The spectral radiant intensity distribution and totalspectral flux are important parameters for the char-acterization of light sources. The spectral radiant in-tensity distribution describes how properties such astheperceived color of a light source change indifferentdirections, which is of critical importance inmany sig-nalling and illumination applications. Such informa-tion about a light source can also be used tomodel theinfluence of the source when used within any definedenvironment or as part of another optical system.When combined with a measurement of the powerconsumption, the spectral radiant intensity, spectraltotal flux, and associated luminous quantities enablethe efficacy and energy efficiency of a source to be de-termined.The spectral total flux of a light source can be mea-
sured using either an integrating sphere-based or agoniometric approach [1]. In the case of the former,
the output of the source in different directions is aver-aged over 4π sr by multiple reflections from the coat-ing on the inside surface of the integrating sphere.Goniometric techniques, on the other hand, rely onmeasuring the irradiance or radiant intensity of thelight source at a large number of different points inspace and then integrating these measured valuesover a surface enclosing the source. One significantadvantageofgoniometricmethods is that theyprovideinformation about the spectral radiant intensitydistribution of the source as well as the spatiallyaveraged properties.
In the past theNational Physical Laboratory (NPL)has realized scales of total luminous and spectral fluxusing a goniometric approach [2,3], and disseminatedthese scales using large diameter integrating spheresin the case of spectral total flux, and using both inte-grating spheres and a goniophotometer in the case oftotal luminous flux. With the development of a newgoniospectroradiometer, NPL is now able to dissemi-nate both spectral and luminous flux scales directlyusing a goniometric technique.
TheNPLgoniospectroradiometer isbasedonaGO-DS2000 mirror goniophotometer system manufacturedbyLMT[4],whichhasbeenspeciallydesigned toallowthe detector head to be positioned in a fixed location ina tunnel at a distance ≤ 17m from the center of thelight source as with a typical mirror goniophotometerconfiguration [5], or at the center of the mirror withthe mirror itself being covered with a low-reflectanceblackmaterial, effectively converting it to a type 2 go-niophotometer with a moving head [5] and a source–receiver distance of ∼1:7m. The latter configurationallowsmeasurement of compact, relatively low-powersources, which can be difficult to measure with a longpath length due to low signal levels. Additionally, andmore importantly, removing themirror from the lightpath eliminates measurement uncertainties arisingfrom the mirror, such as effects due to polarizationand surface nonuniformities. Furthermore, sincethemirror is back-silvered,measurements can be car-ried out further into the UVwithout the mirror in thelight path between the source and the receiver.The light source being measured is mounted on an
arm, the orientation of which is controlled by a pair ofrotation stages allowing the zenith angle (γ) of themeasurement to be set without changing the orienta-tion of the lampwith respect to the laboratory. A thirdrotation stage allows rotation of the source about avertical axis passing through its photometric centerand sets the azimuthal angle (C) of the measurement(Fig. 1).In both configurations (i.e., with and without the
mirror in the lightpath), thespectral radiant intensityof the source ismeasured by replacing the photometer
detector head, which is used for luminous intensitydistribution and luminous flux measurements, by aCCD array spectrometer. For measurements usingthe mirror this substitution is straightforward sincethe photometer is in a fixed position and can be easilyreplaced by the spectrometer. For direct measure-ments of compact sources without the mirror in thelight path, a more complex arrangement is necessary.In this case the receiver is a 50:8mm diameterBaSO4-coated integrating spheremounted at the cen-ter of the (covered)mirror such that the distance fromthe center of its12:7mmdiameter entranceport to thecenter of the light source is the same at all measure-ment positions (see Fig. 2). The integrating sphere iscoupled to thespectrometerusinganoptical fiberbun-dle, which has a round-to-line format to maximizelight collection into the spectrometer. The tips of theindividual fibers are distributed uniformly across acircular field at the receiver end to match the config-uration of the sphere exit port and form a vertical lineat the spectrometer end to match the configuration ofthe entrance slit. The sphere exhibits good cosine-response characteristics for radiation incident at <35° to the normal to the entrance port, and effectivelydepolarizes the light before it enters the fiber bundle.The slit at the output end of the fiber bundle is imagedonto the entrance slit of the spectrograph by a lensthat matches the F=# of the radiation exiting thefiber (NA ¼ 0:22, F=2:3) to that of the spectrograph(NA ¼ 0:125, F=4). The spectrometer is mounted asclose to the integrating sphere as practically possiblegiven theavailable spaceand loadbearing constraintsof the goniometer, while also ensuring that during theoperation of the goniometer there is no relativemove-
Fig. 1. Schematic diagramof theNPLgoniospectroradiometer showing the configurationwhenmeasuring the spectral radiant intensity ofa light source at different zenith angles for compact light sources (i.e., without themirror in the light path). A section of themirror has beenremoved to show the mounting of the array spectrometer behind the mirror.
