Polarized Light Microscope, PLM
A polarized light microscope visualizes birefringent materials by generating contrast through the differential interaction of plane polarized light with the material and an upstream polarizing filter (Analyzer).
Generic PLM microscope. A: Polarizer; B: Analyzer; C: Compensator
In a polarized light microscope light is polarized by the substage Polarizing filter (usually a Polaroid filter, "Pol"). With no sample in place, the polarized light impinges on the upper polarizing filter (Analyzer), which is set at 90° to the Pol filter. Interaction of the plane polarized light and the Analyzer decreases light amplitude, producing a Polarized Dark Field. The Analyzer is said to be at Extinction.

If a birefringent specimen is placed after the Polarizer, two orthogonal beams of polarized light are formed from each impinging ray. One of the rays will be vibrating parallel to the transmittance plane of the Analyzer, and will not destructively interact with its birefringent crystals. Thus a birefringent sample will be seen as a bright object on a polarized dark field.

When imaging samples with a very small optical path length, such as biological samples, it is useful to add a third birefringent device after the sample and before the Analyzer. This device, the Retardation Plate, is designed to retard the phase of polarized light a precisely small amount (usually 5–10nm). This known amount of phase retardation added to the sample retardation increases the total Optical Path Length from the sample (refractive index times geometric distance) at the image plane, which results in an increased brightness of the sample.

A different device, the Compensator, may be placed in the same position as the retardation plate. This optical device can differentially retard the e- and o- rays leaving the sample. An example is the quartz wedge. When the two rays are recombined at the Analyzer, the interaction of the fast ray (E-ray) and phase retarded O-ray can produce interference colors at the image plane. The relationship of interference color and optical path is defined in the Michel-lévy Interference Color Chart. Compensator types are the Full Wave, or Red plate, and the de Sénarmont Compensator.