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When
any form of polarized light is incident on a birefringent waveplate, it is
resolved into two linearly polarized components which vibrate
perpendicular to each other and travel with different velocities in the
crystal. The beams emerge with a phase delay. This phase delay or
retardation is dependent on birefringence of the crystal. Birefringence at
a specified wavelength is the difference in refractive indices of the
extraordinary and ordinary resolved components. Because of dispersion of
birefringence i.e. it's dependence on wavelength, there is a variation of
retardation with wavelength. The shorter the wavelength, the more is the variation
of retardation with wavelength. A zero order double plate retarder is
made of two plates of single crystal magnesium fluoride with fast axes at
90 degrees to each other. Thickness difference of the two gives the
desired zero order retardation at a desired wavelength. Zero
order quarter or half waveplates for a specified wavelength from 150 to
6000 nm are available in standard clear aperture diameters of 12 mm and 22
mm in air-spaced version only. These are
supplied uncoated. Other clear apertures including eighth wave retarders
are available on special order.
Quarter
and half wave retarders made for wavelengths below 220 nm are made on a
best effort basis. At present, we cannot make a direct measurement of
retardation at these wavelengths. For these retarders, retardation is
measured at 220 to 300 nm and the results are extrapolated.
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