ment between the two ends of the fiber that could po-tentially lead to changes in its transmittance. In prac-tice this means the spectrometer is fixed behind themirror on the rotating arm that supports the integrat-ing sphere and the mirror.Changing the position of the two rotation stages (γ
between 0° and 180°, and C between 0° and 360°) al-lows the integrating sphere to collect radiationemitted by the source in any direction. Equivalentlythe receiver canbe considered tobe sampling the spec-tral irradianceof the light sourceatdifferent points onthesurfaceofanotional spherecenteredonthesource.By measuring at a sufficiently large number of posi-tions on this notional sphere, the total flux of thesource can be determined by spatial integration ofthe measured intensity values. For a small angularsubtense around the lamp cap, radiation emitted bythe source is shielded from the receiver by the lampmount and the goniometer arm. To correct for this ob-scuration a small additional contribution to the totalflux is calculated from consideration of the geometryandthemeasured radiant intensitydistributionof thelight source,aswell as thedegreeof obscurationby thegoniometer arm, and added to the measured flux. Fora tungsten filament flux standard lamp, this obscura-tion or “cap correction” is typically ∼0:2%.The spectrograph is an Andor Shamrock SR303i
with a Czerny–Turner configuration and a focallength of 303mm. After passing through the entranceslit, light entering the spectrograph is collimated by atoroidalmirrorontoadiffractiongrating.Asecondtor-oidalmirror forms an image of the entrance slit on thedetector array of a Peltier-cooled Andor DV401-UVCCDcameramountedat the focal planeof the spectro-graph. The two-dimensional (2D) detector array iscomposed of 256 × 1024 pixels, each 26 μm wide.Counts from the 256 pixels in each column of the de-tector array are summed to give spectral values at1024wavelengths.A six position filterwheel fitted be-hindthespectrographentranceslit containsanumber
of different blocking filters that are used for ordersorting and to improve the stray light rejection ofthe spectrometer (see Section 3).
The spectrometer includes a facility for adjustingthe position of the diffraction grating by means ofa rotatable grating turret, allowing different spectralregions to be sampled if required. In practice, how-ever, the spectrometer is operated with the gratingturret in a fixed position to minimize potential wave-length errors in the acquired spectra arising from themovement of the grating and also to speed up dataacquisition. The diffraction grating used has a linedensity of 150 l=mm (500nm blaze), giving a lineardispersion of ∼21nm=mm at the focal plane of thespectrograph, meaning each detector pixel spans0:56nm and the bandpass across the entire detectorarray is ∼570nm. The grating orientation is set suchthat the 546:1nm emission line from a mercury dis-charge lamp falls in the center of the detector array,and wavelengths between 260 and 830nm are simul-taneously incident on the detector array. The en-trance slit of the spectrograph is set to a width of50 μm, which (for a monochromatic input) is imagedwith 1∶1 magnification onto the CCD detector array.The spectral resolution of the combined detectionsystem is determined by both the width of the en-trance slit and the effective slit width defined by eachpixel in the detector array. With the settings de-scribed above, the instrumental slit function is trape-zoidal in shape with a measured full width athalf-maximum of ∼1:7nm across the detector array.
3. Stray Light Errors in the Array Spectrometer
Using array-based spectrometers for radiometric ap-plications presents a number of difficulties, one ofthe most significant of which is the relatively poorstray light rejection of these devices when comparedwith scanning double monochromators. However, insituations where measurement speed is of criticalimportance, array spectrometers offer a significantadvantageover scanning systemsdue to their capabil-ityof simultaneouslyacquiringspectraldataata largenumber of wavelengths over a broad wavelengthrange. This is the primary reason for incorporatingan array detector into the goniospectroradiometersince acquiring spectral information in the manyspatial positions required to characterize a lightsource becomes extremely time consuming using ascanningmonochromator system (a typical array sys-tem is capable of measuring over the entire visible re-gion several orders of magnitude faster than a typicalscanning system).
