulationzone. Movement detectors must be suitably posi-tioned. The energy consumption of lighting incorridors and stairways can be further reduced inconjunction with daylight-linked control.

Streets and Public SpacesIllumination is necessary for reasons of safety andorientation in streets and public spaces. Switchingthe lighting on and off in conjunction with theavailable daylight has long been a common prac-tice. Nowadays, it is also possible to reducebrightness levels at hours of the night when trafficdensity decreases. Control is performed centrallyfor streets, districts, and public spaces.

Movement detectors can be installed to ensurethat the brightness of the lighting anticipates theapproach of pedestrians and cyclists. While thegeneral brightness level is lowered, it can beincreased at the point of use to ensure safecirculation.

DomesticBeing able to vary the lighting takes on particularemotional importance in the home setting. Well-being and work, social interaction, and intimacyall take place within the confines of a small space.Access to different lighting moods at the press of abutton enables the occupants to set the scene fortheir private space with area, accent, and moodlighting. Blinds are frequently integrated into thecontrol system to regulate solar heat gain. Timecontrol is also a security feature which can be usedto make the house appear occupied when theresidents are away on holiday.

Individual lighting scenes can be selected froma central control panel. Switches incorporating areduced number of scenes on doors and in readilyaccessible locations allow intuitive operation ofthe lighting installation.

The possibilities and the requirements of theapplication are taken into account in the lightingconcept. The lighting designer incorporates theoperator control units, the control devices, andthe choice, number, and arrangement of lumi-naires, blinds, and other actuators.

Interfaces for Lighting Management

Generally, there are interfaces between theuser, the environment, and the lightinginstallation.

The user can actively intervene using differentoperator control units:

• Standard or momentary action switch, with orwithout dimming function: The simplest formis a switch and/or dimmer frequently fitted nearthe door or in another readily accessible loca-tion in the room.

• Multifunction switch: Also near the door or in areadily accessible location, for calling up dif-ferent lighting scenes.

• Remote control: This can be an IR or wirelessunit or the user’s mobile phone, for calling upindividual lighting scenes.

• Workstation control: In this case, operation isthrough the user’s own computer or using acontrol unit directly at the workstation.

• Central control unit: At a defined location,trained personnel can select specific scenes ortimelines for the lighting installation, oftenusing a touch screen.

The environment can be integrated by meansof sensors or defined timelines:

• Daylight sensors: Sensors installed outdoors ornear daylight openings detect the level of day-light brightness and transmit the appropriatesignals to control or regulate the lighting instal-lation (luminaires and blinds).

868 Lighting Controls

• Presence or absence detectors: These sensewhether there is a person within their detectionzone. The signals they transmit switch thelighting installation on or off.

• Time signals: The lighting can be switched topreselected scenes at set times. The simplestcase is to switch off all the lights at specifictimes. It is also possible to call up timelines toautomatically tune the lighting in accordancewith programmed strategies.

• User behavior sensors: Signals based not onlyon presence or absence but also on user move-ment patterns can be used to initiate situation-based tuning of the lighting situation.

• Environmental signals: Other predefined datacan be used for control purposes, e.g., to main-tain constant luminous flux on the basis of setmaintenance factor curves or to reduce instal-lation output if the energy demand exceeds aspecific limit.

Behind each operator function, there are sig-nals which are processed by the lighting manage-ment system. This means, for instance, that if asensor detects that a person has left the room,switch off can be delayed.

Intelligent control is based on the following logic:for each activity, a lighting scene is defined, saved,called up, and modified as required. Each scene isassigned a name and a symbol which are shown onthe operator control units. The lighting scenes areactivated automatically by means of presence detec-tors or simple time inputs. The basic philosophy isalways the same: it must be possible to manuallyoverride any activated lighting scene.

Strategies for Lighting Management

What strategies can be used to achieve usefulobjectives with a lighting management system?Firstly, those objectives have to be defined andsecondly, acceptance of the usefulness of theobjectives put to the test.

One objective is to increase energy efficiency:

1. Individual switch on and off: This simplestobjective is dependent on a switch being

located near the user. Users appreciate thispossibility but frequently do not use it.

2. Daylight-linked control – switching function:Automatic switch on and off; thecontrol input is the daylight measured by asensor.

3. Daylight-linked control – switching function:Automatic switch off and manual/individualswitch on; this requires a daylight sensor anda switch.

4. Daylight-linked control – dimming: Auto-matic setting at a constant value; the controlinput is the daylight measured by a sensor.

5. Daylight-linked control – dimming: Auto-matic switch off and individual switch on;this requires a daylight sensor and a switch.

6. Occupancy-based control – switching func-tion: Automatic switch on and off; the controlinput is delivered by a sensor which detectshuman presence.

7. Occupancy-based control – switching func-tion: Automatic switch off and individualswitch on; this requires a presence detectorand a switch.

8. Constant light control (without daylight):A constant level of luminous flux ismaintained to compensate for the decreasein light output with increasing age of theinstallation; the control input is delivered bya programmed timeline or a sensor for theemitted luminous flux.

9. Time-based control: Programmed lightingscenes are called up, and the installation isswitched on or off at set times.

10. Load shedding: Automatic limitation ofenergy demand.

Another objective is to improve the quality oflight:

11. Activity-related control: Individual setting ofa lighting scene to suit an activity; thisincludes the switching and dimming of indi-vidual blinds, luminaires, and groups of lumi-naires in the adjacent area.

12. Individual daylight control: Individual opera-tion of blinds to avoid disturbances such asglare and heat.

Lighting Controls 869

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13. Automatic daylight control: Automatic con-trol of blinds to reduce heat gain and glarefrom direct sunlight and excessive outdoorbrightness; this requires a daylight sensor.

