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Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and...

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Chapter 7 Chapter 7 Components of Components of Optical Optical Instruments Instruments
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Page 1: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Chapter 7Chapter 7Components of Components of Optical InstrumentsOptical Instruments

Page 2: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

IInstruments for the ultraViolet nstruments for the ultraViolet (UV)(UV),,ViSibleViSible , and infrared , and infrared ((IRIR)) regions have enough features inregions have enough features in

common that they are often common that they are often called optical instruments even called optical instruments even

though the human eye is not though the human eye is not sensitive to ultraviolet or sensitive to ultraviolet or

infrared wavelengths.infrared wavelengths.

Page 3: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

7A7A GENERAL GENERAL DESIGNS OFDESIGNS OF

OPTICAL OPTICAL INSTRUMENTSINSTRUMENTS

Page 4: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Optical spectroscopic methods Optical spectroscopic methods are are based upon based upon sixsix phenomena: phenomena:

11. Absorption. Absorption

22. Fluorescence. Fluorescence

33. Phosphorescence. Phosphorescence

44. Scattering. Scattering

55. Emission. Emission

66. Chemiluminescence. Chemiluminescence

Page 5: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Components of typical Components of typical spectroscopic instruments:spectroscopic instruments:

1.1. A stable source of radiant energy (sources A stable source of radiant energy (sources of radiation).of radiation).

2.2. A transparent container for holding the A transparent container for holding the sample (sample cell).sample (sample cell).

3.3. A device that isolates a restricted region A device that isolates a restricted region of the spectrum for measurement of the spectrum for measurement (wavelength selector, monochromator or (wavelength selector, monochromator or grating).grating).

4.4. A radiation detector, which converts A radiation detector, which converts radiant energy to a usable electrical radiant energy to a usable electrical signal.signal.

5.5. A signal processor and readout, which A signal processor and readout, which displays the transduced signal.displays the transduced signal.

Page 6: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE7-1FIGURE7-1 Components of various Components of various types of instruments for types of instruments for optical spectroscopy. In optical spectroscopy. In

(a),(a),the arrangement for the arrangement for

absorption absorption measurements is shown. measurements is shown.

Note that source Note that source radiation of theradiation of the

selected wavelength is selected wavelength is sent through the sample, sent through the sample,

and the transmitted and the transmitted radiation is measuredradiation is measuredby the detector-signal by the detector-signal

processing-readout unit. processing-readout unit. With some instruments, With some instruments,

the position of thethe position of thesample and wavelength sample and wavelength selector is reversed. In selector is reversed. In

(b), the configuration for (b), the configuration for fluorescence fluorescence

measurementsmeasurementsis shown. Here, two is shown. Here, two

wavelength selectors are wavelength selectors are needed to select the needed to select the

excitationexcitationand emission and emission

wavelengths. The wavelengths. The selected source radiation selected source radiation is incident on the sample is incident on the sample

and theand theradiation emitted is radiation emitted is measured, usually at measured, usually at right angles to avoid right angles to avoid scattering. In (c), the scattering. In (c), the

configurationconfigurationfor emission for emission

spectroscopy is shown. spectroscopy is shown. Here, a source of Here, a source of

thermal energy, such as thermal energy, such as a flame ora flame or

plasma, produces an plasma, produces an analy1evapor that emits analy1evapor that emits radiation isolated by the radiation isolated by the

wavelength selectorwavelength selectorand converted to an and converted to an

electrical signal by the electrical signal by the detector.detector.

Page 7: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE7-2FIGURE7-2 (a)construction materials and (b) wave (a)construction materials and (b) wave length selectors for spectroscopic instruments.length selectors for spectroscopic instruments.

Page 8: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

7B7B Sources of Sources of RadiationRadiation

Page 9: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Sources of RadiationSources of Radiation

In order to be suitable for In order to be suitable for spectroscopic studies, a source spectroscopic studies, a source must generate a beam of radiation must generate a beam of radiation with sufficient power for easy with sufficient power for easy detection and measurement and its detection and measurement and its output power should be stable for output power should be stable for reasonable periods. Sources are of reasonable periods. Sources are of two typestwo types..

11. . Continuum sourcesContinuum sources

22. . Line SourcesLine Sources

Page 10: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

7B-17B-1 Continuum Continuum SourcesSources

Page 11: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Continuum Sources:Continuum Sources:

Continuum sources emit radiation that Continuum sources emit radiation that changes in intensity only slowly as a changes in intensity only slowly as a function of wavelength. It is widely used function of wavelength. It is widely used in in absorption absorption andand fluorescence fluorescence spectroscopy. For the spectroscopy. For the ultravioletultraviolet region, region, the most common source is the the most common source is the deuterium deuterium lamplamp. High pressure gas filled arc lamps . High pressure gas filled arc lamps that contain argon, xenon, or mercury that contain argon, xenon, or mercury serve when a particular intense source is serve when a particular intense source is required. For the required. For the visible region visible region of the of the spectrum, the spectrum, the tungsten filament lamp tungsten filament lamp is is used universally. The common infrared used universally. The common infrared sources are inert solids heated to 1500 to sources are inert solids heated to 1500 to 2000 K.2000 K.

Page 12: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

7B-2 7B-2 Line SourcesLine Sources

Page 13: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Line Sources:Line Sources:

Sources that emit a few discrete lines Sources that emit a few discrete lines find wide use in find wide use in atomic absorption atomic absorption spectroscopy, spectroscopy, atomicatomic and and molecular molecular fluorescence fluorescence spectroscopy, and spectroscopy, and RamanRaman spectroscopy. Mercury and sodium spectroscopy. Mercury and sodium vapor lamps provide a relatively few vapor lamps provide a relatively few sharp lines in the ultraviolet and visible sharp lines in the ultraviolet and visible regions and are used in several regions and are used in several spectroscopic instruments. spectroscopic instruments. Hollow Hollow cathode lamps cathode lamps and and electrodelesselectrodeless discharge lamps discharge lamps are the most are the most important line sources for atomic important line sources for atomic absorption and fluorescence methods.absorption and fluorescence methods.

Page 14: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

7 B-3 7 B-3 Laser SourcesLaser Sources

Page 15: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Laser SourcesLaser Sources

The term ‘LASER’ is an acronym The term ‘LASER’ is an acronym for for LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of RRadiation. Laser are highly adiation. Laser are highly useful because of their very high useful because of their very high intensities, narrow bandwidths, intensities, narrow bandwidths, single wavelength, and coherent single wavelength, and coherent radiation. Laser are widely used radiation. Laser are widely used in high-resolution spectroscopyin high-resolution spectroscopy..

Page 16: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-2 FIGURE 7-2 (a) sources and (b) detectors for (a) sources and (b) detectors for spectroscopic in struments.spectroscopic in struments.

Page 17: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Components of Components of LasersLasers

Page 18: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Component of Lasers:Component of Lasers:

The important components of The important components of laser source are lasing medium, laser source are lasing medium, pumping source, and mirrors. The pumping source, and mirrors. The heart of the device is the lasing heart of the device is the lasing medium. It may be a solid crystal medium. It may be a solid crystal such as ruby, a semiconductor such as ruby, a semiconductor such as gallium arsenide, a such as gallium arsenide, a solution of an organic dye or a solution of an organic dye or a gas such as argon or krypton.gas such as argon or krypton.

Page 19: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

““LASER”LASER” LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of

RRadiationadiation Emits very intense, Emits very intense, monochromaticmonochromatic light at light at

high power (intensity)high power (intensity) All waves All waves in phasein phase (unique), and parallel (unique), and parallel All waves are polarized in one planeAll waves are polarized in one plane Used to be expensiveUsed to be expensive Not useful for Not useful for scanningscanning wavelengths wavelengths

Page 20: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

LaserLaser SetupSetup

FIGURE 7-2 FIGURE 7-2 schematic schematic representation of a typical laser representation of a typical laser

source.source.

Page 21: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Lasing MechanismLasing Mechanism

Page 22: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Four processesFour processes in in Lasing Lasing

MechanismMechanism::

11. Pumping. Pumping

22. Spontaneous emission . Spontaneous emission

(fluorescence)(fluorescence)

33. Stimulated emission. Stimulated emission

44. Absorption. Absorption

Page 23: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

1.1. PumpingPumping Molecules of the active Molecules of the active

medium are excited to higher medium are excited to higher

energy levelsenergy levels

Energy for excitation Energy for excitation

electrical, light, or chemical electrical, light, or chemical

reactionreaction

Page 24: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PumpingPumping

Page 25: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Spontaneous: Incoherent radiationDiffers in direction and phase

FIGURE 7·5 FIGURE 7·5 Four processes Four processes important in laser action: (a) important in laser action: (a)

pumping (excitation by pumping (excitation by electrical,electrical,

radIant, or chemical energy), radIant, or chemical energy), (b) spontaneous emission, (c) (b) spontaneous emission, (c)

stimulated emission, andstimulated emission, and(d) absorption.(d) absorption.

Page 26: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

22.. Spontaneous EmissionSpontaneous Emission A molecule in an excited state can A molecule in an excited state can

lose excess energy by emitting a lose excess energy by emitting a

photon (this is fluorescence)photon (this is fluorescence)

E = hE = h = hc/ = hc/; E = E; E = Eyy – E – Exx

E (fluorescence) < E (absorption) E (fluorescence) < E (absorption)

(fluorescence) > (fluorescence) > (absorption) (absorption)

[fluorescent light is at longer [fluorescent light is at longer

wavelength than excitation light]wavelength than excitation light]

Page 27: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Spontaneous EmissionSpontaneous Emission

Page 28: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

33. . Stimulated EmissionStimulated Emission Must have stimulated emission to have Must have stimulated emission to have

lasing lasing Excited molecules interact with Excited molecules interact with

photons produced by emissionphotons produced by emission Collision causes excited molecules to Collision causes excited molecules to

relax and emit a photon (i. e., emission)relax and emit a photon (i. e., emission) Photon energy of this emission = Photon energy of this emission =

photon energy of collision photon photon energy of collision photon now there are 2 photons with same now there are 2 photons with same energy (in same phase and same energy (in same phase and same direction)direction)

Page 29: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

A photon incident on an excited A photon incident on an excited state species causes emission of a state species causes emission of a second photon of the same second photon of the same frequencyfrequency, which travels in , which travels in exactly the same direction, and is exactly the same direction, and is precisely in phase with the first precisely in phase with the first photo.photo.