To determine which of the commercially availablearray spectrometers was most suitable for use on thegoniospectroradiometer, a number of devices wereevaluated at NPL. To assess their stray light rejec-tion, the different spectrometers were used with aquartz halogen tungsten projector lamp to measurethe transmittance of a series of colored glass “cut-on”(long pass) glass filters [6]. For an ideal system, theratio of the measured signals for the lamp-plus-filter
Fig. 2. Schematic diagram of the NPL goniospectroradiometerconfigured for measurements on compact light sources (i.e., with-out the mirror in the light path). A section of the mirror has beenremoved to show the mounting of the array spectrometer behindthe mirror.
to those for the unfiltered lamp (with correspondingdark signals subtracted) will give a result equal tothe transmittance of the cut-on filter; any deviationfrom this ideal behavior provides information aboutthe performance of the spectrometer and, in particu-lar, its stray light rejection. Cut-on filters of sufficientthickness have extremely low transmittance atwavelengths shorter than the cut-on wavelength,and any significant measured signal at these wave-lengths therefore indicates the presence of straylight. The stray light errors for a given array spectro-meter depend on the spectral power distribution ofthe source being measured and for this reason thesemeasurements only provide information about themagnitude of the errors associated with measuringthis particular type of source. However, the use ofa tungsten lamp for stray light tests is useful fortwo reasons. First, tungsten lamps are widely usedas reference sources for the calibration of spectrora-diometric measurement systems due to their excel-lent stability and reproducibility. It is thereforeimportant to know the stray light performance withsuch a source. Second, use of a tungsten lamp repre-sents a reasonably stringent test of stray light perfor-mance, so if good results are obtained with thissource, results with other types of source are alsolikely to be reasonably good. This is because the spec-tral irradiance of a tungsten lamp is several orders ofmagnitude higher at longer visible to near-infraredwavelengths than at shorter visible or ultravioletwavelengths (for a 2856K Planckian radiator,< 1% of the total radiant power emitted between250 and 1100nm is emitted at wavelengths shorterthan 435nm). The problem is compounded for sili-con-based detector arrays, which have a significantlyhigher spectral responsivity at longer visible andnear-infrared wavelengths than at shorter wave-lengths. Thus if even a small fraction of the longerwavelength radiation is scattered onto the shortwavelength sensing part of the detector array, itcan dominate the signal measured at these shorterwavelengths.Six different commercially available array spectro-
meters were tested at NPL using a series of cut-onfilters as described above. It is important to useseveral filters since stray light performance can vary
significantly with wavelength. Figure 3 shows an ex-ample of the results of these tests; in this case using a3mm thick piece of GG435.
All six spectrometers show significant levels ofstray light at short wavelengths with even the bestdevice producing errors > 2% in measured transmit-tance at all wavelengths shorter than the cut-onwavelength, rising to 100% at the shortest wave-lengths. In other words, even for the best spectro-meter tested here, between 2% and 100% of themeasured signal at short wavelengths is due to straylight. This being the case, none of the spectrometersshown in Fig. 3 can be considered suitable for mea-suring the irradiance of a tungsten source at wave-lengths shorter than 400nm without correction.
The Andor spectrograph and CCD camera used onthe goniospectroradiometer were selected partly onthe basis of reasonable stray light performance, incomparison to other array spectrometers tested,and partly because the combined system offeredthe flexibility to meet the other requirements (spec-tral resolution, wavelength interval, sensitivity, etc.).Nevertheless, significant stray light was observedduring the tests that would severely compromisethe calibration and use of the device. One solutionto the stray light problem is to determine a straylight correction matrix based on the results of inves-tigations in which the spectrometer is irradiatedwith tunable narrowband radiation [7]. An alterna-tive is to reduce the stray light level using appropri-ate blocking filters, and it is this latter approach thathas been implemented for the NPL goniospectroradi-ometer system.