14. Algorithmic lighting: Automatic sequence ofvariations in the lighting based onprogrammed rules.

15. Room-related scene setting: Selection of pre-set static or dynamic lighting scenes.

User Acceptance and Energy EfficiencyWhile the various strategies might bring greatbenefits from a planning perspective, they alsohave to be accepted by the users. Many fieldstudies have looked at the question of useracceptance.

One of the essential requirements is the abilityto manually override each activated lightingscene. In comparison with automated processes,however, this makes it difficult to calculate theparameters for possible energy savings, the deter-mination of which is a major challenge in view ofthe many unknown factors.

The behavior of energy-conscious occupantsreceives too little attention in calculations of thiskind. Making the groups of luminaires as small aspossible provides greater opportunity to changethe lighting situation manually and individually tosuit requirements. Large rooms with severalgroups of luminaires do not lend themselves tothis approach as it is difficult for large numbers ofusers to agree the preferred setting of the lightinginstallation. For this reason, a smaller group ofluminaires and operator control units must beinstalled in close proximity to enable the user tomake spontaneous changes.

Changing weather conditions andunforeseeable user behavior make it difficult topredict energy usage. The time delay for presencedetectors also plays a role.

In view of their complexity, the dynamicchanges taking place in the world of work and inbuilding use are continually being analyzed andmeasured in field studies carried out by differentindustries. In the context of a lighting manage-ment system, human behavior will always bemore difficult to predict than technically definedparameters (Table 1).

Technical Requirements

The transmission of information from the operatorcontrol units and control devices to the actuators isat the heart of any lighting management system.Actuators are luminaires, blinds, or other equip-ment integrated into the lighting managementsystem.

Information transmission means the sendingand receiving of signals, which can be achievedusing various means:

• The signal “on/off/dim” is sent via thepower line.

• Signal transmission between operator controlunits and control devices takes place using thepower line.

• Signal transmission between operator controlunits and control devices and to the poweredluminaire/actuator uses a separate control linewith standardized control signals (DALI,DMX)

• Operator control units and control devices(e.g., computers) are networked via data lines.Communication is by means of bus (LON,KNX, Luxmate, etc.) or TCP/IP protocols.

DefinitionsBus protocols such as LON, KNX, and Luxmate:Bus systems network all technical services in thebuilding and provide central control for heating,ventilation, window blinds, and security systems.

TCP/IP: Transmission Control Protocol/Inter-net Protocol (TCP/IP) is the set of communicationprotocols used for the Internet and is consequentlyalso referred to as the Internet protocol suite.

DALI stands for Digital Addressable LightingInterface and is a standardized digital interface forcontrol gear and electronic ballasts. DALI can beused with a small number of lines to address largenumbers of control circuits over large distances.

DMX is an acronym for Digital Multiplex, astandard for digital communication networkscommonly used to control stage and event light-ing. Nowadays, the DMX protocol is also fre-quently used by architects and lighting designersas it enables over 500 channels to be controlledindividually with rapid signal sequences from asingle central control unit.

Lighting Controls, Table 1 The table shows the effect of individual strategies on energy efficiency and light quality andassesses the associated user acceptance

Strategy OperationEnergyefficiency

Lightquality Acceptance

1 Individual Low High High

2 Daylight-linked control – switching Auto off, auto on Medium Medium Low

3 Daylight-linked control – switching Auto off, manualon

High Medium Medium

4 Daylight-linked control – dimming Auto off, auto on High Medium Medium

5 Daylight-linked control – dimming Auto off, manualon

Very high Medium High

6 Occupancy-basedcontrol – switching

Auto off, auto on High Medium Medium

7 Occupancy-basedcontrol – switching

Auto off, manualon

Very high Medium High

8 Constant light Auto High Medium Medium

9 Time-based control Auto Very high Medium High

10 Load shedding Auto High Medium Medium

11 Activity-related control Manual Medium Very high Very high

12 Individual daylight control Manual Medium High Very high

13 Automatic daylight control Auto High High High

14 Dynamic lighting Auto High Very high Very high

15 Scene setting Manual Medium Very high Very high

Auto automatic

870 Lighting Controls

Luminaires, blinds, and other actuators inte-grated into the lighting management system haveto be compatible with the existing communicationprotocols and understand the signals. The actua-tors are assigned individual or group addresses sothat they can be uniquely indentified within thenetwork and to enable them to communicate.

In the case of classic lighting installationsequipped with fluorescent lamps, dimmable bal-lasts have to be used. High-intensity dischargelamps, on the other hand, can rarely be dimmed,at best switched in stages. As a rule, LED lightingincorporates dimming. With relatively simplemeans, it also offers the opportunity to vary thelight color (with a range from 2,700 to over8,000 K) or even to set different colors usingRGB diodes.

Lighting Design

The decision regarding a lighting managementsystem should be made at an early stage of theproject. Lighting management is an integral part

of the overall building management system. Forthis reason, the lighting management system willoften come under a different budget to the actuallighting installation itself.

The choice and layout of the luminaires shouldallow different lighting scenes to be selected.

The location of operator control units, the lay-out of the luminaires, and in particular the inte-gration of daylight should be part of a holisticdesign approach. Service and long-term supportfor the lighting management system will ensurethat the operators obtain the expected benefits.

Factors Driving Lighting Management

The growing demands for lighting installations tobe energy efficient are currently a strong driver forthe use of lighting management. European greenbuilding directives are leading to national regula-tions for energy requirements in buildings, whichcannot be met without the use of lighting manage-ment. Daylight-linked control and presence detec-tors are therefore a necessity.

Lippmann, Jonas Ferdinand Gabriel 871

Green building certification schemes also lookto reduce environmental impact with the aid ofintelligent lighting management. Well-knownschemes include LEED, BREEAM, DGNB, andCASBEE, among others. As well as helping toreduce energy consumption, the integration oflighting management can improve the quality oflight for the users.