M* + hM* + hM + 2hM + 2h

Page 30: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Stimulated EmissionStimulated Emission

Page 31: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

44. . AbsorptionAbsorption Competes with stimulated Competes with stimulated

emission emission A molecule in the ground state A molecule in the ground state

absorbs photons and is absorbs photons and is promoted to the excited state promoted to the excited state

Same energy level as pumping, Same energy level as pumping, but now the photons that were but now the photons that were produced for lasing are goneproduced for lasing are gone

Page 32: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

AbsorptionAbsorption

Page 33: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Population Inversion and Light Population Inversion and Light AmplificationAmplification

To have light amplification in a laser, the To have light amplification in a laser, the number ofnumber of

photons produced by stimulated emission photons produced by stimulated emission must exceedmust exceed

the number lost by absorption. This the number lost by absorption. This condition prevailscondition prevails

only when the number of particles in the only when the number of particles in the higher energyhigher energy

state exceeds the number in the lower; in state exceeds the number in the lower; in other words,other words,

there must be a there must be a population inversion from population inversion from the normalthe normal

distribution of energy states. Population distribution of energy states. Population inversions arcinversions arc

created by pumping.'created by pumping.'

Page 34: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Population Inversion:Population Inversion: Must have population inversion to Must have population inversion to

sustain lasing. sustain lasing. Population of molecules is inverted Population of molecules is inverted

(relative to how the population (relative to how the population normally exists).normally exists).

Normally: there are more molecules Normally: there are more molecules in the ground state than in the in the ground state than in the excited state (need > 50 %).excited state (need > 50 %).

Population inversion: More molecules Population inversion: More molecules in the excited state than in the in the excited state than in the ground state.ground state.

Page 35: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Why is it important?Why is it important? More molecules in the ground More molecules in the ground

state state more molecules that can more molecules that can

absorb photonsabsorb photons

Remember: absorption competes Remember: absorption competes

with stimulated emission with stimulated emission

Light is attenuated rather than Light is attenuated rather than

amplifiedamplified

More molecules in the excited More molecules in the excited

state state net gain in photons net gain in photons

producedproduced

Page 36: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Population Inversion Population Inversion Necessary for AmplificationNecessary for Amplification

Population inversions are obtained by pumping

FIGURE 7-6 FIGURE 7-6 Passage of radiation through (a) a noninverted population Passage of radiation through (a) a noninverted population and (b) an invertedand (b) an inverted

population created by excitation of electrons into virtual states by an population created by excitation of electrons into virtual states by an external energy sourceexternal energy source

(pumping).(pumping).

Page 37: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Three- and Four-Level Three- and Four-Level Laser SystemsLaser Systems

Page 38: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

How to achieve population inversion?How to achieve population inversion? Laser systems: 3-level or 4-LevelLaser systems: 3-level or 4-Level 4-level is better 4-level is better easier to sustain easier to sustain

population inversionpopulation inversion 3-level system: lasing transition is 3-level system: lasing transition is

between Ebetween Ey y (excited state) and the (excited state) and the ground stateground state

4-level system: lasing transition is 4-level system: lasing transition is between two energy levels (neither of between two energy levels (neither of which is ground state)which is ground state)

All you need is to have more molecules All you need is to have more molecules in Ein Eyy than E than Exx for population inversion for population inversion (4-level system) (4-level system) easier to achieve easier to achieve than more molecules in Ethan more molecules in Eyy than ground than ground state (3-level system)state (3-level system)

Page 39: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

In the In the three-level systemthree-level system, , the transition responsible the transition responsible

for laser radiation is for laser radiation is between an excited state between an excited state Ey Ey

and the ground state E0;and the ground state E0;in in a four-level systema four-level system, on , on

the other hand. radiation is the other hand. radiation is generated by a transition generated by a transition from from Ey to a state Ex that Ey to a state Ex that has has a greater energy than a greater energy than

the ground state.the ground state.

Page 40: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

OverallOverall

Easy population inversion

FIGURE 7-7 FIGURE 7-7 Energy level diagrams for two types of laserEnergy level diagrams for two types of lasersystemssystems

Page 41: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Advantages of LasersAdvantages of Lasers• Low Beam Divergence (“Small dot”)Low Beam Divergence (“Small dot”)

• Nearly Monochromatic (“narrow bandwidth”)Nearly Monochromatic (“narrow bandwidth”)

• Coherent (“constructive interference”)Coherent (“constructive interference”)

Page 42: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Types of LasersTypes of Lasers Solid state lasersSolid state lasers

Nd:YAG Nd:YAG neodymium yttrium aluminum garnetneodymium yttrium aluminum garnet 1064 nm1064 nm

Gas lasersGas lasers lines w/ specific lines w/ specific s in UV/vis/IRs in UV/vis/IR

He/NeHe/Ne ArAr++, Kr, Kr++

COCO22

eximers (XeFeximers (XeF++,….),….)

Dye lasersDye lasers limited tunability in the visiblelimited tunability in the visible

Semiconductor diode lasersSemiconductor diode lasers limited tunability in the IR, redlimited tunability in the IR, red

Page 43: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Semiconductor Semiconductor Diode LasersDiode Lasers

Page 44: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

An increasingly importantAn increasingly important

source of nearly monochromatic radiation is thesource of nearly monochromatic radiation is the

laser diode. 7 Laser diodes are products of laser diode. 7 Laser diodes are products of modern semiconductormodern semiconductor

technology. We can understand their mechanismtechnology. We can understand their mechanism

of operation by considering the electrical of operation by considering the electrical conductionconduction

characteristics of various materials as illustrated characteristics of various materials as illustrated inin

Figure 7-8.Figure 7-8.

Page 45: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 46: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-9 FIGURE 7-9 A distributed Bragg-reftector laser diode. (From D. W.Nam A distributed Bragg-reftector laser diode. (From D. W.Nam and R. G. Waarts, and R. G. Waarts, LaserLaser

Focus World, 1994, 30 (8),52. Reprinted with permission of Focus World, 1994, 30 (8),52. Reprinted with permission of PennWellPublishing Company.)PennWellPublishing Company.)

Page 47: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Nonlinear Optical Nonlinear Optical Effects with LasersEffects with Lasers

Page 48: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

We noted in Section 6B-7 that when an electromagneticWe noted in Section 6B-7 that when an electromagnetic

wave is transmitted through a dielectric· medium,wave is transmitted through a dielectric· medium,

the electromagnetic field of the radiation causesthe electromagnetic field of the radiation causes

momentary distortion, or polarization, of the valencemomentary distortion, or polarization, of the valence

electrons of the molecules that make up the medium.electrons of the molecules that make up the medium.

For ordinary radiation the extent of polarization For ordinary radiation the extent of polarization P isP is

directly' proportional to the magnitude of the electricdirectly' proportional to the magnitude of the electric

field field E of the radiation. Thus, we may writeE of the radiation. Thus, we may write

P=aEP=aE

where" is the proportionality constant.where" is the proportionality constant. observed, and the observed, and the relationship between polarizationrelationship between polarization

and electric field is given byand electric field is given by

P = aE + f3E' + yE' + . . . (7-1)P = aE + f3E' + yE' + . . . (7-1)

Page 49: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7·10 FIGURE 7·10 A frequency-doubling system for convertingA frequency-doubling system for converting975-nm laser output to 490 nm. (From D. W.Nam975-nm laser output to 490 nm. (From D. W.Namand R. G. Waarts, and R. G. Waarts, Laser Focus World, 1994,30 (8),Laser Focus World, 1994,30 (8),

52. Reprinted with permission of PennWeli Pubtishing52. Reprinted with permission of PennWeli PubtishingCompany.)Company.)

Page 50: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

WAVELENGTH WAVELENGTH SELECTORSSELECTORS

7 B:7 B:

Page 51: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Wavelength SelectorsWavelength Selectors

Need to select wavelengths (Need to select wavelengths () of light for optical ) of light for optical measurements. The output from a wavelength measurements. The output from a wavelength selector would be a radiation of a single wavelength selector would be a radiation of a single wavelength or frequency. There are two types of wavelength or frequency. There are two types of wavelength selector:selector:

11. . FiltersFilters

22. . Monochromators Monochromators 33. Gratings. Gratings

• 4 4 . . Michelson InterferometerMichelson Interferometer

Page 52: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Wavelength SelectorsWavelength Selectors…..…..

Used to select the wavelength (or wavelength range) of light Used to select the wavelength (or wavelength range) of light that eitherthat either impinges on the sample (fluorescence and phosphorescence)impinges on the sample (fluorescence and phosphorescence) is transmitted through the sample (absorption and emission)is transmitted through the sample (absorption and emission)

This selected wavelength then strikes the detectorThis selected wavelength then strikes the detector the ability to select the wavelength helps you to discriminated the ability to select the wavelength helps you to discriminated

between phenomena caused by your between phenomena caused by your analyteanalyte and that caused by and that caused by interfering or interfering or non-relevantnon-relevant species. species.

Are often combined with a set of Are often combined with a set of SLITSSLITS (discussed later) (discussed later)

Various typesVarious types based on filters (CHEAP COLORED GLASS)based on filters (CHEAP COLORED GLASS) based on prisms (LIMITED APPLICATIONS)based on prisms (LIMITED APPLICATIONS) based on gratings…. (GREAT STUFF)based on gratings…. (GREAT STUFF)

Page 53: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-11 FIGURE 7-11 Out put of typical wave length selsctor.Out put of typical wave length selsctor.

Page 54: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FILTERSFILTERS

7 C-17 C-1

Page 55: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FiltersFilters Simple, rugged (no moving parts in general)Simple, rugged (no moving parts in general)

Relatively inexpensiveRelatively inexpensive

Can select some broad range of wavelengthsCan select some broad range of wavelengths

Most often used in Most often used in field instrumentsfield instruments simpler instrumentssimpler instruments instruments dedicated to monitoring a single wavelength range.instruments dedicated to monitoring a single wavelength range.

Two types of filters:Two types of filters: Interference filters Interference filters depend on destructive interference of the impinging depend on destructive interference of the impinging

light to allow a limited range of wavelengths to pass through them (more light to allow a limited range of wavelengths to pass through them (more expensive)expensive)

Absorption filters Absorption filters absorbabsorb specific wavelength ranges of light (cheaper, specific wavelength ranges of light (cheaper, more common)...more common)...