Stray light in an array spectrometer is caused byinterreflections and scattering of radiation withinthe body of the device, resulting in radiation at otherwavelengths reaching parts of the detector array towhich they do not correspond. The problem is exacer-bated in compact array devices by the relativelysmall distances between the various optical surfaces,which often makes baffling and effective rejection ofother diffracted orders particularly difficult. Theprinciple behind using blocking filters to reducethe stray light errors in such a device is to limitthe bandwidth of radiation entering the spectro-meter; this allows measurements to be made overa truncated spectral range without the influence ofradiation at wavelengths outside this range. In thecase of measuring the irradiance of a tungsten lamp,for example, removal of the longer visible and near-infrared wavelengths when the UV and shorter visi-ble range is measured can lead to a dramatic reduc-tion in the level of stray light, since even a smallamount of scattered radiation from longer wave-lengths has a significant effect on the measured sig-nal. The remaining stray light will be due only to thesmall fraction of light scattered at short wave-lengths, which represents a correspondingly smallfraction of the measured signal.
The type and number of different stray light block-ing filters represents a compromise between im-
Fig. 3. (Color online) Transmittance of a 3mm thick GG435 co-lored glass filter measured using six different array spectrometers.
proved stray light performance (using more filters,each with a narrower bandpass, generally means bet-ter stray light rejection) andmeasurement time (mea-surements must be made using each filter in turn soincreasing the number of filters increases the overallmeasurement time). Asmentioned previously, it is de-sirable to keep themeasurement time as short as pos-sible to minimize effects due to lamp aging ortemporal drift in output. Bymaking some simplifyingapproximations about the stray light distribution inthe array spectrometer, it is possible to estimatethe fractional error due to stray light when the irra-diance froma tungsten lampoperatingatagiven colortemperature is measured through a blocking filter ofdefined transmittance. The assumptions are
a. The stray light signal counts are the same ateach pixel across the entire detector array.b. The total summed stray light counts are a
fixed fraction of the total integrated signal counts.
Both of these are crude approximations and ignorethe fact that the stray light signal distribution acrossthe array is both nonuniformand dependent upon thespectral irradiance at the spectrograph entrance slit.However, they enable an order of magnitude assess-ment of the effect of using a blocking filter with de-fined transmittance with the spectrometer. Afterexperimenting with different transmittance curves,a set of four Gaussian profiles were identified which,according to the simple model, together gave straylight errors of < 1% across the range of 350 to830nm. The characteristics (the center wavelengthand fullwidthathalf-maximum) of thesehypotheticalGaussian filters were then used as a guide for the de-sign of real sets of blocking filters.The four stray light blocking filters shown in Fig. 4
have been incorporated into a filter wheel housedinside the Andor spectrograph used on the NPLgoniospectroradiometer. Each of these blocking filtersis a combination of commercially available coloredglass and interference filters chosen such that to-gether they have high transmittance over a certainwavelength range and good out-of-band blocking.Since the filterwheel is positioned behind the spectro-
graph entrance slit, it is important that all four filtershave approximately the same optical path length forall wavelengths from350 to 830nm, otherwise the en-trance slit will not be focused onto the CCD array inthe same way for each filter. This is achieved by usingadditional fused silica plates to increase the overalloptical path length of each combination to ∼9mm.To measure the spectral characteristics of a lightsource from 350 to 830nm using the blocking filters,fourdifferentacquisitionsare taken,one througheachblocking filter.Acombinedspectrumis then formedbythe addition of these four spectra over adjacent, non-overlapping wavelength ranges. For example, mea-surements using blocking filter 1 provide data inthe range of 350nm < λ ≤ 467nm, measurementsusing blocking filter 2 provide data in the range of467nm < λ ≤ 578nm, and so on. Overall spectra de-termined in this way (see Figs. 8, 12, and 16) do notcontain significant discontinuities at the boundariesbetweendataobtainedusingdifferentblocking filters.
To assess the degree of improvement in stray lightperformance achieved by use of the blocking filters,the tests described previously were repeated. The re-sults shown in Fig. 5 indicate a significant reductionin stray light errors when using the blocking filters(data points) compared with the unfiltered spectro-meter (solid and dashed curves). For example, usingthe blocking filters shows stray light levels of 2.2% at350nm and 0.4% at 380nm when measuring thetransmittance of a 3mm thick piece of GG435, com-pared with levels of 57% at 350nm and 24% at380nm when the measurements are made withoutthe blocking filters. In this case the stray light block-ing filters reduce the stray light errors by more thanan order of magnitude.
