Tint plates

Quarter wave plate, full wave sensitive tint plate and quartz wedge

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 (λ = 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.