Page 56: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Interference filtersInterference filters

Page 57: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Interference FiltersInterference Filters Dielectric layer between two metallic Dielectric layer between two metallic

filmsfilms Radiation hits filter Radiation hits filter some reflected, some reflected,

some transmitted (transmitted light some transmitted (transmitted light reflects off bottom surface)reflects off bottom surface)

If proper radiation If proper radiation reflected light in reflected light in phase w/incoming radiation: other phase w/incoming radiation: other undergo destructive interferenceundergo destructive interference

i.e., i.e., s of interest s of interest constructive constructive interference (transmitted through interference (transmitted through filter); unwanted filter); unwanted s s destructive destructive interference (blocked by filter)interference (blocked by filter)

Result: narrow range of Result: narrow range of s transmitted s transmitted

Page 58: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-12 FIGURE 7-12 (a) Schematic cross section of an(a) Schematic cross section of aninterference filter.Note that the drawing is not to scaleinterference filter.Note that the drawing is not to scale

and that the three central bands are much narrowerand that the three central bands are much narrowerthan shown. (b) Schematic to show the conditionsthan shown. (b) Schematic to show the conditions

for constructive interference.for constructive interference.

Page 59: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Fabry-Perot Filters (Interference Filters)Fabry-Perot Filters (Interference Filters)

Douglas A. Skoog and James J. Leary, Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.College Publishing, Fort Worth, 1992.

Calcium or Magnesium Calcium or Magnesium Fluoride (FLUORITE!)Fluoride (FLUORITE!)

tFrom 1 to 1’: From 1 to 1’:

For reinforcement to occur at For reinforcement to occur at point 2, point 2,

cos

t

'cos

2

nt

N is order of interference (a small whole number)

A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields.

Page 60: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Fabry-Perot Filters (Interference Filters)Fabry-Perot Filters (Interference Filters)

n

t 2

n

t 2

When When approaches zero approaches zero nn’ = 2t’ = 2t

Snell’s law: Snell’s law: //’’ = = ’/

then then = = ’’

is the wavelength passing the filter and is the refractive index of the dielectric medium

t

Are we missing something?

Page 61: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Interference WedgesInterference Wedges

Page 62: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

An interference wedge An interference wedge consists of a pair of consists of a pair of

mirrored,mirrored,partially transparent partially transparent plates separated by a plates separated by a

wedgeshapewedgeshapelayer of a dielectric layer of a dielectric

material.material.

Page 63: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-13 FIGURE 7-13 Transmission characterics of typical Transmission characterics of typical interference filters. interference filters.

Page 64: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Absorption filtersAbsorption filters

Page 65: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Absorption FiltersAbsorption Filters Colored glass (broader bandwidth: ~50-100 Colored glass (broader bandwidth: ~50-100

nm vs. ~10-nm with interference filters)nm vs. ~10-nm with interference filters) Glass absorbs certain Glass absorbs certain s while transmitting s while transmitting

othersothers TypesTypes

Bandpass: passes 50-100 nmBandpass: passes 50-100 nm Cut-off (e.g., high-pass)Cut-off (e.g., high-pass)

Passes high wavelengths, blocks low Passes high wavelengths, blocks low wavelengthswavelengths

Type of filter could be used for Type of filter could be used for emission or fluorescence (since emission or fluorescence (since excitation light is lower excitation light is lower and should and should be blocked; emission is higher be blocked; emission is higher and and should be collected).should be collected).

Page 66: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Characteristics of Characteristics of absorption filterabsorption filter

Cheaper than interference filterCheaper than interference filter Worse than interference filterWorse than interference filter But widely usedBut widely used

Absorption some spectral rangeAbsorption some spectral range Effective bandwidth = 30 ~ 250 nmEffective bandwidth = 30 ~ 250 nm %T = less than 10%%T = less than 10% Cut-off filterCut-off filter

typestypes Dye suspended in gelatinDye suspended in gelatin Colored glass (stable to heat)Colored glass (stable to heat)

Page 67: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Cut off filteresCut off filteres

Page 68: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Cut off filteres : Cut off filteres : have have transmittances of nearly 100% transmittances of nearly 100% over a over a portion of the visible spectrum but portion of the visible spectrum but

then rapidly decrease to zero then rapidly decrease to zero transmittance over the remainder. A transmittance over the remainder. A narrow spectral band can be isolated narrow spectral band can be isolated

by coupling a cutoff filter with a second by coupling a cutoff filter with a second filter (see Figure 7-15). Figure 7-filter (see Figure 7-15). Figure 7-

1414shows that the performance shows that the performance characteristics characteristics of absorption filters are of absorption filters are

significantly inferior to those of significantly inferior to those of interference-type filters.interference-type filters.

Page 69: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Two basic filter functions….Two basic filter functions…. cutoff cutoff filters absorb light in a specific filters absorb light in a specific

range of wavelengths. They “cutoff” this range of wavelengths. They “cutoff” this range from the detectors (e.g. cutoff for range from the detectors (e.g. cutoff for 550 nm)550 nm)

Absorption filters are cutoffs.Absorption filters are cutoffs.

bandpassbandpass filters absorb light outside of filters absorb light outside of a specific range (e.g. 350-550 nm)a specific range (e.g. 350-550 nm)

Interference filters are bandpass or you can Interference filters are bandpass or you can make a bandpass from a combination of two make a bandpass from a combination of two cutoff filters!cutoff filters!

Page 70: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Comparison of various types of absorptionComparison of various types of absorptionfilters for visible radiation.filters for visible radiation.

Page 71: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 72: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

MONOCHROMATORSMONOCHROMATORS A)A) PRISM PRISM

MONOCHROMATORS MONOCHROMATORS B)B)GRATING GRATING MONOCHROMATORSMONOCHROMATORS C)C)ECHELLE ECHELLE MONOCHROMATORS.MONOCHROMATORS.

7 C-27 C-2

Page 73: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Monochromators: Monochromators:

For many spectroscopic methods, it is necessary or For many spectroscopic methods, it is necessary or desirabledesirable

to be able to continuously vary the wavelengthto be able to continuously vary the wavelength

of radiation over a broad range. This process is calledof radiation over a broad range. This process is called

scan ing a spectrum. Monochromators are designedscan ing a spectrum. Monochromators are designed

for spectral scanning. Monochromators for ultraviolet,for spectral scanning. Monochromators for ultraviolet,

visible, and infrared radiation arc all similar in visible, and infrared radiation arc all similar in mechanicalmechanical

const ruction in the sense that they use slits,const ruction in the sense that they use slits,

lenses, mirrors, windows, and gratings or prisms. Thelenses, mirrors, windows, and gratings or prisms. The

materials from which these components are fabricatedmaterials from which these components are fabricated

depend on the wavelength region of intended usedepend on the wavelength region of intended use

Page 74: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Components of Components of Monochromators Monochromators Figure 7-18 Figure 7-18

illustrates the optical elements found illustrates the optical elements found in all monochromators, which include in all monochromators, which include the following: (I) an entrance slit the following: (I) an entrance slit that provides a rectangular optical that provides a rectangular optical

image, image, (2) a collimating lens or mirror that (2) a collimating lens or mirror that produces a parallel beam of radiation, produces a parallel beam of radiation, (3) a prism or a grating that disperses (3) a prism or a grating that disperses

the radiation into its component the radiation into its component wavelengths, wavelengths,

(4) a focusing element that reforms the (4) a focusing element that reforms the image of the entrance slit and focuses image of the entrance slit and focuses

it on a planar surface called a it on a planar surface called a focal focal plane, andplane, and

(5) an exit slit in the focal (5) an exit slit in the focal plane that plane that isolates the desired spectral band.isolates the desired spectral band.

Page 75: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-18 FIGURE 7-18 Two types of monochromators: (a) Czerney-Tumer grating Two types of monochromators: (a) Czerney-Tumer grating monochromator andmonochromator and

(b) Bunsen prism monochromator. (Inboth instances, Al > A,.)(b) Bunsen prism monochromator. (Inboth instances, Al > A,.)

Page 76: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 77: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

A) A) Prism Prism MonochromatorsMonochromators

Page 78: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Prism MonochromatorsPrism Monochromators

Prisms can be used to disperse ultraviolet, visible, andPrisms can be used to disperse ultraviolet, visible, and

infrared radiation. The material used for their constructioninfrared radiation. The material used for their construction

differs, however, depending on the wavelengthdiffers, however, depending on the wavelength

region (see Figure 7-2b). Figure 7-20 shows the two most common types region (see Figure 7-2b). Figure 7-20 shows the two most common types of prism designs_ The first is a 60° prism, which is usuallyof prism designs_ The first is a 60° prism, which is usually

fabricated from a single block of material. When crystallinefabricated from a single block of material. When crystalline

(but not fused) quartz is the construction material, however, the prism is (but not fused) quartz is the construction material, however, the prism is usually formed by cementing two 30° prisms together, as shown in usually formed by cementing two 30° prisms together, as shown in Figure 7-20a; one is fabricated from right -handed quartz and theFigure 7-20a; one is fabricated from right -handed quartz and the

second from left-handed quartz in this way, the opticallysecond from left-handed quartz in this way, the optically

active quartz causes no net polarization of theactive quartz causes no net polarization of the

emitted radiation; this type of prism is called a emitted radiation; this type of prism is called a Cornu prism. Figure 7-Cornu prism. Figure 7-18b shows a Bunsen monochromator, 18b shows a Bunsen monochromator, which uses a 60° prism, likewise which uses a 60° prism, likewise often made of quartz.often made of quartz.

Page 79: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-20 FIGURE 7-20 Dispersion by a Dispersion by a prism: prism: (a) (a) quartz Cornu type quartz Cornu type

and and (b)(b) Littrow type. Littrow type.

Page 80: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PrismsPrisms

First type of widely used, “scanning” First type of widely used, “scanning” wavelength selection devices (TURN PRISM)wavelength selection devices (TURN PRISM)

Often made of salts such as sodium chloride, Often made of salts such as sodium chloride, fluorites etc (Remember figure 7-2b).fluorites etc (Remember figure 7-2b).

VERY delicateVERY delicate. Often subject to damage in . Often subject to damage in humidity and wide heat ranges.humidity and wide heat ranges.

Not widely used today in spectroscopy Not widely used today in spectroscopy equipment.equipment. Great demonstration tools for kidsGreat demonstration tools for kids Nice on the cover of a Pink Floyd albumNice on the cover of a Pink Floyd album

Page 81: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PrismsPrisms

Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

Page 82: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PrismPrism

Page 83: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

B) B) Grating Grating MonochromatorMonochromator

ss

Page 84: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Grating Monochromators (scan a Grating Monochromators (scan a spectrum)spectrum)

Scan spectrum = vary Scan spectrum = vary continuously continuously Materials for construction = Materials for construction = range of range of

interestinterest Components of a grating monochromatorComponents of a grating monochromator

1. Entrance slit (rectangular image)1. Entrance slit (rectangular image)

2. Collimating optic (parallel beam)2. Collimating optic (parallel beam)

3. Grating (disperses light into separate 3. Grating (disperses light into separate s) s)

4. Focusing optic (reforms rectangular 4. Focusing optic (reforms rectangular image)image)

5. Exit slit at focal plane of focusing optic 5. Exit slit at focal plane of focusing optic (isolates desired spectral band) (isolates desired spectral band)

Page 85: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Reflection GratingsReflection Gratings

Widely used in instruments today.Widely used in instruments today.