Although using blocking filters in the way de-scribed above does substantially improve the straylight rejection of the spectrometer, the techniquedoes have some limitations. The major problem isthat, as mentioned previously, overall measurementtime is increased significantly since four separatedata acquisitions are required to span the range of350 to 830nm. Acquisition of a full spectrum overthis range using the blocking filters typically takes6–17 s, depending on the intensity of the light source.
Fig. 4. (Color online) Measured regular transmittance of fourstray light blocking filters used on the NPL goniospectroradi-ometer.
Fig. 5. (Color online) Transmittance of colored glass filters mea-sured using tungsten lamp and Andor array spectrometer withand without stray light blocking filters.
One solution to this problemwould be to fit the filtersdirectly in front of the corresponding areas of the de-tector array. However, this does present practical dif-ficulties; not least that introducing an additionaloptical element into the spectrometer is likely tochange its stray light characteristics.Any significant temperature dependence in the
transmittance of the filters will change the effectiveresponsivity of the combined spectrometer detectionsystem. However, the impact of this is small because
a. The spectrometer is mounted several metersfrom the light source in a temperature-stabilized la-boratory and is therefore not subject to significantchanges in temperature.b. Temperature coefficients of spectral filters are
generally only significant in spectral regions wherethe transmittance of the filter varies strongly withwavelength. The filters have been designed so thatthere is some overlap between them in regions wherethe transmittance is changing rapidly, thus ensuringthat any change in the temperature of the filters hasa negligible effect on the final measured values.
4. Data Acquisition and Analysis
A. Data Acquisition
Two personal computers are used to operate the go-niospectroradiometer; one to control the motion ofthe goniometer and the other to set up and acquiredata from the array spectrometer. The angular coor-dinates (C and γ) of each required measurement po-sition are set using the goniometer control software.The number and distribution of the spatial measure-ment positions depend on the type of source beingmeasured. When measuring the flux of highly direc-tional sources such as LEDs, it is more effective toconcentrate measurements in the directions of highradiant intensity rather than distribute the mea-surement positions uniformly over the entire 4π sr.Each time the goniometer reaches one of the mea-surement positions defined by the control program,it pauses and sends a transistor–transistor logicpulse to trigger the spectrometer. It then waits to al-low the spectrometer to acquire the spectral data be-fore moving the rotary stages to the next definedcoordinates, and so on. The spectrometer control soft-ware sets the CCD acquisition parameters (integra-tion time, pixel binning, etc.), the spectrographsettings (slit width, grating position, etc.), and storesthe measured spectra.Dark measurements are performed with a small
black baffle mounted between the entrance port ofthe integrating sphere and the light source withthe baffle positioned such that it blocks only directlight from the source from entering the sphere.The dark signal measured in this way includes theambient stray light, that is any light reaching the de-tector due to scattering off the floor, walls, and ceilingof the laboratory or off the body of the goniospectror-adiometer, and light from any sources other than the
test source, as well as the dark current of the CCDcamera. Individual dark measurements are requiredwith the goniometer in each spatial measurementposition to determine the relevant background signalspectrum, which can be subtracted point-by-pointfrom the corresponding raw lamp signals at that po-sition. In practice it is often possible to determine thebackground signals by interpolation of a subset ofmeasurements at a limited number of different ele-vation (γ) angles since the ambient stray light tendsto be reasonably isotropic and represents only asmall fraction of the overall measured signal. Deter-mining the background signal from a smaller subsetof measurements in this way has the advantage thatthe overall lamp burn time is reduced, thus reducinglamp aging effects. In a laboratory with low levels ofambient stray light, such as that used at NPL, thishas a negligible effect on the measurement resultsand uncertainties.
