From left to right: Quarter
wave plate; full wave sensitive tint plate; quartz wedge
Full wave sensitive tint
plate (also known as red tint plate)
A sensitive tint plate can
be used to introduce colour contrast in polarised light
images, and consists of a slice of birefringent material,
usually gypsum, mica or quartz.
The slice is cut or cleaved
parallel to the optic axis of the crystal, to such a
thickness that the O-rays and E-rays for green light (l
= 540 nm) are out of phase by
exactly one wavelength. The analyser therefore extinguishes
green light, but permits other wavelengths to pass through
to some extent.
When using white light this
causes the field of view to appear red (white light minus
green light). Isotropic, non-birefringent materials
also appear red.
The sensitive tint plate
increases the observed birefringence by one wavelength. The
path difference between the O-rays and E-rays emerging from
an anisotropic crystal adds or subtracts from this
single wavelength path difference. Individual grains appear
to exhibit differences in colour, depending on their
composition and orientation.
Quarter wave plate
A quarter wave plate is
made form a flake of mica that is cleaved to such a
thickness that the O-rays and E-rays emerge a quarter of a
wavelength out of phase. This corresponds to a pale grey
interference colour.
This plate is especially
useful for examining specimens showing bright interference
colours, because they are moved only a short distance along
the scale. The plate can be used to enhance the contrast
between regions of the specimen.
Quartz wedge
The quartz wedge is cut so
that it varies in thickness from about 0.01 mm to about
0.08 mm and covers several orders of retardation colours. As
the wedge is inserted into the slot in the microscope it
produces progressively higher retardations, and the position
at which complete extinction occurs is noted.
Michel Levy produced a
colour chart which plots the thickness of an isotropic
specimen, its birefringence and its retardation in
nanometres. Once two of these variables is known, the third
can be easily determined.