Light reflected off a surface, and not Light reflected off a surface, and not cancelled out by destructive interference, is cancelled out by destructive interference, is used for selection of wavelengthsused for selection of wavelengths

Constructed of various materials….Constructed of various materials…. Polished glass, silica or polymer substratePolished glass, silica or polymer substrate Grooves milled or laser etched into the surfaceGrooves milled or laser etched into the surface Coated with a reflective material (silvered) such as Coated with a reflective material (silvered) such as

a shiny metala shiny metal

VERY FRAGILE!! VERY FRAGILE!! Sealed inside the instrument. DO NOT TOUCH!Sealed inside the instrument. DO NOT TOUCH!

Page 86: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Reflection (Diffraction) Reflection (Diffraction) Gratings...Gratings...

Widely used in instruments today.Widely used in instruments today. Light reflected off a surface, and not cancelled out by Light reflected off a surface, and not cancelled out by

destructive interference, is used for selection of destructive interference, is used for selection of wavelengthswavelengths

Constructed of various materials….Constructed of various materials…. Polished glass, silica or polymer substratePolished glass, silica or polymer substrate Grooves milled or laser etched into the surfaceGrooves milled or laser etched into the surface Coated with a reflective material (silvered) such as a shiny Coated with a reflective material (silvered) such as a shiny

metalmetal VERY FRAGILE!! VERY FRAGILE!!

Sealed inside the instrument. DO NOT TOUCH!Sealed inside the instrument. DO NOT TOUCH! Laser Cut have 100’s - 1000’s of lines (blazes) per mmLaser Cut have 100’s - 1000’s of lines (blazes) per mm High resolution (<0.01 nm) if neededHigh resolution (<0.01 nm) if needed Most expensive optical part of an instrumentMost expensive optical part of an instrument

Page 87: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Reflection GratingsReflection Gratings

Light hits grating and Light hits grating and

light is dispersedlight is dispersed

Tilt grating to vary which Tilt grating to vary which

is passed at exit slit is passed at exit slit

during the scanduring the scan

Page 88: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

1.The 1.The EchelletteGratingEchelletteGrating2 . Concave 2 . Concave Gratings Gratings 3.Holographic 3.Holographic GratingsGratings

Grating Grating MonochromatorsMonochromators

Page 89: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Construction of Gratings…..Construction of Gratings….. The substrate is formed and polished. It is then The substrate is formed and polished. It is then

blazed by one of a number of techniques…blazed by one of a number of techniques… Cut using mechanical tools Cut using mechanical tools

Poor reproducibility in shape and spacing of the blazesPoor reproducibility in shape and spacing of the blazes Etched using chemicalsEtched using chemicals

Better but still not very goodBetter but still not very good Laser etched blazes (aka holographic gratings).Laser etched blazes (aka holographic gratings).

Best method for productionBest method for production Closely spaced blazes (high # of lines/mm) means a greater Closely spaced blazes (high # of lines/mm) means a greater

capacity to separate light into component wavelengthscapacity to separate light into component wavelengths Good reproducibility from blaze to blaze means that the Good reproducibility from blaze to blaze means that the

grating produces fewer “defects” such as double imagesgrating produces fewer “defects” such as double images Most common method todayMost common method today

The substrate is then coated with a very thin (few The substrate is then coated with a very thin (few molecules or atoms thick) film of reflective molecules or atoms thick) film of reflective materialmaterial

The grating is then mounted in a holder and will The grating is then mounted in a holder and will never be touched by anything if correctly cared fornever be touched by anything if correctly cared for

Page 90: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

1. echellette -type grating 1. echellette -type grating Figure 7-21 is a schematic representationFigure 7-21 is a schematic representation

of an of an echellette -type grating, which isechellette -type grating, which isgrooved, or grooved, or blazed, such that it has relatively broadblazed, such that it has relatively broad

faces from which reflection occurs and narrow unusedfaces from which reflection occurs and narrow unusedfaces. This geometry provides highly efficient diffractionfaces. This geometry provides highly efficient diffractionof radiation, and the reason for blazing is to concentrateof radiation, and the reason for blazing is to concentrate

the radiation in a preferred direction . Eachthe radiation in a preferred direction . Eachof the broad faces can be considered to be a line sourceof the broad faces can be considered to be a line source

of radiation perpendicular to the plane of the page; thusof radiation perpendicular to the plane of the page; thusinterference among the reflected beams 1,2, and 3 caninterference among the reflected beams 1,2, and 3 can

occur. For the interference to be constructive, it is occur. For the interference to be constructive, it is necessary that the path lengths differ by an integral necessary that the path lengths differ by an integral multiplen of the wavelength A of the incident beam.multiplen of the wavelength A of the incident beam.

Page 91: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

echellette -type gratingechellette -type grating

Page 92: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Echelle grating Echelle grating AdvantagesAdvantages

The advantage of an echelle The advantage of an echelle high efficiency and low polarization effects over high efficiency and low polarization effects over

large spectral intervalslarge spectral intervals Together with high dispersion, this leads to Together with high dispersion, this leads to

compact, high-resolution instruments.compact, high-resolution instruments. An important limitation of echelleAn important limitation of echelle

the orders overlap unless separated optically, for the orders overlap unless separated optically, for instance by a cross-dispersing element. instance by a cross-dispersing element.

A prism or echelette grating is often used for this A prism or echelette grating is often used for this purpose. purpose.

For broad spectral range, to use many sucessive For broad spectral range, to use many sucessive ordersorders

http://www.gratinglab.com/library/http://www.gratinglab.com/library/technotes/technote6.asptechnotes/technote6.asp

Page 93: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Example 7-1

Grating with 1450 blazes/mmPolychromatic light at i = 48 deg

L of the monochromatic reflected light at R = +20,+10 and 0 deg?

dd(sin i + sin r) = n(sin i + sin r) = n1) Calculate “d”1) Calculate “d”

d= 1 mm/1450 blazes convert to nm x106 689.7 nm per groove!

dd(sin i + sin r) = n(sin i + sin r) = n2) Calculate “2) Calculate “” for n=1 at +20 ” for n=1 at +20 degdeg

= 689.7 nm ( sin 48 + sin 20)/1 = 748.4 nm!

Grating will give a monochromatic beam of light of 748.4 nm at 20 deg, 632 nm at 10 deg and 513 nm at 0 deg. For n=1!

Page 94: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.

ResolutionResolution

New holographic gratings can have up to 64K New holographic gratings can have up to 64K grooves!grooves!

More grooves = better resolving powerMore grooves = better resolving power

R = R = //(1K to 10K)(1K to 10K)

R = nR = nN (N = grooves!)N (N = grooves!)

How well can you focus on two adjacent How well can you focus on two adjacent wavelengths!wavelengths!

Page 95: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

2. Concave Gratings.2. Concave Gratings.

Gratings can he formed on a concaveGratings can he formed on a concave

surface in much the same way as on a plane surface.surface in much the same way as on a plane surface.

A concave grating permits the design of a A concave grating permits the design of a monochromatormonochromator

without auxiliary collimating and focusingwithout auxiliary collimating and focusing

mirrors or lenses because the concave surface bothmirrors or lenses because the concave surface both

disperses the radiation and focuses it on the exit slit.disperses the radiation and focuses it on the exit slit.

Such an arrangement is advantageous in terms of cost:Such an arrangement is advantageous in terms of cost:

in addition. the reduction in number of optical surfacesin addition. the reduction in number of optical surfaces

increases the energy throughput of a monochromatorincreases the energy throughput of a monochromator

that contains a concave grating.that contains a concave grating.

Page 96: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

3. Holographic 3. Holographic GratingsGratings

. . Holographic gratings are appearingHolographic gratings are appearing

in ever-increasing numbers in modern in ever-increasing numbers in modern optical instruments, even some of the less optical instruments, even some of the less expensive ones.expensive ones.

Page 97: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Grating EquationGrating Equation

Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.

Grooved or blazed. Provides highly efficient diffraction (small enough in size compared to wavelength) of radiation. Each broad face is considered as a point source.

d: Spacing between the reflecting surfaces

Page 98: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Beam 2travels a greater distance than beam 1, for constructive interferences to occur,CB + BD = nangle i = CAB, angle r = DAB

CB = dsini, BD = dsinr

n = d(sini + sinr)

d: Spacing between the reflecting surfaces

Page 99: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Performance Characteristics Performance Characteristics the relationship of Grating the relationship of Grating

MonochromatorsMonochromators11.Spectral Purity..Spectral Purity.

22.Dispersion of Grating .Dispersion of Grating Monochromators.Monochromators.

33.Resolving Power of .Resolving Power of Monochromators.Monochromators.

44.light-Gathering power of .light-Gathering power of Monochromators.Monochromators.

Page 100: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

The quality of a monochromator depends on The quality of a monochromator depends on the purity of its radiant output, its ability to the purity of its radiant output, its ability to resolve adjacent wavelengths, its light-resolve adjacent wavelengths, its light-gathering power, and its spectral gathering power, and its spectral bandwidth.bandwidth.

Page 101: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

1. Spectral Purity1. Spectral Purity

The exit beam of a monochromator The exit beam of a monochromator is usually contaminated with is usually contaminated with small amounts of scattered or small amounts of scattered or stray radiation with wavelengths stray radiation with wavelengths far different from that of the far different from that of the instrument selling.instrument selling.

Page 102: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

2. Dispersion of Grating2. Dispersion of GratingDispersion of Grating: Monochromators. The abilityDispersion of Grating: Monochromators. The ability

of a monochromator to separate different wavelengthsof a monochromator to separate different wavelengths

depends on its depends on its dispersion. The angular dispersion isdispersion. The angular dispersion is

given by given by drlddrld λ λ, where dr is the change in the angle of, where dr is the change in the angle of

reflection or refraction with a change in wavelengthreflection or refraction with a change in wavelength

ddλλ..

Page 103: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

3. Resolving3. Resolving PowerPower of of Monochromators.Monochromators.

The resolving power R of a monochromator describes the limit of its ability to separate adjacent images that have a slight difference in wavelength.