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Cross-References

▶Architectural Lighting▶Electrical Control Gear for Lamps▶ Interior Lighting▶Road Lighting

References

1. Craig, D.L.: Lighting Controls Handbook. TheFairmount Press, Lilburn (2007)

2. Simpson, R.S.: Lighting Control. Focal Press, Oxford(2003)

3. David, D.L., Houser, K.W., Mistrich, R.G., Steffy, G.R.: The Lighting Handbook, 10th edn. IES, New York(2011)

Lighting Design

▶Architectural Lighting

Lighting for Indoor Spaces

▶ Interior Lighting

Lighting for Interior Spaces

▶ Interior Lighting

Lighting Management

▶Lighting Controls

Linguistic Influences on ColorPerception

▶Effect of Color Terms on Color Perception

Linguistic Relativism

▶Color Category Learning in Naming-GameSimulations

Lippmann, Jonas Ferdinand Gabriel

Mark D. FairchildCollege of Science, Rochester Institute ofTechnology, Rochester, NY, USA

Gabriel Lippmann was a French inventor andphysicist (born in Luxembourg) who created thefirst color photographs using what was certainly

872 Lorenz-Mie Theory

the first spectral imaging system. Lippmann’s sys-tem of color photography was conceived in 1886and then refined for several years due to the com-plex nature of its theory and implementation. Thesystem works by placing a very fine-grain photo-graphic plate in contact with mercury that acts as amirror. Light waves pass through the emulsion,reflect from the mercury backing, and then createan interference pattern within the emulsion. Thedeveloped plate then has an interference filter builtinto it due to the properly spaced layers of silver inthe emulsion (created by the interference patternexposure). The plates are then viewed with direc-tional lighting, and the observer sees the samewavelengths that were present in the scene – aspectral image reproduction. Lippmann presentedthe first color photograph using his system in 1891and then presented several nearly flawless photo-graphs made by Auguste and Louis Lumière cre-ated with the process. The process was difficultand time-consuming, and few have been able toreplicate the stunning photographs. Lippmann’swork certainly presaged modern photographicand holographic processes, and for it he receivedthe 1908 Nobel Prize in Physics.

Despite ending up a full professor at theSorbonne (University of Paris), Lippmann had noformal education beyond high school. He was astudent at the École Normale, but failed the exam-ination that would have qualified him as a teacherdue to his penchant for concentrating only on thework that interested him and neglecting the rest.However, he was appointed to a government sci-entific mission to Germany where he was able towork with the likes of Kirchhoff and Helmholtz. Atabout the same time, in 1873, he invented theLippmann capillary electrometer for precise mea-surements of extremely small electrical voltages. Itserved as the basis for early echocardiographs.Lippmann joined the Faculty of Science in Parisin 1878, became Professor of Mathematical Phys-ics in 1883, and was later appointed Professor ofExperimental Physics and Director of the ResearchLaboratory. He made many contributions in vari-ous fields of physics including electricity, thermo-dynamics, optics, and photochemistry.

In addition to his Nobel Prize for the Lippmannprocess of full-color photography, Gabriel

Lippmann served as Marie Curie’s thesis advisorat the Sorbonne and let her use his laboratory forher thesis work in radioactivity and helped herfind other sources of support. Lippmann died atsea in 1921 while returning from a voyage toCanada. There is no record of the cause of death.

Lippmann’s Nobel lecture on color photogra-phy can be found at the following link:

http://www.nobelprize.org/nobel_prizes/physics/laureates/1908/lippmann-lecture.html

Lorenz-Mie Theory

▶Rayleigh and Mie Scattering

Lorenz-Mie-Debye Theory

▶Rayleigh and Mie Scattering

Lovibond, Joseph Williams

Mark D. FairchildCollege of Science, Rochester Institute ofTechnology, Rochester, NY, USA

Low-Pressure Mercury Gas Discharge Lamps 873

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Joseph Lovibond was a British chemist andbrewer and is credited with inventing the com-mercial colorimetry, the Lovibond Tintometer. InLovibond’s Light and Colour Theories [1], weread “the writer was formerly a brewer, and thiswork had its origin in an observation that the finestflavour of beer was always associated with a col-our technically called ‘golden amber,’ and that, asthe flavour deteriorated, so the colour assumed areddish hue.” Such observations of the relation-ships between beer quality and observed color ledLovibond to his work on color standards as areliable means of reference and to the develop-ment of an instrument, the visual colorimeter, inwhich such standards could be systematically andobjectively applied.

The general form of the Lovibond Tintometerwas a visual colorimeter in which a split field isviewed. One half of the split field represented thetest sample, perhaps a cuvette of beer placed in abeam of light. The other side of the split field wascomposed of subtractive primaries of adjustabledensity (e.g., a series of cyan, magenta, yellow,and neutral glass filters) that could be adjusted toselect the density and overlapped one another inthe adjacent beam of light. By adjusting the den-sity of the filters, observers would match the testcolor stimulus and record the densities of thestandard filters required for the match. Noteagain that the Tintometer system was one of sub-tractive color mixing of standard materials ratherthan the more common additive color mixing ofstandard lights typically found in visualcolorimetry.

While brewing beer might have been the moti-vation for the Tintometer, by 1914, the systemwasin use in a wide variety of additional industriesincluding tanning, wine and spirits, dyeing andprinting, paint, water chemistry, ceramics, variousoils, and hematology. In many cases, specific ver-sions of the instrument and the reference stan-dards were produced for a given application. Thesystem was also presented with a number ofawards by international juries (including twogold, five silver, and two bronze medals) alongwith significant recognition from ten scientificsocieties (one gold, three silver, and five bronzemedals and a diploma). Lovibond worked

tirelessly in promoting his system through lec-tures and demonstrations around the world. Hecreated a technically successful system that alsomet with commercial success. In fact, TheTintometer Limited still exists to this day withproducts such as color standards and scales, visualcolorimeters (including versions of the LovibondTintometer), and photoelectric colorimeters andspectrophotometers. For example, one can stillpurchase instruments and standard color scalesdesigned for American (ASBC) and European(EBC) methods of specifying beer color.Lovibond’s initial inspiration is still beingaddressed by the progeny of his instrumentsusing his very techniques.

