If our sample is wedged shaped, as shown above, instead of flat, the thickness of the sample and the corresponding retardation will vary along the length of the wedge.

Examination of the wedge under crossed polars, gives an image as shown below, and reveals:

1. dark areas where retardation is a whole number of wavelengths.
2. light areas where the two rays are out of phase,
3. brightest illumination where the retardation of the two rays is such that they are exactly ½, 1½, 2½ wavelengths and are out of phase.

The percentage of light transmitted through the upper polarizer is a function of the wavelength of the incident light and retardation.

If a mineral is placed at 45° to the vibration directions of the polarizers the mineral yields its brightest illumination and percent transmission (T).