R=λ/∆λ R=λ/∆λ =nN

Page 104: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

4. light-Gathering power 4. light-Gathering power of Monochromatorsof Monochromators

To in crease the signal-to-noise ratio of a To in crease the signal-to-noise ratio of a spectrometer, it is necessary that the spectrometer, it is necessary that the radiant energy that reaches the detector be radiant energy that reaches the detector be as large as possible. The as large as possible. The number F. or number F. or speed speed provides a measure of the ability of provides a measure of the ability of a monochromator To collect the radiation a monochromator To collect the radiation that emerges from the entran slit. The that emerges from the entran slit. The f.number is defined by:f.number is defined by:

Page 105: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

CC) ) Echelle Echelle MonochromatorsMonochromators

Page 106: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Echelle monochromatorsEchelle monochromators

contain two dispersing elements arranged in contain two dispersing elements arranged in series. The first of these elements is a special series. The first of these elements is a special type of grating called an type of grating called an echelle grating. The echelle grating. The second, which follows, is usually a second, which follows, is usually a low-low-dispersion prism, or sometimes a grating. The dispersion prism, or sometimes a grating. The echelle grating, which was first described by echelle grating, which was first described by G. R. Harrison in 1949, provides higher G. R. Harrison in 1949, provides higher dispersion and higher resolution than an dispersion and higher resolution than an echellette of the same sizeechellette of the same size..

Page 107: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Echelle grating: Light is reflected Echelle grating: Light is reflected off the short side of the blazes off the short side of the blazes

(grooves) in the grating.(grooves) in the grating.

Page 108: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

`

Page 109: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

2-D distribution of light and detectionusing an array of transducers (for later)

Page 110: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

MONOCHROMATMONOCHROMATOR OR SLITSSLITS

7C-37C-3

Page 111: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Slits = hole in the wallSlits = hole in the wall Control the entrance of light into and out from the Control the entrance of light into and out from the

monochromator. They control quality!monochromator. They control quality!

EntranceEntrance slits control the slits control the intensityintensity of light entering the of light entering the monochromator and help control the range of monochromator and help control the range of wavelengths of light that strike the gratingwavelengths of light that strike the grating Less important than exit slitsLess important than exit slits

Exit Exit slights help select the slights help select the range of wavelengthsrange of wavelengths that that exit the monochromator and strike the detectorexit the monochromator and strike the detector More important than entrance slitsMore important than entrance slits

Can be:Can be: Fixed (just a slot)Fixed (just a slot) Adjustable in Adjustable in widthwidth (effective bandwidth and intensity) (effective bandwidth and intensity) Adjustable in Adjustable in heightheight (intensity of light) (intensity of light)

Page 112: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Monochromator SlitsMonochromator Slits

Good slitsGood slits Two pieces of metal to give sharp Two pieces of metal to give sharp

edgesedges Parallel to one anotherParallel to one another Spacing can be adjusted in some Spacing can be adjusted in some

modelsmodels Entrance slitEntrance slit

Serves as a radiation sourceServes as a radiation source Focusing on the slit planeFocusing on the slit plane

Page 113: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Effect of Slit Width on Effect of Slit Width on ResolutionResolution

Page 114: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Effect of slit width on Effect of slit width on resolutionresolution

BandwidthBandwidth Defined as a span of Defined as a span of

monochromator monochromator setting setting

needed to move the needed to move the image of the image of the entrance slit across entrance slit across the exit slitthe exit slit

Effective bandwidthEffective bandwidth effeff

½½ of the bandwidth of the bandwidth When two slits are When two slits are

identicalidenticalFIGURE 7-24 Illumination of an exit slit by monochromatic radiation λ at various monochromator settings. Exit and entrance slits are identical.

Page 115: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 116: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Calculating slit widthCalculating slit width Effective bandwidth(Effective bandwidth(effeff) and D) and D-1-1

DD-1-1 = = yy When When y = w = (slit width)y = w = (slit width) DD-1-1 = = eff eff /w/w

ExampleExample Recpiprocal linear dispersion = 1.2nm/mmRecpiprocal linear dispersion = 1.2nm/mm Sodium lines at 589.0 nm and 589.6 nmSodium lines at 589.0 nm and 589.6 nm Required slit width?Required slit width? eff eff = = ½½ (589.6-589.0) = 0.3 nm (589.6-589.0) = 0.3 nm W = 0.3 nm/(1.2 nm/mm) = 0.25 mmW = 0.3 nm/(1.2 nm/mm) = 0.25 mm Practically, narrower than the theoretical Practically, narrower than the theoretical

values is necessary to achieve a desired values is necessary to achieve a desired resolutionresolution

Page 117: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Wider slits =

greater

intensity,

More signal trading off

withPoorer resolution

Narrow

er slits =

lower intensity,

Less signal trading

off with

Better resolutionFIGURE 7-25 FIGURE 7-25 The effect of the slit width on spectra. The entrance slit is The effect of the slit width on spectra. The entrance slit is

illuminated with A" A"illuminated with A" A"and A 3 only. Entrance and exit slits are identical. Plots on the right show changes in and A 3 only. Entrance and exit slits are identical. Plots on the right show changes in

emittedemittedpower as the setting of monochromator is varied.power as the setting of monochromator is varied.

Page 118: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Wider slits = greater intensity,

Poorer resolution

Narrower slits = lower intensity,

Better resolution

Page 119: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Choice of slit widthsChoice of slit widths

Variable slits for Variable slits for effective bandwidtheffective bandwidth

Narrow spectrumNarrow spectrum Minimal slit widthMinimal slit width Bet decrease in the Bet decrease in the

radiant powerradiant power Quantitative Quantitative

analysisanalysis Wider slit width Wider slit width for for ““moremore”” radiant radiant

powerpower

Page 120: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Effect of Effect of bandwidthbandwidth on on spectral detail for spectral detail for

benzene vaporbenzene vapor

Page 121: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

SAMPLE HOLDERS SAMPLE HOLDERS (CELLS)(CELLS)

7D7D

Page 122: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Sample Holders (Cells)Sample Holders (Cells) Must:Must:

contain the sample without chemical interactioncontain the sample without chemical interaction be be more-or-lessmore-or-less transparent to the wavelengths of light in use transparent to the wavelengths of light in use be readily cleaned for reusebe readily cleaned for reuse be designed for the specific instrument of interest….be designed for the specific instrument of interest….

ExamplesExamples quartz is good from about 190-3000 nmquartz is good from about 190-3000 nm glass is a less expensive alternative from about glass is a less expensive alternative from about 300300-900 nm-900 nm NaCl and KBr are good to much higher wavelengths (NaCl and KBr are good to much higher wavelengths (IR rangeIR range))

Cells can be constructed to:Cells can be constructed to: transmit light absorbed at 180 degrees to the incident lighttransmit light absorbed at 180 degrees to the incident light allow emitted light to exit at 90 degrees from the incident lightallow emitted light to exit at 90 degrees from the incident light contain gases (lower concentrations) and have long path contain gases (lower concentrations) and have long path

lengths (1.0 and 10.0 cm cells are most common)lengths (1.0 and 10.0 cm cells are most common)

Page 123: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Sample ContainersSample Containers

The cells or cuvettes that hold the The cells or cuvettes that hold the samples must be made of material that samples must be made of material that is transparent to radiation in the is transparent to radiation in the spectral region of interest. Quartz or spectral region of interest. Quartz or fused silica is required for work in the fused silica is required for work in the ultraviolet region (below 350 nm), both ultraviolet region (below 350 nm), both of these substances are transparent in of these substances are transparent in the visible region. Silicate glasses can the visible region. Silicate glasses can be employed in the region between 350 be employed in the region between 350 and 2000 nm. Plastic containers can be and 2000 nm. Plastic containers can be used in the visible region. Crystalline used in the visible region. Crystalline NaCl is the most common cell windows NaCl is the most common cell windows in the i.r region. in the i.r region.

Page 124: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Absorbance: usually in a matched pair!

Fluorescence, Phosphorescence, Chemiluminescence

Page 125: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Different Shapes and Different Shapes and Sizes of CellsSizes of Cells

Page 126: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

RADIATIONRADIATION TRANSDUCERSTRANSDUCERS

7 E7 E

Page 127: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

RADIATION RADIATION TRANSDUCERSTRANSDUCERS

7 E-17 E-1

Page 128: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Radiation TransducersRadiation TransducersIntroductionIntroduction

The detectors for early The detectors for early

spectroscopic instruments were spectroscopic instruments were

the human eye or a the human eye or a

photographic plate or film. Now photographic plate or film. Now

a days more modern detectors a days more modern detectors

are in use that convert radiant are in use that convert radiant

energy into electrical signal.energy into electrical signal.

Page 129: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

properties of the Ideal properties of the Ideal TransducerTransducer

The ideal transducer would have a high sensitivity, The ideal transducer would have a high sensitivity, a high signal-to-noise ratio, and a constant a high signal-to-noise ratio, and a constant response over a considerable range of response over a considerable range of wavelengths. In addition, it would exhibit a fast wavelengths. In addition, it would exhibit a fast response time and a zero output signal in the response time and a zero output signal in the absence of illumination, Finally, the electrical absence of illumination, Finally, the electrical signal produced by the ideal transducer would be signal produced by the ideal transducer would be directly proportional to the radiant power directly proportional to the radiant power P.P.

Page 130: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Types of Radiation TransducersTypes of Radiation Transducers

As indicated in Figure 7-3b, there arc two generalAs indicated in Figure 7-3b, there arc two general

types of radiation transducers.2o One type responds totypes of radiation transducers.2o One type responds to

photons, the other to heat. All photon transducers (alsophotons, the other to heat. All photon transducers (also

called called photoelectric or quantum detectors) have anphotoelectric or quantum detectors) have an

active surface that absorbs radiation. [n some types, the absorbed active surface that absorbs radiation. [n some types, the absorbed energy causes emission of electrons andenergy causes emission of electrons and

the production of a photocurrent. In others, the radiationthe production of a photocurrent. In others, the radiation

promotes electrons into conduction bands: detectionpromotes electrons into conduction bands: detection

here is based on the resulting enhanced conductivityhere is based on the resulting enhanced conductivity

(photo conduction), Photon transducers are used(photo conduction), Photon transducers are used

largely for measurement of UV, visible, and near infraredlargely for measurement of UV, visible, and near infrared

radiation.radiation.

Page 131: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

the relative spectral response ofthe relative spectral response ofthe various kinds of transducers that are the various kinds of transducers that are

useful for UV,useful for UV,visible, and IR spectroscopy.visible, and IR spectroscopy.