In the history of The Tintometer Limited, it isstated that “The Company was founded in 1885 byJoseph Lovibond, a prominent brewery owner whodeveloped the ‘colorimeter’ as a means of ensuringthe quality of his beer. By 1893 he had perfected hisresearch and introduced the first instruments.Muchhas developed since then. Today, the company isbringing colour measurement to the next genera-tion. While still recognizing the importance of tra-ditional methods, The Tintometer Ltd isintroducing new techniques to bring measurementand quality control to an even higher level, devel-oping creative solutions to ensure the continuedreputation of the Lovibond® brand” [2].

References

1. Lovibond, J.W.: Light and Colour Theories and TheirRelation to Light and Colour Standardization. E. & F. NSpon, Haymarket, London (1921)

2. www.lovibond.com

Low-Pressure Glow Discharge Lamps

▶Neon Lamp

Low-Pressure Mercury Gas DischargeLamps

▶Tubular and Compact Fluorescent Lamp

874 Lumen Depreciation

Lumen Depreciation

Wout van BommelNuenen, The Netherlands

Synonyms

Light depreciation

Definition

Light losses during the use of a lighting installa-tion, caused by the decrease of the luminous fluxof lamps, the dirt accumulation on and loss ofreflectivity and or transparency of luminaires,and dirt accumulation on or discoloration ofroom surfaces (in interiors).

Lamp Lumen Depreciation

The light output of virtually all lamp types declinesgradually duringoperation.The causes of light outputdepreciation are numerous. With incandescentlamps, it is especially the blackening of thebulb – caused by evaporation of the filament. Dis-charge lamps also suffer fromblackening, in this casedue to scattering of the electrode material, whichsettles on the wall of the discharge tube. With fluo-rescent lamps, high-pressure mercury lamps, andsolid state, LED, and OLED light sources with afluorescent coating, the major cause of light outputdepreciation is a gradual exhaustion of thefluorescentpowder, which slowly loses its effectiveness.

Because of the different causes of lamp lumendepreciation, the actual rate of decline of lumenoutput is different per lamp type. Lamp manufac-turers should be able to supply lamp lumen depre-ciation curves for their lamps.

Luminaire Depreciation

The gradual reduction of output of a luminaire iscaused by dirt that is gradually deposited on lamps

and on or in the luminaires and by a gradual loss ofreflectivity of reflectors and mirrors or loss of trans-parency of refractors due to corrosion and discolor-ation. Where the effects of dirt accumulation can beoffset by regular cleaning, the output loss due tocorrosion and discoloration cannot be regained.The rate of output reduction depends on the mate-rials used in the luminaire, the construction of theluminaire, and on the nature of airborne dirt in theatmosphere. The degree of protection provided bythe construction of the luminaire is classifiedaccording to the international protection code, IP,as described in an international IEC standard.

Room Surface Depreciation

Gradual dirt collection on room surfaces and/ordiscoloration of these surfaces gradually reducesthe interreflected component of light from theinstallation. This may especially have a noticeableeffect in interiors of smaller dimensions where,because of the relative large wall area,interreflection components are relatively large.

Maintenance Factor

The maintenance factor is the ratio of illuminanceor luminance produced by the lighting installationat the moment that maintenance operations arebeing carried out to the illuminance or luminanceproduced by that installation when new. Themaintenance factor takes into account all lightlosses described above together with the averagelight loss of the installation due to failed lamps notbeing replaced. The actual value is of course alsodependent on length of the period between main-tenance operations. The term light loss factor hassometimes been used instead of maintenance fac-tor. Depreciation factor has been formerly used todesignate the reciprocal of the above ratio.

The recommended lighting level (in terms ofilluminance or luminance) for a lighting design isbased on “maintained lighting level” which is theaverage lighting level at the moment that mainte-nance is being carried out. This implies that theinitial lighting level has to be:

Luminaires 875

Linitial ¼ Lmaintained=Maintenance factor

It is the responsibility of the lighting designer tomake an economical balance between therecommended length of the maintenance intervaland the extra light required for the initial installa-tion. Shorter maintenance intervals increase themaintenance cost but allow for lower initial light-ing levels with lower initial investment and lowerenergy costs.

Cross-References

▶ Interior Lighting▶Luminaires▶Road Lighting▶ Sports Lighting

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Luminaires

Wout van BommelNuenen, The Netherlands

Synonyms

Fitting (Anglo-Saxon English); Fixture(American English)

Definition

Devices that control the distribution of the lightemitted by a lamp or lamps and that include all theitems necessary for fixing and protecting thelamps (and sometimes the gear, too) and forconnecting them to the electricity-supplycircuit [1].

Luminaire Characteristics

The principal characteristics of luminaires can belisted under the following headings:

• Optical• Mechanical• Electrical• Thermal• Aesthetics

Optical Characteristics

The optical characteristics of a luminaire deter-mine the shape of its light beam, or light-outputdistribution. The light distribution of a luminairedefines how the luminous flux radiated by thelamp or lamps is distributed in the various direc-tions within the space around it. Different lightingapplications require different light distributionsand thus different luminaires.

The desired light distribution of a luminaire isobtained through the application of one or more ofthe physical phenomena: reflection, refraction,and diffuse transmission. Many luminaires alsomake use of shielding in one form or another,principally to obtain the required degree of glarecontrol and to limit light pollution. The shieldingfunction may be performed by refractors or dif-fusers or by mirror reflectors, by white-paintedsurfaces, or, where very stringent glare control isrequired, by black surfaces.

ReflectorsMany conventional luminaires are provided witha reflector (sometimes in conjunction with anotherlight-control element) in order to create the appro-priate light distribution. The reflecting materialthat is used for reflectors can be specular, spread,or diffuse.