B. Measurement Procedure
Traceability for the goniospectroradiometer comesvia calibration using specially designed luminous in-tensity standard lamps (Polaron Special Lamps) cali-brated against theNPL spectral irradiance scale thatwas established in 2003using a 3050K blackbody anda set of filter radiometers [8]. Two different referencelamps are usedwith the goniometer arm set such thatthe lamp is viewed by the spectrometer in the sameorientation as that for which it was calibrated. Mea-surements are made using each of the four differentstray light blocking filters and are corrected for themeasured dark signals. The mean of the resultsfrom the two reference lamps is used to determinethe spectral radiant intensity responsivity of thegoniospectroradiometer (in units of counts · W−1 · sr·nm) for each blocking filter, and subsequentmeasure-ments onother sources are then correctedaccordingly.
Measurements of each test source are recorded ateachmeasurementangle requiredasdescribedabove,and the raw data are processed offline using a pro-gram written in MATLAB. This program first sub-tracts the measured background counts from thetest lamp signal counts and stores the result in athree-dimensional data array. Next, the four spectrameasured at each spatial position (one for each differ-ent stray light blocking filter) are corrected for the ca-librated spectral responsivity of the spectrometer.The data from each of these four different spectra,each covering a different restricted spectral range,are then combined as described in Section 3 to deter-mine the full spectral radiant intensity distribution ofthe light source at each spatialmeasurement positionfrom 350 to 830nm.
A number of useful quantities can be derived fromthe spectral radiant intensity distribution results.The luminous intensity distribution and x and y chro-maticity coordinates of the source are calculatedfrom the VðλÞ photopic response function and the10° color matching functions [9], which have been lin-early interpolated and evaluated at the center wave-
lengths of each pixel of the CCD array. The correlatedcolor temperature (CCT) of the source is calculatedfrom the chromaticity coordinates by determiningthe temperature of the Planckian radiator with theclosest color coordinates to those of the test sourcein ðu; vÞ color space [9].The total spectral flux, ΦeðλÞ, of the source is
calculated by integration of the measured spectralradiant intensity values, IeðλÞ, over 4π sr:
ΦeðλÞ ¼Z
2π
0
Z π
0Ieðλ;C; γÞ sin γdγdC: ð1Þ
In practice, this is carried out by definingN nonover-lapping polygons that cover the entire ðC; γÞ space,each containing a single measured radiant intensityvalue. Calculating the flux in this way allows for anarbitrary angular spacing of the measured intensityvalues in the γ and C directions. The integration canthen be approximated by regarding the value of IeðλÞas being constant throughout each polygon (equal tothe measured value at the appropriate measurementposition) and calculating the integral of sin γ withineach polygon (Pn) using an analytical formula
ΦeðλÞ ¼XNn¼1
Ieðλ; γn;CnÞZPn
sin γdA: ð2Þ
The polygons are assigned by a 2D Voronoi tessella-tion of the radiant intensity data set carried outusing the MATLAB routine “voronoin.” For a givenpoint p, the Voronoi polygon is defined by the bound-ary enclosing all intermediate points lying closer to pthan to any other point. The main drawback to theVoronoi tessellation is that it can generate infinitepolygons. If a point is on the edge of the set of pointsbeing tessellated, the polygon surrounding it is infi-nite. To avoid this problem, the data set is expandedto include extra dummy points. The data are orderedsuch that the real measurement points always formthe first N points to be meshed so that the MATLABcommand will return the polygons surrounding thesepoints first and it is clear which polygons are needed.The main source of uncertainty associated with
calculating the flux in this way is that the valuesof IeðλÞ will not, in reality, be constant over the solidangle defined by each polygon. Such sampling errorsare minimized by an appropriate choice of both thenumber and the distribution of spatial measurementpoints. Where no prior information about the ex-pected intensity distribution for a particular sourceis available, it may be necessary to carry out a pre-liminary measurement to determine a suitable set ofspatial sampling points.
5. Calibration and Uncertainties
The wavelength scale of the spectrometer system isdetermined at a number of positions using severaldifferent low-pressure discharge lamps. The wave-length corresponding to the center of each detector
pixel is then calculated by interpolation [6]. The re-sulting wavelength error has been checked using anumber of other spectral emission lines and foundto be < 0:2nm over the full spectral range used. Lin-earity errors evaluated using the double aperturetechnique have been found to be < 0:2% over therange of use. This has been restricted to less thanthe full dynamic range of the CCD.