Page 132: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PHOTON PHOTON TRANSDUCERSTRANSDUCERS

7E-27E-2

Page 133: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

photon transducersphoton transducers

Several types of photon transducers are availableSeveral types of photon transducers are available, including , including (I) (I) photovoltaic cells, in which the radiant energy generates a photovoltaic cells, in which the radiant energy generates a

current at the interface of a semiconductor layer and a metal; current at the interface of a semiconductor layer and a metal; (2) (2) phototubes, in which radiation causes emission of phototubes, in which radiation causes emission of

electrons from a photosensitive solid surface;electrons from a photosensitive solid surface; (3) (3) photomultiplier tubes, which contain a photoemissive photomultiplier tubes, which contain a photoemissive

surface as well as several additional surfaces that emit a cascade of electrons surface as well as several additional surfaces that emit a cascade of electrons when struck by electrons from the photosensitive area;when struck by electrons from the photosensitive area;

(4) (4) photoconductivity transducers in which absorption of radiation by a photoconductivity transducers in which absorption of radiation by a semiconductor produces electrons and holes, thus leading to enhanced semiconductor produces electrons and holes, thus leading to enhanced conductivity;conductivity;

(5) (5) silicon photodiodes. in which photons cause the formation silicon photodiodes. in which photons cause the formation ofelectron-hole pairs and a current across a reversebiasedofelectron-hole pairs and a current across a reversebiased

pn junction; and pn junction; and `̀(6) (6) charge-transfer transducers, charge-transfer transducers, in which the charges developed in a in which the charges developed in a

silicon crystal as a result of absorption of photons are collected and measured.silicon crystal as a result of absorption of photons are collected and measured.

Page 134: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

a) Photovolatic cella) Photovolatic cell StructureStructure

metal-semiconductor-metal-semiconductor-metal sandwichesmetal sandwiches

produce voltage when produce voltage when irradiatedirradiated 350-750 nm350-750 nm 550 nm maximum 550 nm maximum

responseresponse 10-100 microA10-100 microA

Barrier-layer cellBarrier-layer cell Low-priceLow-price Amplification difficultyAmplification difficulty Low sensensitivity for Low sensensitivity for

weak radiationweak radiation Fatigue effectFatigue effect

Page 135: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

b) Vacuum Phototubeb) Vacuum Phototube StructureStructure

Wire anode and semi Wire anode and semi cylinder cathode in a cylinder cathode in a vacuum tubevacuum tube

Photosensitive materialPhotosensitive material electrons produced by electrons produced by

irradiation of cathode irradiation of cathode travel to anode. travel to anode.

l response depends on l response depends on cathode material (200-cathode material (200-1000 nm)1000 nm) High sensitivityHigh sensitivity Red responseRed response UV responseUV response Flat responseFlat response FIGURE 7-29 A phototube and op amp

readout. Thephotocurrent induced by the radiation causes a voltagedrop across R, which appears as "0 at the output of thecurrent-to-voltage converter. This voltage may be displayedon a meter or acquired by a data-acquisitionsystem.

Page 136: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

What do we want in a transducer?What do we want in a transducer? High sensitivityHigh sensitivity High S/NHigh S/N Constant response over many Constant response over many s s

(wide range of wavelength)(wide range of wavelength) Fast response timeFast response time S = 0 if no light presentS = 0 if no light present S S P (where P = radiant power) P (where P = radiant power)

Photon transducers: light Photon transducers: light electrical electrical signalsignal

Thermal transducers: response to Thermal transducers: response to heat heat conduction bands (enhance conduction bands (enhance conductivity)conductivity)

Page 137: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

c) Photomultiplier Tube c) Photomultiplier Tube (PMT)(PMT)

Extremely sensitive (use for low light Extremely sensitive (use for low light applications).applications).

Light strikes photocathode (photons strike Light strikes photocathode (photons strike emits electrons); several electrons per photon.emits electrons); several electrons per photon.

Bias voltage applied (several hundred volts) Bias voltage applied (several hundred volts) electrons form current.electrons form current.

Electrons emitted towards a dynode (90 V more Electrons emitted towards a dynode (90 V more positive than photocathode positive than photocathode electrons electrons attracted to it).attracted to it).

Electrons hit dynode Electrons hit dynode each electron causes each electron causes emission of several electrons.emission of several electrons.

These electrons are accelerated towards dynode These electrons are accelerated towards dynode #2 (90 V more positive than dynode # 1) …etc.#2 (90 V more positive than dynode # 1) …etc.

Page 138: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

d) Photomultiplier tubes (found in d) Photomultiplier tubes (found in more advanced, scanning UV-VIS and more advanced, scanning UV-VIS and

spectroscopic instruments)spectroscopic instruments) Also function based on the photoelectric effectAlso function based on the photoelectric effect

Additional signal is gained by multiplying the number of electrons produced by the initial reaction in the detector.Additional signal is gained by multiplying the number of electrons produced by the initial reaction in the detector.

Each electron produces as series of photo-electrons, multiplying its signal. Thus the name PMT!Each electron produces as series of photo-electrons, multiplying its signal. Thus the name PMT!

Very sensitive to incoming light.Very sensitive to incoming light. Most sensitive light detector in the UV-VIS range.Most sensitive light detector in the UV-VIS range. VERY rugged. They last a long time.VERY rugged. They last a long time. Sensitive to excessive stray light (room light + powered PMT = DEAD PMT)Sensitive to excessive stray light (room light + powered PMT = DEAD PMT)

Always used with a scanning or moveable wavelength selector (grating) in a monochromatorAlways used with a scanning or moveable wavelength selector (grating) in a monochromator

Page 139: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE7-31 Photomultiplier tube: (a), photograph of a typical commercial tube; (b), cross:FIGURE7-31 Photomultiplier tube: (a), photograph of a typical commercial tube; (b), cross:sectional view; (c), electrical diagram illustrating dynode polanzatlon and photocurrent measectional view; (c), electrical diagram illustrating dynode polanzatlon and photocurrent mea

surement. Radiation striking the photosensitive cathode (b) gives nse to photoelectrons by thesurement. Radiation striking the photosensitive cathode (b) gives nse to photoelectrons by thehotoelectric effect. Dynode D1 is held at a positive voltage Withrespect to the photocathode.hotoelectric effect. Dynode D1 is held at a positive voltage Withrespect to the photocathode.

~Iectrons emitted by the cathode are attracted to the first dynode and accelerated In the fteld.~Iectrons emitted by the cathode are attracted to the first dynode and accelerated In the fteld.Each electron striking dynode D1 thus gives rise to two to four secondary electrons. TheseEach electron striking dynode D1 thus gives rise to two to four secondary electrons. These

are attracted to dynode D2, which is again positive with respect to dynode D1. The resultingare attracted to dynode D2, which is again positive with respect to dynode D1. The resultingamplification at the anode can be 106 or greater. The exact amplification factor depends onamplification at the anode can be 106 or greater. The exact amplification factor depends on

the number of dynodes and the voltage difference between each. ThiSautomatic Internalthe number of dynodes and the voltage difference between each. ThiSautomatic Internalamplification is one of the major advantages of photomultiplier tubes. With modern Instrumentation,amplification is one of the major advantages of photomultiplier tubes. With modern Instrumentation,

the arrival of individual photocurrent pulses can be detected and counted Insteadthe arrival of individual photocurrent pulses can be detected and counted Insteadof being measured as an average current. This technique, called photon counting, ISof being measured as an average current. This technique, called photon counting, IS

advantageous at very low light levels.advantageous at very low light levels.

Page 140: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 141: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.

8–19 dynodes (9-10 is 8–19 dynodes (9-10 is most common).most common).

Gain (m) is # eGain (m) is # e-- emitted emitted per incident eper incident e-- ( () to the ) to the power of the # of power of the # of dynodes (k).dynodes (k).

m = m = kk

e.g. 5 ee.g. 5 e-- emitted / incident e emitted / incident e--

10 dynodes.10 dynodes.

m = m = kk = 5 = 51010 1 x 10 1 x 1077

Typical Gain = 10Typical Gain = 1044 - 10 - 1077

Page 142: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 143: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

e) Silicon Diodese) Silicon Diodes

Constructed of charge depleted and charge rich Constructed of charge depleted and charge rich regions of silicon (silicon doped with other ions)regions of silicon (silicon doped with other ions)

Light striking the detector causes charge to be Light striking the detector causes charge to be created between the p and n regions.created between the p and n regions.

The charge collected is then measured as current The charge collected is then measured as current and the array is ‘reset’ for the next collectionand the array is ‘reset’ for the next collection

Used most frequently these days in instruments Used most frequently these days in instruments where the grating is fixed in one position and light where the grating is fixed in one position and light strikes an array of silicon diodes (aka the diode strikes an array of silicon diodes (aka the diode arrayarray Can have thousands of diodes on an arrayCan have thousands of diodes on an array Each diode collects light from a specific wavelength Each diode collects light from a specific wavelength

rangerange The resolution is generally poorer than with a PMTThe resolution is generally poorer than with a PMT However, you can scan literally thousands of times a However, you can scan literally thousands of times a

minute since there are NO moving parts! Then, the minute since there are NO moving parts! Then, the scans are averaged (ensemble or boxcar averaging) to scans are averaged (ensemble or boxcar averaging) to give a resulting spectra.give a resulting spectra.

Page 144: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PhotodiodesPhotodiodes

Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.

Page 145: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

the relative spectral response ofthe relative spectral response ofthe various kinds of transducers that are the various kinds of transducers that are

useful for UV,useful for UV,visible, and IR spectroscopy.visible, and IR spectroscopy.

Page 146: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Skoog et al. 2007

Page 147: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Forward biasing Reverse biasing

High resistant

e-

Page 148: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

MULTICHANNEL MULTICHANNEL PHOTON PHOTON TRANSDUCERSTRANSDUCERS

7E-37E-3

Page 149: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Multichannel photon Multichannel photon transducerstransducers

The first multichannel detector used in spectroscopy was a The first multichannel detector used in spectroscopy was a photographic plate or a film strip that was placed along the photographic plate or a film strip that was placed along the length of the focal plane of a spectrometer so that all the length of the focal plane of a spectrometer so that all the lines in a spectrum could be recorded simultaneously. lines in a spectrum could be recorded simultaneously. Photographic detection is relatively sensitive, with some Photographic detection is relatively sensitive, with some emulsions that respond to as few as 10 to 100 photons. The emulsions that respond to as few as 10 to 100 photons. The primary limitation of this type of detector, however, is the primary limitation of this type of detector, however, is the time required to develop the image of the spectrum and time required to develop the image of the spectrum and convert the blackening of the emulsion to radiant convert the blackening of the emulsion to radiant intensities. Modern multichannel transducers 24 consist of intensities. Modern multichannel transducers 24 consist of an array of an array of small photosensitive elements arranged either small photosensitive elements arranged either linearly or in a two-dimensional pattern on a single linearly or in a two-dimensional pattern on a single semiconductor chip.semiconductor chip.

