17. Source of radiation
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Requirements of an ideal source
It should be stable and should not allow fluctuations.
It should emit light of continuous spectrum of high and uniform intensity
over the entire wavelength region in which it’s used.
It should provide incident light of sufficient intensity for the transmitted
energy to be detected at the end of optic path.
It should not show fatigue on continued use.
1. Tungsten Halogen Lamp:
Its construction is similar to a house hold lamp.
The bulb contains a filament of Tungsten fixed in evacuated condition and
then filled with inert gas.
The filament can be heated up to 3000 k, beyond this Tungsten starts
sublimating.
It is used when polychromatic light is required.
18. DEMERIT:
It emits the major portion of its radiant energy in near IR region of
the spectrum.
2. Hydrogen Discharge Lamp:
In Hydrogen discharge lamp pair of electrodes is enclosed in a
glass tube (provided with silica or quartz window for UV radiation
to pass trough) filled with hydrogen gas.
When current is passed trough these electrodes maintained at high
voltage, discharge of electrons occurs which excites hydrogen
molecules which in turn cause emission of UV radiations in near
UV region.
They are stable and widely used.
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19. 3. Xenon Discharge Lamp:
It possesses two tungsten electrodes separated by some distance.
These are enclosed in a glass tube (with quartz or fused silica) and
xenon gas is filled under pressure.
An intense arc is formed between electrodes by applying high
voltage. This is a good source of continuous plus additional
intense radiation. Its intensity is higher than the hydrogen
discharge lamp.
DEMERIT:
The lamp since operates at high voltage becomes very hot during
operation and hence needs thermal insulation.
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