A device with known, fixed or variable retardation, and vibration direction, used for determining the amount of retardation (hence, the thickness and birefringence) of an anisotropic substance. It is also used to determine the sign of elongation of elongated anisotropic specimens. The most common compensator is the first-order red (530-550 nm retardation), but other compensators frequently used include the quartz wedge (1-6 orders), the Berek (1-3, 1-5, 1-30 orders), the quarter-wave (~137 - 147 nm), and the Sénarmont. Compensators are typically introduced into the light path through a bodytube slot between the objective and the eyepiece.
A birefringent slab of optical quartz, mica, or similar material that is positioned almost anywhere between the polarizer and analyzer in a polarized microscope, but limited to the exterior of the prisms and other imaging components of a DIC microscope. The slab can be tilted and/or rotated to achieve varying optical effects. A compensator changes the optical path difference between orthogonal wavefronts, and can be utilized to perform quantitative measurements of the relative ordinary and extraordinary wavefront retardations, or for qualitative purposes in adjusting image contrast and brightness. Compensator plates are often used in DIC microscopy for optical staining of transparent specimens.