The goniospectroradiometer measurement techni-que requires that the distance from the photometriccenter of the source to the center of the receiver aper-ture be the same for measurement of the intensityreference standard and the test source. This beingthe case, knowledge of the absolute source to receiverdistance is not necessary for calculation of the radi-ant intensity. A low power laser fitted inside the ro-tating arm of the goniometer is aligned with the axisof the rotation stage that controls the elevation of thelamp with respect to the laboratory and intersectsthe lamp rotation (C) axis. Using this laser and a ser-ies of manual kinematic stages on the lamp mount, itis possible to align the center of most types of tung-sten filament lamp to within 2mm of the intersectionof the laser beam and the C axis, which is equal to0.12% of the total source to receiver distance.
For measurement of the spectral total flux of atungsten source, the combined (k ¼ 2) measurementuncertainty is < 3:6% over the entire visible rangeand < 3% at wavelengths longer than 450nm. Thelargest single contribution to this uncertainty comesfrom the spectral radiant intensity scale, howeverthis is expected to decrease significantly with theplanned realization of a new NPL spectral irradiancescale in 2008.
6. Results
The NPL goniospectroradiometer system has beenused to measure the spectral radiant intensity distri-butions of a 1kW tungsten flux standard, a cluster ofwhite LEDs, and a compact fluorescent source. Foreach of these sources the luminous intensity andCCTdistributionsandthetotalspectralfluxhavebeencalculated from the measured spectral radiant inten-sity distributions. In the luminous intensity and colortemperature plots shown in Subsections 6.A–6.C, themeasured value in a given direction is indicated bothby the radial distance fromthe origin of the coordinatesystem and the shading of the surface. All threesourcesweremeasuredwith the lampcappointingup-wardandtheluminousintensityandCCTdistributionplotshavebeenoriented in this sameway.All themea-surements presented here were made with the inte-grating sphere receiver mounted directly on themirror cover with the goniospectroradiometer in theconfiguration as shown in Fig. 2.
A. Measurement of a Tungsten Flux Standard
The measurements presented in this section wereperformed on a 1kW tungsten flux standard (PolaronSpecial Lamps). The lamp comprises an open wreathtype tungsten filament arranged in a planar hepta-
gonal configuration with one side removed where theends of the filament connect to the lamp electrodes,encased in a large glass envelope. The lamp was or-iented cap up, such that the filament lay in a horizon-tal plane. As the lamp has been designed to have areasonably isotropic intensity distribution, measure-ments were made in directions spaced uniformly inelevation (γ) and azimuthal (C) angles.Figure 6 shows that the luminous intensity of the
tungsten flux standard is lower in the plane of thefilament where the section of the filament nearestthe receiver obscures light from the back part. Thedirection of greatest intensity is directly below thecenter of the bulb (γ ¼ 0°). In the opposite direction,γ ¼ 180°, obscuration of the emitted radiation by thelamp cap, and partly by the goniometer arm, resultsin a lower measured radiant intensity. Some aniso-tropy in the C direction is caused by the geometricstructure of the filament.TheCCTof the tungsten flux standard (seeFig. 7) is
relatively isotropic in the azimuthal direction; how-ever, a clear trend is observable in the zenith (γ) direc-tion where the source has a lower CCT in directionsnear the lamp cap, γ ¼ 180°. This can be attributedto the presence of a mica disk fitted inside the lampenvelope between the cap and the filament. The micadisk has a lower transmittance at shorter wave-lengths, hence when the filament is viewed thoughit the shorter, bluer wavelengths are preferentiallyabsorbed causing a reduction in the measured CCT.Figure 8 shows the measured spectral total flux of
the tungsten flux standard, which exhibits the typi-cal Planckian distribution expected for a lamp ofthis type. The luminous flux calculated from thetotal spectral flux of this source measured on thegoniospectroradiometer agrees with a measurementof the luminous flux made using the NPL 5m inte-
grating sphere to within 0.4%, which is well withinthe combined uncertainties of the two instruments.
B. Measurement of a White LED Cluster
The white LED cluster was made up of 75 individualwhite LED elements encapsulated in a clear plasticenvelope. The individual LEDs were mounted on twoboards fixed back-to-back inside the envelope. Thespectral radiant intensity of the LED cluster wasmeasured in 302 directions around the source. Dueto the extremely directional output of the cluster,these measurement directions were primarily inthe hemisphere below the source (0° ≤ γ ≤ 90°), andwere more closely spaced in the directions nearthe peak intensity.