Specular ReflectorsSpecular reflectors (also called high-gloss mirrorreflectors) are used when a precise form of lightdistribution is required, as in floodlights, spot-lights, and road-lighting luminaires. The reflectorcreates multiple images of the light source. Themost widely used material is sheet aluminum,which has the strength needed to produce a stablereflector. Reflectance values are around 0.70.Alternatively, commercial-grade aluminum canbe clad with a thin layer of super-purity aluminum

Luminaires, Fig. 1 A spread finish as produced by brushing (left) or by hammering (right) a specular surface

Plane reflectors Facetted reflectorsCurved reflectors

Luminaires, Fig. 2 Basicreflector forms

876 Luminaires

or silver. With aluminum, reflectance values of upto 0.80 can be obtained, while with silver areflectance of more than 0.90 is possible. Finally,there is vacuum metalizing, in which a specularlayer of aluminum is deposited on a suitablysmooth substrate (metal, glass, or plastics). Theresulting reflectance, which is somewherebetween 0.80 and 0.90, is dependent on both thesubstrate material and the quality of the metalizingprocess.

Spread ReflectorsWith spread reflectors (sometimes also calledhalf-matt reflectors), there is no sharp mirrorimage of the light source. They are employedwhere a moderate degree of optical control isrequired, with the emphasis on producing abeam with smooth transitions. Such reflectorsalso help to smooth out discontinuities in thelight distribution caused by inaccuracies in theshape of the reflector. Spread reflection is pro-duced by brushing or etching or by hammeringvery small dimples and bumps into a specularsurface (Fig. 1).

Diffuse ReflectorsAt the other extreme from specular reflection is dif-fuse reflection, which is also called matt reflection.Here light incident on the reflector is scattered in alldirections, so there is no mirror image of the lightsource. Matt reflectors cannot provide sharp beamcontrol, but are employed where diffuse ornon-focused light distributions are required. Matt-finished metals and white glossy paints on metal orglossy-white plastics provide near-diffuse reflection.The small specular component due to the gloss is ofno practical optical significance; the gloss merelyserves to facilitate cleaning. Reflectance values canbe in the range 0.85–0.90. Ceramic materials orfinishes have completely diffuse reflection character-istics with extremely high reflectances of up to 0.98.

Reflector FormsThere are three basic reflector forms: plane,curved, and faceted (Fig. 2).

Plane ReflectorsWhen using a simple plane (or straight-sided)reflector, the light emitted by the light source is

Luminaires, Fig. 3 Bending of light by refraction fromthe incident angle to the refraction angle

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reflected according to the material of the reflectingsurface, viz., specularly or diffusely.

Plane reflectors are often used to screen off thedirect light from the light source. Accurate beamshaping is scarcely possible with plane reflectors,but by changing the symmetry of the reflectors,the direction in which the bulk of the light isemitted can be changed.

Curved ReflectorsThe best optical performance is obtained whenusing a curved reflector. Depending on the curva-ture, many different types of beams can be cre-ated. A curved reflector may be cylindrical,parabolic, elliptical, hyperbolic, or some othercontour to suit a particular application. The circu-lar and parabolically shaped reflectors are the onesmost commonly used.

The most important optical property of aparabolic reflector is that a point source oflight placed in its focus will produce a parallelbeam of reflected rays with the greatestintensity in its center. If the light source is not atthe focus but in front or behind it, the reflectedrays are no longer parallel. Thus, by choosing theposition of the light source relative to thefocus point, the desired beam shape (narrow towide) can be created. Since a lamp is never areal point source, deviations from the theoreticalbeam shape for a point light source assketched above will always occur. Thesmaller the light source relative to the size of thereflector, the more accurately can the beam beshaped.

Faceted ReflectorsSmooth-curved reflectors have to be produced to ahigh degree of accuracy, because even smalldeviations from their intended shape willproduce undesirable discontinuities in theirlight distribution (striations). This will not occurwith a faceted reflector. A faceted reflector con-sists of a number of adjacent, plane or curved,facets that together approximate a curve that isan approximation of a parabolic curve. Thewidth of beam produced by the faceted reflectoris somewhat greater than that of a smooth-curvedreflector.

RefractorsRefractors are used to create the desired luminairelight distribution by passing the light from thesource through a refractor (Fig. 3). The anglethrough which the light is bent is dependent onboth the shape of the refractive material and itsrefractive index (Snell’s law).

sin ðaiÞsin ðarÞ ¼

nairnmaterial

Refracting devices are either lenses or prisms.The type of refractor most commonly employed inindoor lighting is the lens found in tubularfluorescent-lamp luminaires intended for generallighting. It consists of a horizontal plastic panelwhich is mounted just below the lamp. The panelis flat on the top and has a special pyramidal(prism) or lens structure on the underside, whichdirects the light in certain directions and reducesthe brightness under specific angles. Where in thepast prismatic controllers were used, we see todaymore advanced lens-type refractors that give moreaccurate possibilities to shape the light distribu-tion. These types of refractors are also used toproduce LED luminaires for general indoor light-ing and for road lighting (Fig. 4). Refracting glassbowls were in the past sometimes used for high-pressure mercury and sodium road-lighting lumi-naires. They have become obsolete because theyare heavy, but more so because lighting control inthe upward direction, and therefore control oflight pollution, is not easily attainable.

878 Luminaires

DiffusersTranslucent diffusers enlarge the apparent size ofthe light source. They scatter the light of the lampin all directions without defining its light distribu-tion. They serve mainly to reduce the brightnessof the luminaire and thus the glare created byit. Diffusers are made of opal glass or translucentplastic, commonly acrylic or polycarbonate. Thematerial should be such that it scatters the lightwhile producing the minimum amount ofabsorption.

