2012 Spectrometry 5 Optical

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BASIC COMPONENT

OF SPECTROMETER(OPTICAL INSTRUMENTS)


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Basic element of single beam spectrometer


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Source of light The commonly used source of visible

light is tungsten filament lamp.Wavelength range : 325/350 nm 3um

For UV region, a low pressure hidrogen or

deuterium discharge tube is generally

used. Ranging from 185

375 nm. UVsources must have a quartz windows,

glass is not transparent to UV radiation.

IR radiation is essentially heat, so hot

wires, light bulbs or glowing ceramics can

be used as IR sources. Range 2 15 um.


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Monochromator Lenses or mirror focus radiation Slits restrict unwanted radiation Dispersing medium to separate

the wavelength polychromatic

radiation. Two types : prisms &diffraction grating.


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MirrorsWhen light reflects off of a rear surface mirror, the light

first passes through the glass substrate, resulting in

reflection losses, secondary reflections, and a

change in apparent distance. First surface mirrors

avoid this by aluminizing the front, and coating it with

a thin protective SiO coating to prevent oxidation and

scratching.


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Concave MirrorsConcave mirrors are

often used to focus light

in place of a lens. Just as

with a lens, a concave

mirror has a principal

focus, f, through which all

rays parallel to the opticalaxis pass through. The

focal length of a spherical

concave mirror is one

half the radius of thespherical surface.


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PrismsPrisms use glass with a high index of refraction

(different from air) to exploit the variation ofrefraction with wavelength. Blue light refracts

more than red, providing a spectrum that can

be isolated using a narrow slit.


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Diffraction GratingsMost monochromators use

gratings to disperse lightinto the spectrum. This

consist of a large number of

parallel lines (grooves) ruled

on a highly polished surfacesuch as aluminium.

The wavelength of reflected

light varies with angle, as

defined by the grating

equation, where m is the

order of the spectrum (an

integer).


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Optical filters

Various types of filter may be used to isolatecertain wavelength :

1. narrow-bandpass

2. sharp-cut

Made from glass & contain dyes that

absorbs unwanted radiation


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DETECTORSensitivity to the band of interest is a primary consideration

when choosing a detector. You can control the peak

responsivity and bandwidth through the use of filters, butyou must have adequate signal to start with. Filters cansuppress out of band light but cannot boost signal.

Another consideration is blindness to out of band radiation. Ifyou are measuring solar ultraviolet in the presence ofmassive amounts of visible and infrared light, for example,you would select a detector that is insensitive to the longwavelength light that you intend to filter out.

Lastly, linearity, stability and durability are considerations.

Some detector types must be cooled or modulated toremain stable. High voltages are required for other types. Inaddition, some can be burned out by excessive light, orhave their windows permanently ruined by a fingerprint.


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Silicon photodiodePlanar diffusion type silicon photodiodes are perhaps the most

versatile and reliable sensors available. The P-layer material

at the light sensitive surface and the N material at thesubstrate form a P-N junction which operates as aphotoelectric converter, generating a current that isproportional to the incident light. Silicon cells operate linearlyover a ten decade dynamic range, and remain true to their

original calibration longer than any other type of sensor.Silicon photodiodes are best used in the short-circuit mode,

with zero input impedance into an op-amp. The sensitivity ofa lightsensitive circuit is limited by dark current, shot noise,and Johnson (thermal) noise. The practical limit of sensitivityoccurs for an irradiance that produces a photocurrent equalto the dark current (Noise Equivalent Power, NEP = 1).


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Solar-Blind Vacuum PhotodiodesThe phototube is a light sensor that is based on the

photoemissive effect. The phototube is a bipolar tube

which consists of a photoemissive cathode surface thatemits electrons in proportion to incident light, and ananode which collects the emitted electrons. The anodemust be biased at a high voltage (50 to 90 V) in order toattract electrons to jump through the vacuum of the tube.

Some phototubes use a forward bias of less than 15volts, however.

The cathode material determines the spectral sensitivity ofthe tube. Solar-blind vacuum photodiodes use Cs-Tecathodes to provide sensitivity only to ultraviolet light,providing as much as a million to one long wavelengthrejection. A UV glass window is required for sensitivity inthe UV down to 185 nm, with fused silica windowsoffering transmission down to 160 nm.


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Multi-Junction ThermopilesThe thermopile is a heat sensitive device that measures

radiated heat. The sensor is usually sealed in a vacuum to

prevent heat transfer except by radiation. A thermopileconsists of a number of thermocouple junctions in serieswhich convert energy into a voltage. Thermopiles areconvenient sensor for measuring the infrared, because theyoffer adequate sensitivity and a flat spectral response in asmall package. More sophisticated bolometers and

pyroelectric detectors need to be chopped and are generallyused only in calibration labs.

Thermopiles suffer from temperature drift, since the referenceportion of the detector is constantly absorbing heat. Thebest method of operating a thermal detector is by choppingincident radiation, so that drift is zeroed out by themodulated reading.

The quartz window in most thermopiles is adequate fortransmitting from 200 to 4200 nm, but for long wavelengthsensitivity out to 40 microns, Potassium Bromide windowsare used.


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2012 Spectrometry 5 Optical