The luminous intensity distribution of the LEDcluster (see Fig. 9) is extremely anisotropic and ishighest in the γ ¼ 0° direction. A small amount ofthe total output is emitted in the direction of the cap.
Figure 10 shows the CCT distribution of the whiteLED cluster measured between γ ¼ 0° and γ ¼ 90°.The CCT at larger elevation angles could not be re-
Fig. 6. Measured luminous intensity distribution of a 1kW tung-sten filament flux standard.
Fig. 7. Measured CCT distribution of a 1kW tungsten filamentflux standard.
Fig. 8. Measured spectral total flux of a 1kW tungsten filamentflux standard.
liably calculated due to the low radiant intensity ofthe source in these directions. The CCT of the sourcevaries significantly with elevation and is highestclose to the equatorial plane, γ ¼ 90°, decreasingwith elevation angle before increasing again directlybeneath the lamp, γ ¼ 0°. The scatterplot of mea-sured chromaticity coordinates (see Fig. 11) furtherillustrates that the color of the source varies signifi-cantly depending on the direction from which it isviewed.The measured spectral total flux of the LED clus-
ter (see Fig. 12) shows how its white appearancearises from a combination of the LED emission atshorter wavelengths centered at around 460nmand the broadband emission of the phosphor coatingat longer wavelengths.
C. Measurement of a Compact Fluorescent Lamp
The compact fluorescent lamp measured on the go-niospectroradiometer had a 2D configuration suchthat with the lamp cap up, the light emitting tubeslay in a horizontal plane. Figure 13 shows the lumi-nous intensity distribution of the source measured in308 directions equally spaced in C and γ. Similarly asfor the tungsten flux standard (see Fig. 6), masking ofthe far sections of the discharge tube by those nearerthe receiver result in a lower luminous intensity inthe plane of the tube (equatorial plane). The CCT dis-tribution for the compact fluorescent source (seeFig. 14) is relatively isotropic, although slightly high-er in directions close to the lamp cap. The scatterplotof the x and y chromaticity coordinates for this source(see Fig. 15) further illustrates that the perceived col-or does not change significantly with viewing angle.It is interesting to compare this latter figure with thecorresponding figures for the white LED cluster (seeFig. 11), where the variation in color (in the x, y colorspace) is significantly larger. The measured totalspectral flux of the compact fluorescent lamp isshown in Fig. 16.
Fig. 9. Measured luminous intensity distribution of a white LEDcluster.
Fig. 10. Measured CCT distribution of a white LED cluster.
Fig. 11. Scatterplot showing x and y chromaticity coordinates fora white LED cluster measured in different directions around thesource.
Fig. 12. Measured spectral total flux of a white LED cluster.
A goniospectroradiometer has been developed atNPL for measurement of the spectral radiant inten-sity distribution of a variety of different source typesfrom 350 to 830nm. Additional source parameterssuch as luminous intensity distribution, spectraland luminous flux, and chromaticity and CCT distri-butions are calculated from the measured spectralradiant intensity distribution. The system incorpo-rates a CCD array spectrometer with a series ofblocking filters that have been shown to improvethe stray light rejection of the device by greater thanan order of magnitude. Work is ongoing to comparespectral and luminous flux measurements made onthe goniospectroradiometer with similar measure-
ments made using NPL’s integrating sphere basedinstrument on a variety of different source types.
The authors acknowledge the help and advice oftheir colleagues within the Optical Technologiesand Scientific Computing Team at the NationalPhysical Laboratory, in particular, Louise Wrightfor developing the spatial flux integration methodand goniometric data visualization tools. The identi-fication of certain commercial equipment does notimply recommendation or endorsement by NPL,nor does it imply that the equipment identified isthe best available for the purpose. This work wasfunded by the National Measurement System Direc-torate of the United Kingdom Department for Inno-vation, Universities, and Skills.
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Fig. 13. Measured luminous intensity distribution of a 2D com-pact fluorescent lamp.
Fig. 14. Measured CCT of a 2D compact fluorescent lamp.
Fig. 15. Scatterplot showing x and y chromaticity coordinates fora compact fluorescent lamp measured in different directionsaround the source.
Fig. 16. Measured spectral total flux of a 2D compact fluorescentlamp.
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