Screenings

Screening the Lamp from Direct ViewScreening is employed to hide the bright lamp orlamps from direct view. The higher the brightness

Luminaires, Fig. 5 Lampshielding by the reflectoritself (left) and by aninternal baffle (right)

Luminaires, Fig. 4 Micro-lens type of refractor for aLED luminaire for general indoor lighting

(luminance) of the lamp, the stricter the require-ments for the shielding.

The luminaire reflector housing itself, or abuilt-in baffle, can provide the screening function(Fig. 5). When the sole purpose of the louvre is toshield the lamp from view, diffuse-reflectingmaterial is used, such as a white-plastic louvreor, in the case of floodlights, matt-black metalrings (Fig. 6).

Shielding devices are often combined with thefunction of defining the light distribution, inwhich case highly reflective material is used forthe louvre.

Color FiltersIn certain lighting applications, in particular dis-play lighting and decorative floodlighting, color issometimes used to help achieve the desired aes-thetic effect. In the past, color filters attached toluminaires containing white light sources wereextensively used for this purpose. Both absorptionand dichroic (interference) filters were used,although absorption filters in particular (Fig. 7)lower the efficacy of the total lighting system.Typical transmittance values are as follows: forblue absorption filters 5 %, for green absorptionfilters 15 %, and for red absorption filters 20 %.The consequence of the light absorption is thatthese filters become warm, which with high-power floodlights may damage the filter. The solu-tion where such floodlights are employed is to usedichroic filters, which are a more expensive alter-native. Today, colored LED light sources are nor-mally employed where colored lighting isrequired. Here the color comes directly from the

Luminaires,Fig. 6 Simple louvre (left)to shield the lamp in afluorescent-lamp luminaireand (right) a floodlightlouvre

Luminaires, Fig. 7 Absorption-type color filters

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lamp itself, so the efficacy of the lighting system ismuch higher.

Light-Distribution CharacteristicsThe light distribution of a luminaire defines howthe luminous flux radiated by the luminaire isdistributed in the various directions within thespace around it. It is the result of the opticaldevices used in the luminaire as describedabove. This is also called luminous intensity dis-tribution, since it is specified in terms of luminousintensities in all the directions in which the lumi-naire radiates its light (Fig. 8). The luminousintensity diagram is in fact the light fingerprint

of a luminaire, in digital form (I-Table), and is thebasis of all lighting calculations.

Basic photometric data that can be calculatedfrom the light distribution are the beam spread andthe luminaire light-output ratio. For all types ofluminaires and for all types of application, thesedata provide an insight into the photometric qual-ity of the luminaire.

Mechanical Characteristics

The mechanical function of the luminaire housingis threefold: it accommodates the various compo-nent parts of the luminaire, such as the opticalsystem and the various components of the electri-cal system; protects these against external influ-ences; and provides the means of mounting theluminaire in the installation.

Material

Sheet SteelSheet steel is generally chosen for the manufac-ture of tubular fluorescent luminaire housings foruse indoors. The pre-painted sheet steel from theroll is white with diffuse reflection properties.Thus, after having been shaped in the luminairefactory into the desired luminaire form, nofinishing-off operations are required.

Stainless SteelStainless steel is widely used for many of thesmall luminaire components, such as clips,hinges, mounting brackets, nuts, and bolts, thathave to remain corrosion free.

Luminaires, Fig. 8 Lightdistribution of a luminairegiven by its luminousintensity diagram. Thearrows represent theluminous intensities in thedirections specified. Herethe light distribution isgiven for all planes,although it is usually onlygiven for one (e.g., the bluecurve) or two, mutuallyperpendicular, planes

Luminaires, Fig. 9 Mirror reflector of sheet aluminumprotected by a plastic film, which must be removedbefore use

880 Luminaires

Aluminum AlloysAluminum alloys, in which other elements havebeen added to the pure aluminum to improve theirmechanical, physical, and chemical protectiveproperties, are used to manufacture cast, extruded,and sheet-metal luminaires. Cast aluminum refersto the process in which molten aluminum alloy ispoured (cast) in a mold. Extrusion is the process inwhich softened aluminum alloy is pressed throughthe openings of a die. Cast and extruded alumi-num alloys are much used in housings forfloodlighting and road- and tunnel-lighting lumi-naires because they can be employed in humidand damp atmospheres without having to addprotective finishes. Sheet aluminum is chieflyemployed in luminaires for reflectors (Fig. 9).The reflectors are anodized to improve theirreflection properties and to protect them frombecoming matt.

PlasticsPlastics are used for complete luminaire housings,for transparent or translucent luminaire covers,and for many smaller component parts.All-plastic houses can of course only be employed

for light sources that have a relatively low operat-ing temperature.

Plastic covers are of methacrylate or polycar-bonate. Methacrylate maintains its high lighttransmission properties over a long period, but

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its impact resistance is relatively low. The impactresistance of polycarbonates is very high and thusoffers a high degree of protection against vandal-ism. It can be chemically treated to protect it fromyellowing under the influence of ultravioletradiation.

GlassAlthough glass is heavy, glass covers are usedwhere these have to be positioned close to a lightsource having a high operating temperature. Thisis the case, for example, with HID flat-cover road-lighting luminaires and with most floodlightingluminaires. Two sorts of glass are used:

• Normal glass, where no special demands areplaced on heat resistance.

• Hard glass, where heat resistance, chemicalstability, and resistance to shock are important.Should hard glass break, it will disintegrateinto small pieces.

Luminaires made completely out of glass areextremely heavy and nowadays are seldomemployed.

CeramicsCeramic material is used in compact housings thatare exposed to very high temperatures such asvery compact halogen lamp luminaires.