Page 150: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Multichannel Photon TransducersMultichannel Photon Transducers

Photographic plate or a film stripPhotographic plate or a film stripPlace along the focal plane of a spectrometerPlace along the focal plane of a spectrometer

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Photodiode ArraysPhotodiode Arrays

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Photodiode ArraysPhotodiode Arrays

In a PDA, the individual photosensitive In a PDA, the individual photosensitive elements are small silicon photodiodes, elements are small silicon photodiodes, each of which consists of a reverse-biased each of which consists of a reverse-biased pn junctionpn junction

Page 153: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Photodiode TransducerPhotodiode Transducer A silicon photodiode transducer A silicon photodiode transducer

consists of a consists of a Reversed Biased Reversed Biased pnpn junctionjunction formed on a silicon chip formed on a silicon chip

A photon promotes an electron from A photon promotes an electron from the valence bthe valence boond (filled orbitals) to the nd (filled orbitals) to the conduction bconduction boond (unfilled orbitals) nd (unfilled orbitals) creating an electron(-) - hole(+) paircreating an electron(-) - hole(+) pair

The concentration of these electron-The concentration of these electron-hole pairs is dependent on the amount hole pairs is dependent on the amount of light striking the semiconductorof light striking the semiconductor

Page 154: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Photodiode ArrayPhotodiode Array

Semiconductors (Silicon and Semiconductors (Silicon and Germanium)Germanium) Group IV elementsGroup IV elements Formation of holes (via thermal Formation of holes (via thermal

agitation/excitation)agitation/excitation) DopingDoping n-type:n-type: Si (or Ge) doped with group V Si (or Ge) doped with group V

element (As, Sb) to add electrons.element (As, Sb) to add electrons.As:As: [Ar]4S[Ar]4S223d3d10104p4p33

p-type:p-type: Doped with group III element (In, Doped with group III element (In, Ga) to added holesGa) to added holesIn: In: [Kr]5S[Kr]5S224d4d10105p5p11

Skoog et al, p43

Page 155: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIGURE 7-33 FIGURE 7-33 A reverse-biased linear diode-arrayA reverse-biased linear diode-arraydetector: (a)cross section and (b)top view.detector: (a)cross section and (b)top view.

Page 156: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Photodiode Arrays

Page 157: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 158: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 159: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Charge-Transfer Charge-Transfer DeviceDevice

Page 160: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Charge-Transfer Device Charge-Transfer Device (CTD)(CTD)

Important for multichannel detection Important for multichannel detection (i.e., spatial resolution); 2-dimensional (i.e., spatial resolution); 2-dimensional arrays.arrays.

Sensitivity approaches PMT.Sensitivity approaches PMT. An entire spectrum can be recorded as An entire spectrum can be recorded as

a “snapshot” without scanning.a “snapshot” without scanning. Integrate signal as photon strikes Integrate signal as photon strikes

element. element. Each pixel: two conductive electrodes Each pixel: two conductive electrodes

over an insulating material (e.g., SiOover an insulating material (e.g., SiO22).). Insulator separates electrodes from n-Insulator separates electrodes from n-

doped silicon. doped silicon.

Page 161: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Semiconductor capacitor: stores Semiconductor capacitor: stores charges that are formed when charges that are formed when photons strike the doped silicon. photons strike the doped silicon.

101055 –10 –1066 charges/pixel can be stored charges/pixel can be stored (gain approaches gain of PMT).(gain approaches gain of PMT).

How is amount of charge measured?How is amount of charge measured? Charge-injection device (CID): Charge-injection device (CID):

voltage change that occurs from voltage change that occurs from charge moving between electrodes.charge moving between electrodes.

Charge-coupled device (CCD): Charge-coupled device (CCD): charge is moved to amplifier.charge is moved to amplifier.

Page 162: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PHOTO CONDUCTIVITY PHOTO CONDUCTIVITY TRANSDUCERSTRANSDUCERS

7E-47E-4

Page 163: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Photo conductivity Photo conductivity TransducersTransducers

The most sensitive transducers for The most sensitive transducers for monitoring radiation 10 the near-infrared monitoring radiation 10 the near-infrared region (0.75 to 3 /µm) are semiconductors region (0.75 to 3 /µm) are semiconductors whose resistances decrease when theywhose resistances decrease when they

absorb radiation within this range.absorb radiation within this range.

Page 164: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

THERMAL THERMAL TRANSDUCERSTRANSDUCERS

7E-57E-5

Page 165: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Thermal TransducersThermal Transducers

Thermal Transducers are used in Thermal Transducers are used in

infrared spectroscopy. infrared spectroscopy.

Phototransducers are not Phototransducers are not

applicable in infrared because applicable in infrared because

photons in this region lack the photons in this region lack the

energy to cause photoemission of energy to cause photoemission of

electrons. Thermal transducers are electrons. Thermal transducers are

– Thermocouples, Bolometer – Thermocouples, Bolometer

(thermistor(thermistor).).

Page 166: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

ThermocouplesThermocouples

In its simplest form, a thermocouple consists of a pairIn its simplest form, a thermocouple consists of a pair

of junctions formed when two pieces of a metal such asof junctions formed when two pieces of a metal such as

copper are fused to each end of a dissimilar metal suchcopper are fused to each end of a dissimilar metal such

as constantan as shown in Figure 3-13. A voltage developsas constantan as shown in Figure 3-13. A voltage develops

between the two junctions that varies with thebetween the two junctions that varies with the

difference in their temperatures. difference in their temperatures. A well- A well-designed thermocouple transducer is capabledesigned thermocouple transducer is capable

of responding to temperature differences ofof responding to temperature differences of

10-6 K. This difference corresponds to a potential difference10-6 K. This difference corresponds to a potential difference

of about 6 to 8 of about 6 to 8 µV/µW.µV/µW.

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ThermocouplesThermocouples

Page 168: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

bolometerbolometer

A bolometer is a type of resistance A bolometer is a type of resistance thermometer constructed of strips of thermometer constructed of strips of metals, such as platinum or nickel, or of a metals, such as platinum or nickel, or of a semiconductor. Semiconductor bolometers semiconductor. Semiconductor bolometers are often called are often called thermistors .thermistors .

Page 169: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Pyroelectric transducersPyroelectric transducers

Pyroelectric transducers are constructed from Pyroelectric transducers are constructed from single crystalline wafers of single crystalline wafers of pyroelectric pyroelectric materials, which are materials, which are insulators (dielectric insulators (dielectric materials) with very special thermal and materials) with very special thermal and electrical properties. Triglycine sulfate electrical properties. Triglycine sulfate (NH(NH22CHCH22COOH)COOH)33· H· H22SOSO44 (usually deuterated or(usually deuterated or

with a fraction of the glycines replaced with with a fraction of the glycines replaced with alanine), is the most alanine), is the most important pyroelectric important pyroelectric materialmaterial used in the construction of infrared used in the construction of infrared transducers.transducers.

Page 170: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

SIGNAL SIGNAL PROCESSORSPROCESSORS

AND AND READOUTSREADOUTS

7F7F

Page 171: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Signal Processors and Signal Processors and ReadoutsReadouts

The signal processor is ordinarily an The signal processor is ordinarily an electronic device that amplifies the electronic device that amplifies the electrical signal from the transducer. In electrical signal from the transducer. In addition, it may alter the signal from dc addition, it may alter the signal from dc to ac (or the reverse), change the phase to ac (or the reverse), change the phase of the signal, and filter it to remove of the signal, and filter it to remove unwanted components. Furthermore, the unwanted components. Furthermore, the signal processor may be called upon to signal processor may be called upon to perform such mathematical operations perform such mathematical operations on the signal as differentiation, on the signal as differentiation, integration, or conversion to a integration, or conversion to a logarithm.logarithm.

Page 172: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PHOTON COUNTINGPHOTON COUNTING7F-17F-1

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Photon countingPhoton counting

The output from a photomultiplier tube consists ofThe output from a photomultiplier tube consists of

a pulse of electrons for each photon that reaches the a pulse of electrons for each photon that reaches the detectordetector

surface. This analog signal is often filtered to removesurface. This analog signal is often filtered to remove

undesirable fluctuations due to the random appearanceundesirable fluctuations due to the random appearance

of photons at the photocathode and measuredof photons at the photocathode and measured as a de as a de voltage or eurrent.voltage or eurrent.

Page 174: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

FIBER OPTICSFIBER OPTICS

7G7G

Page 175: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Fiber opticsFiber optics

In the late 1960, analytical instruments began to In the late 1960, analytical instruments began to appear on the market that contained fiber optics appear on the market that contained fiber optics for transmiting radiation and images from one for transmiting radiation and images from one component of the instrument to another. Fiber component of the instrument to another. Fiber optics have added a new dimension of utility to optics have added a new dimension of utility to optical instrument designs."optical instrument designs."

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Optical FibersOptical Fibers

Used to transmit light waves over non-Used to transmit light waves over non-linear paths.linear paths.

Often used in remote sensing, solution Often used in remote sensing, solution sampling (dipping probes) and field sampling (dipping probes) and field instrumentsinstruments

Based on the fact that light inside a fiber Based on the fact that light inside a fiber can be continuously (totally internally can be continuously (totally internally reflected) if the angle it strikes the fiber reflected) if the angle it strikes the fiber surface at is correct (determines radius of surface at is correct (determines radius of bends, etc.).bends, etc.).

Used in construction of optodes (optical Used in construction of optodes (optical fiber based chemical sensor)fiber based chemical sensor)

Page 177: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PROPERTIES PROPERTIES OFOF OPTICAL OPTICAL

FIBERSFIBERS

7G-17G-1

Page 178: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Properties of Optical Properties of Optical FibersFibers

Optical fibers are fine strands of glass or plastic thatOptical fibers are fine strands of glass or plastic that

transmit radiation for distances of several hundred feettransmit radiation for distances of several hundred feet

or more. The diameter of optical fibers ranges fromor more. The diameter of optical fibers ranges from

0.05 pm to as large as 0.6 cm. Where images are to be0.05 pm to as large as 0.6 cm. Where images are to be

transmitted, bundles of fibers, fused at the ends, aretransmitted, bundles of fibers, fused at the ends, are

used. A major application of these fiber bundles hasused. A major application of these fiber bundles has

been in medical diagnoses, where their flexibility permitsbeen in medical diagnoses, where their flexibility permits

transmission of images of organs through tortuoustransmission of images of organs through tortuous

pathways to the physician. Fiber optics are usedpathways to the physician. Fiber optics are used

not only for observation but also for illumination ofnot only for observation but also for illumination of

objects. In such applications, the ability to illuminateobjects. In such applications, the ability to illuminate

without heating is often very important.without heating is often very important.

