Sensitive tint plate

The first order red plate is also known as the sensitive tint plate. Its main use is in determining the sign of the birefringence of objects of small retardation, e, which have a grey color in crossed polars. With the first order plate in place the net retardation is (575 + e) nm, which is second order blue, or (575 - e) nm, which is first order yellow. The eye is very sensitive to color changes in this range, so a very small value of e has a visible effect. 

Figures 3.15 and 3.16 show spherulites in crossed polars and with the sensitive tint plate. In crossed polars, the spherulites are bright except at the four perpendicular radial directions where the crystals are in the extinction position. High density polyethylene (see Fig. 3.15) produces spherulites that are unusually perfect, and also have circumferential dark bands. The bands are regions of apparent isotropy where the optic axis is perpendicular to the specimen plane. With the first order red plate placed so that its slow direction is at -l45° (top right to bottom left of the image), the...

The tint plate produces a retardation of about 570 millimicrons, so that the yellow components of white light vanish. The resulting colour of violet-red (or sensitive violef) is changed when an extra retardation is introduced by the strained glass. The eye is very... 

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