StrengthAll luminaires should have housings of sufficientrigidity to withstand normal handling, installa-tion, and use. With indoor-lighting luminairesfor fluorescent lamps, stiffness and rigidity ofconstruction is particularly important, since theselamps are relatively large and awkward to handle.Perhaps the most critical part of a luminaire as faras strength is concerned are the mountingbrackets. The strength required here is coveredby a safety factor: the mounting bracket (s) mustbe able to support at least five times the weight ofthe luminaire itself. With road-lighting and out-door floodlighting luminaires, the mountingbrackets must also be strong enough to withstandthe highest conceivable wind loading for the loca-tion. Here a good aerodynamic shape for the

luminaire (characterized by its so-called shapefactor) can be advantageous, as it also serves toreduce the strength required for the lighting mast.

Under some circumstances, the impact resis-tance of the luminaire itself is also important,particularly where protection against vandalismis called for.

Resistance to Pollution and HumidityThe atmosphere can contain many potentially cor-rosive gases which, in the presence of moisturevapor, will form highly corrosive compounds. Inall areas where this danger exists (notably in out-door applications, indoor swimming pools, andcertain industrial premises), luminaires madefrom corrosion-resistant materials or having pro-tective finishes should be used. In such areas, theluminaire should protect the optical and electricalcomponents it houses. It should, of course, befully enclosed. The degree of protection providedby the luminaire is classified according to theInternational Protection code (IP code) asdescribed in an international IEC standard[2]. The IP code consists of two numerals: IP • •.

• The first numeral classifies the degree of pro-tection against the ingress of solid foreign bod-ies (ranging from fingers and tools to fine dust)and protection against access to hazardousparts.

• The second numeral classifies the degree ofprotection against the ingress of moisture.

The higher the IP values, the better the protec-tion and thus the lower the dirt accumulation andthe lumen depreciation.

Ease of Installation and MaintenanceMany luminaires are of such a shape, size, andweight as to make mounting them a difficult andtime-consuming operation. Mounting, but alsorelamping and cleaning, must usually be carriedout high above ground level. So the ergonomicdesign of the luminaire should be such as to makethese operations as easy and as safe as possible toperform. For example, covers should be hinged sothat the electrician has his hands free to work onthe lamp and gear. A good, ergonomically

Luminaires, Fig. 10 Avariety of lamp holdersmade of both plastics andporcelain

882 Luminaires

designed luminaire is one that can be mounted instages: first the empty housing or a simple mount-ing plate, which is light and easily handled, thenthe remaining parts.

Electrical Characteristics

The electrical function of a luminaire is to providethe correct voltage and current for the properfunctioning of the lamp in such a way as to ensurethe electrical safety of the luminaire.

Lamp HoldersThe most usual types of holder are the Edisonscrew, the bayonet, and the pin (Fig. 10). MostEdison screw and bayonet holders are made ofplastics or porcelain, with metal parts for carryingthe current. Porcelain is resistant to high temper-atures and has a high voltage-breakdown resis-tance, which is important considering the highignition voltage of HID lamps. The pin lampholders for tubular fluorescent and compact fluo-rescent lamps are nearly always made ofplastic. The metal contacts are spring loaded toensure a constant contact pressure.

Electrical WiringThe electrical wiring in a luminaire must be suchas to ensure electrical safety. This necessitatesgreat care in the choice of wire used and its

installation. There are a great many differenttypes of wire available, in both single-core(solid) and multi-core (stranded) versions, allwith various cross-sectional areas and clad withvarious thicknesses and qualities of insulation.The cross-sectional area (thickness) of the wiremust be matched to the strength of the currentflowing through it. The insulation of the wireused must be resistant to the high air temperaturein the luminaire and the temperatures of the lumi-naire materials with which it is in direct contact.This is true not only under normal conditions ofoperation, but also in the presence of a faultcondition.

Mains ConnectionThe method used to connect a luminaire to thepower supply must be both quick and safe. Thepractice generally adopted is to incorporate a con-nection block in the housing, although prewiredluminaires in which the electrical connection tothe mains is automatically made when the unit isplaced in position are also available.

Electrical Safety ClassificationThe electrical safety classification drawn up by theIEC embraces four luminaire classes [2]:

• Class 0: Applicable to ordinary luminairesonly. These are luminaires having functionalinsulation, but not double or reinforced

Luminaires, Fig. 11 Metal halide high-bay luminaire(left) and recessed LED downlight with cooling fins

Luminaires 883

insulation throughout and with no provisionfor earthing.

• Class I: Luminaires in this class, besides beingelectrically insulated, are also provided with anearthing point connecting all those exposedmetal parts that could become live in the pres-ence of a fault condition.

• Class II: This class embraces luminaires thatare so designed and constructed that exposedmetal parts cannot become live. This can beachieved by means of either reinforced or dou-ble insulation. They have no provision forearthing.

• Class III: Luminaires in this class are thosedesigned for connection to extra-low-voltagecircuits (referred to as Safety Extra-Low Volt-age, SELV). They have no provision forearthing.

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Thermal Characteristics

Temperature ControlA considerable amount of the electrical energysupplied to the lamp is converted into heat. Theballast adds to this heating effect within the lumi-naire. With very high-powered lamps, the ballastshould be placed outside the luminaire in a specialballast box.

For a given lamp/ballast combination, theworking temperature reached by the luminaire isdependent upon three factors:

• The volume of the luminaire. The greater thevolume, the lower will be the temperature riseinside the luminaire.

• The ease with which the heat generated withinthe luminaire can be conducted through it tothe surrounding air. One way of promotingairflow through the housing is to make use ofheat-conducting materials in its construction.Most metals are good in this respect, whileplastics, on the other hand, are thermal insula-tors and cannot therefore be employed as hous-ing materials where high-power lamps areinvolved.

• The cooling effect of the surrounding air. Goodheat dissipation calls for large surface areas to

be in contact with the surrounding air. Lumi-naires for high-power lamps, such as high-bayluminaires, floodlights, and some LED lumi-naires that are very sensitive to high tempera-tures, are therefore provided with cooling fins(Fig. 11). Some types of industrial luminairesare provided with air vents in the top of thehousing to allow the warm air to escape.