Page 179: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Optical FiberOptical Fiber

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FIBER-OPTIC SENSORSFIBER-OPTIC SENSORS

7G.27G.2

Page 181: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Fiber-optic sensorsFiber-optic sensors

Fiber-optic sensors, which are sometimes called Fiber-optic sensors, which are sometimes called optrodes, optrodes, consist of a reagent phase immobilized consist of a reagent phase immobilized on the end of a fiber optic. Interaction of the on the end of a fiber optic. Interaction of the analyte with the reagent creates a change in analyte with the reagent creates a change in absorbance, reflectance, fluorescence, or absorbance, reflectance, fluorescence, or luminescence, which is then transmitted to a luminescence, which is then transmitted to a detector via the optical fiber. Fiber optic Sensors detector via the optical fiber. Fiber optic Sensors are generally simple, inexpensive devices that are generally simple, inexpensive devices that are easily miniaturized.are easily miniaturized.

Page 182: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

TYPES OF TYPES OF OPTICAL OPTICAL INSTRUMENTINSTRUMENT

7H7H

Page 183: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Types of Optical Types of Optical InstrumentsInstruments

SpectroscopeSpectroscope Optical instrument used for visual identification of Optical instrument used for visual identification of

atomic emission linesatomic emission lines ColorimeterColorimeter

Human eye acts as detector for absorption Human eye acts as detector for absorption measurementsmeasurements

PhotometerPhotometer Contains a filter, no scanning functionContains a filter, no scanning function

FluorometerFluorometer A photometer for fluorescence measurementA photometer for fluorescence measurement

SpectrographSpectrograph Record simultaneously the entire spectrum of a Record simultaneously the entire spectrum of a

dispersed radiation using plate or filmdispersed radiation using plate or film SpectrometerSpectrometer

Provides information about the intensity of Provides information about the intensity of radiaition as a function of wavelength or frequencyradiaition as a function of wavelength or frequency

More……………(confusing……)More……………(confusing……)

Page 184: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

types of Optical instrumenttypes of Optical instrument

Spectroscope:Spectroscope:an optical instrument used for thean optical instrument used for the

visual identification of atomic emission lines.visual identification of atomic emission lines. We use the We use the term term colorimeter: colorimeter: to designate an instrument to designate an instrument for absorption for absorption measurements in which the human eye serves as the detector measurements in which the human eye serves as the detector using one or more color-comparison standards. using one or more color-comparison standards. spectro graph: spectro graph: is similar in construction to the two is similar in construction to the two monochromators shown monochromators shown in Figure 7-18 except that the sht arrangement is replaced with in Figure 7-18 except that the sht arrangement is replaced with a large aperture that holds a detector or transducer that is a large aperture that holds a detector or transducer that is continuously exposed tn the entire spectrum of dispersed continuously exposed tn the entire spectrum of dispersed radiation. radiation. spectrometerspectrometer :is an instrument that provides :is an instrument that provides informationinformation

about the intensity of radiation as a function of wavelength or about the intensity of radiation as a function of wavelength or frequency.frequency.

Page 185: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

PRINCIPLES OF PRINCIPLES OF FOURIERFOURIERTRANSFORM OPTICALTRANSFORM OPTICALMEASUREMENTSMEASUREMENTS

7I7I

Page 186: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Fourier Transform (FT)Fourier Transform (FT) The instruments we have been talking about work over the The instruments we have been talking about work over the

frequency domain (we are measuring signal vs. frequency or frequency domain (we are measuring signal vs. frequency or wavelength)wavelength)

Fourier transform techniques measure signal vs. time and Fourier transform techniques measure signal vs. time and then convert time to wavelength or frequencythen convert time to wavelength or frequency

FT techniques have much greater resolving power than FT techniques have much greater resolving power than frequency domain techniquesfrequency domain techniques Fewer mechanical partsFewer mechanical parts No “monochromator”No “monochromator” Mathematical deconvolution of the spectrumMathematical deconvolution of the spectrum

FT techniques have higher light throughput because there FT techniques have higher light throughput because there are fewer optical components.are fewer optical components.

Widely used in IR and NMRWidely used in IR and NMR Originally developed to separate out weak IR signals from Originally developed to separate out weak IR signals from

astronomical objects.astronomical objects. An interferometer splits the light beam into two beams and An interferometer splits the light beam into two beams and

then measures the intensity of recombined beamsthen measures the intensity of recombined beams The frequency of these beams is related to the frequency The frequency of these beams is related to the frequency

of the light that caused them…. of the light that caused them….

Page 187: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

HistoryHistory In 1950s, astronomyIn 1950s, astronomy

Separate weak signals from noiseSeparate weak signals from noise Late 1960s, FT-NIR & FT-IRLate 1960s, FT-NIR & FT-IR

Fourier transformFourier transform

Page 188: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Resolution of FT Resolution of FT spectrometerspectrometer

Two closely spaced lines only separated Two closely spaced lines only separated if one complete "beat" is recorded. if one complete "beat" is recorded.

As lines get closer together, As lines get closer together, must must increase.increase.

Page 189: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

INHERENT ADVANTAGES OF INHERENT ADVANTAGES OF FOURIERFOURIER

TRANSFORM TRANSFORM SPECTROMETRYSPECTROMETRY

7I-17I-1

Page 190: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Advantages of FTAdvantages of FT Throughput / Jaquinot advantageThroughput / Jaquinot advantage

Few optics and slitsFew optics and slits Less dispersion, high intensityLess dispersion, high intensity Usually to improve resolution decrease slit width Usually to improve resolution decrease slit width but less light makes spectrum "noisier" (S/N)but less light makes spectrum "noisier" (S/N)

High ResolutionHigh Resolution = 6 ppm= 6 ppm

Short time scaleShort time scale Simultaneously measure all spectrum at once Simultaneously measure all spectrum at once

saves timesaves time frequency scanning vs. time domain scanningfrequency scanning vs. time domain scanning Fellgett or multiplex advantageFellgett or multiplex advantage

Page 191: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

TIME -DOMAIN TIME -DOMAIN SPECTROSCOPYSPECTROSCOPY

7I-27I-2

Page 192: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

time -domain time -domain spectroscopyspectroscopy

Conventional spectroscopy can be termed Conventional spectroscopy can be termed frequency domain frequency domain spectroscopy in that spectroscopy in that radiant power data are recorded as a radiant power data are recorded as a function of frequency or the inversely function of frequency or the inversely related wavelength. In contrast, related wavelength. In contrast, time-time-domain spectroscopy, domain spectroscopy, which can be which can be achieved by the Fourier transform, is achieved by the Fourier transform, is concerned with changes in radiant power concerned with changes in radiant power with with time.time.

Page 193: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Time domain Time domain spectroscopyspectroscopy

Unfortunately, no detector can Unfortunately, no detector can respond on 10respond on 10-14-14 s time scale s time scale

Use Michelson interferometerUse Michelson interferometer to to measure signal proportional to measure signal proportional to time varying signaltime varying signal

Page 194: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Freq-domain / time-domainFreq-domain / time-domain

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ACQUIRING TIME-DOMAIN ACQUIRING TIME-DOMAIN

SPECTRA WITH A SPECTRA WITH A

MICHELSONMICHELSON INTERFEROMETERINTERFEROMETER

7I-37I-3

Page 196: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

modulationmodulation

Velocity of moving Velocity of moving mirror(MM)mirror(MM)

Time to move Time to move /2 /2 cmcm

Bolometer, Bolometer, pyroelectric, pyroelectric, photoconducting photoconducting IR detectors can IR detectors can "see"see““ changes on changes on 1010-4-4 s time scale! s time scale!

Page 197: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

This time domain spectrum is made of different wavelengths of light arriving at the detector at different times.

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Michelson interferometerMichelson interferometer

Page 199: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Analysis of Analysis of interferograminterferogram

Computer needed to Computer needed to turn complex turn complex interferogram into interferogram into spectrumspectrum

Figure 7-43Figure 7-43 (b) resolved lines(b) resolved lines (c) unresolved lines(c) unresolved lines

FTFT Time -> Frequency Time -> Frequency

inverse FTinverse FT Frequency -> TimeFrequency -> Time

Page 200: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 201: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Fourier Transformation of Fourier Transformation of InterferogramsInterferograms

Page 202: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

InterferogramInterferogram

retardation retardation Difference in pathlengthDifference in pathlength

interferograminterferogram Plot signal vs. Plot signal vs. cosine wave with frequency cosine wave with frequency

proportional to light frequency but proportional to light frequency but signal varies at much lower signal varies at much lower frequencyfrequency

One full cycle when mirror moves One full cycle when mirror moves distance distance /2 (round-trip = /2 (round-trip = ))

Page 203: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 204: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

resolutionresolution

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resolutionresolution

The resolution of a Fourier transform The resolution of a Fourier transform spectrometer can be described in terms of spectrometer can be described in terms of the difference in wavenumber between the difference in wavenumber between two lines that can be just separated by the two lines that can be just separated by the instrument. That is,instrument. That is,

Page 206: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.
Page 207: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

Semiconductor Diodes Semiconductor Diodes Diode: is a nonlinear device that has Diode: is a nonlinear device that has

greater conductance in one direction greater conductance in one direction than in anotherthan in another

Adjacent n-type and p-type regionsAdjacent n-type and p-type regions pn junction: the interface between pn junction: the interface between

the two regions the two regions

Page 208: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

This process continues for 9 dynodesThis process continues for 9 dynodes Result: for each photon that strikes Result: for each photon that strikes

photocathode photocathode ~10 ~1066 –10 –1077 electrons electrons collected at anode.collected at anode.

Is there a drawback? Sensitivity Is there a drawback? Sensitivity usually limited by dark current.usually limited by dark current.

Dark current = current generated by Dark current = current generated by thermal emission of electrons in the thermal emission of electrons in the absence of light.absence of light.

Thermal emission Thermal emission reduce by cooling. reduce by cooling. Under optimal conditions, PMTs can Under optimal conditions, PMTs can

detect single photons.detect single photons. Only used for low-light applications; it Only used for low-light applications; it

is possible to fry the photocathode.is possible to fry the photocathode.

Page 209: Chapter 7 Components of Optical Instruments. Instruments for the ultraViolet (UV),ViSible, and infrared (IR) regions have enough features in common.

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