Protection Against FlammabilityThe flammability of a luminaire operating underfault conditions is an issue with luminaires madeof plastics. But the combustion behavior of suchluminaires is not just material dependent; it alsodepends on the shape and thickness of the lumi-naire housing.

Aesthetics

No less important than the functional characteris-tics of a luminaire is what is termed its aestheticalappeal, that is to say its appearance and styling. Ininteriors, all non-recessed luminaires are clearlyvisible, and so whether switched on or not, theirdesign should be in harmony with that of theinterior. In outdoor lighting, it is usually only thedaytime appearance of the luminaires, when theseare clearly visible, that is important: particularly in

Luminaires, Fig. 12 UL and ENEC certification marks.The number in the ENEC mark indicates the country of theinstitute that has given the European approval

884 Luminance Measurement Device

built-up areas, their design can make a positivecontribution to the attractiveness of the locality.

Approval

Luminaires always have to comply with theappropriate safety rules. UL (Underwriters Labo-ratories) is the US mark for demonstrating com-pliance with all US safety standards, and ENEC(European Norms Electrical Certification) is thesimilar European mark (Fig. 12).

Cross-References

▶ Interior Lighting▶Light Distribution▶Light Pollution▶Lumen Depreciation▶Road Lighting▶ Sports Lighting

References

1. CIE Publication: S 017/E: 2011, International lightingvocabulary. International Lighting Commission, CIE,Vienna (2011)

2. IEC International Standard, 60598-1: ed 7.0,Luminaires – part 1: General requirements and tests.(2008)

Luminance Measurement Device

▶Luminance Meter

Luminance Meter

Bor-Jiunn WenDepartment of Mechanical and MechatronicEngineering, National Taiwan Ocean University,Keelung, Taiwan

Synonyms

Luminance measurement device

Definition

A luminance meter is a device used to measure thephotometric unit, luminance, in a particular direc-tion at a solid angle from a surface. The simplestdevices measure the luminance in a single direc-tion, while imaging luminance meters measureluminance in much the same way that a digitalcamera records color images.

Introduction

The luminance of a light source (integratingsphere with known output aperture) is determinedfrom the measurement geometry and illuminancemeasured by a photometer. The luminance of thelight source is obtained as

Lv ¼ EvD2

A(1)

where Ev is the illuminance at a distanceD between the aperture plane of the source andthe photometer; and A is the area of the sourceaperture. The distance A depends on the radius ofthe limiting aperture r1, the radius of the source r2,and the physical distance d between the sourceand the aperture according to Eq. 2.

D2 ¼ r21 þ r22 þ d (2)

Equation 2 is accurate within �0.01 % for dis-tances that are more than one decade greater thanthe radius [1].

Luminance Meter,Fig. 1 Sharpnessmeasurement of FPD byusing a ILMD

Luminance Meter, Fig. 2 Automatic measurement and analysis program for road and tunnel lighting systems

Luminance Meter 885

L

Applications

An imaging luminance measurement device(ILMD) is capable of measuring illuminance in animage. It can automatically quantify the light distri-bution or light uniformity. This is much more pow-erful than the conventional single-point luminance

meter. For example, an ILMD is utilized for the fastimaging luminance measurement of flat panel dis-plays [2]. Figure 1 depicts measurement of theluminance of vertical or horizontal patterns on adisplay with an ILMD in a slightly tilted position[2]. The subsequent image processing includes cal-culating the spatial frequency response (SFR) of the

886 Luminescent Materials

image using a slanted-edge algorithm, obtainingresolution at a specified decrease (e.g., 50 %) inthe SFR of the device under test (DUT), and finally,specifying resolution in the sharpness of the DUT.

In addition, an automatic measurement systembased on an ILMD also measures the photometricproperties of road and tunnel lighting systems[3, 4]. Figure 2 depicts an automatic system forthis purpose. Compared with a single-point lumi-nance meter, an ILMD is more precise and time-saving for measuring road and tunnel lighting.

Since the rapid growth of light-emitting diode(LED) technologies, the adjustable lighting on bill-boards using LED have become increasingly pop-ular. Many of these products are pushed to muchhigher contrast in spatial and/or temporal configu-rations to attract attention. However, the glareand/or flicker of the LED sources may produce anuncomfortable visual experience. Therefore, Hsuet al. [3] proposed a measurement system for reg-ulating between flicker and glare by using percep-tual ratings of LED billboards under variousconditions. In this research the authors performedboth objective and subjective evaluations of flash-ing LED billboards in interior spaces. The objec-tive flicker and glare evaluations were carried outby taking temporal and spatial measurementsrespectively. The properties of the flicker andglare values obtained are reproduced under physi-cal conditions. The visual results were modelled bysimple equations as a function of objective mea-surements in terms of a low-pass flicker index andunified glare rating. Thus, an effective method wasdeveloped for regulating the degree of flashingLED lighting using an ILMD.

Cross-References

▶ Standard Measurement Geometries

References

1. Kostkowski, H.J.: Reliable Spectroradiometry.Spectroradiometry Consulting, La Plata (1997)

2. SID IDMS Information Display Measurements Stan-dard, v1.03, 1 June 2012

3. Hsu, S.W., Chung, T.Y., Pong, B.J., Chen, Y.C., Hsieh,P.H., Lin, M.W.: Relations between flicker, glare, andperceptual ratings of LED billboards under variousconditions. In: CIE Centenary Conference, Paris (2013)

4. CIE 194: On Site Measurement of the PhotometricProperties of Road and Tunnel Lighting (2011)

Luminescent Materials

▶ Phosphors and Fluorescent Powders

Luminous Efficiency Function

▶ Spectral Luminous Efficiency

Luminous Intensity Distribution

▶Light Distribution

Luminous Intensity Meter

▶ Instrument: Photometer

Luxmeter

▶ Illuminance Meter