Compensator, Optical

a device used to import a particular path difference to two light beams or to make an already existing path difference equal to zero or some constant value. Optical compensators are usually equipped with readout devices and thus can serve as path difference meters. A common principle in the design of optical compensators is the possibility of introducing small path differences through comparatively coarse displacements. The most commonly used optical compensators are of two types.

Interferometric compensators are used in double-slit interferometers to equalize path differences in interfering beams. An example of optical compensators of this type is a plane-parallel plate, in which the optical path length of the beam depends on the angle at which the beam is incident on the plate. Plates of equal thickness are usually placed in the path of each of the two interfering beams. If they are strictly mutually parallel the additional path difference contributed by them is zero. One of the plates is equipped with a device that allows it to rotate through a small angle relative to the other plate; the path difference thus imparted can be measured from the angle of rotation. A number of more complex designs also exist, such as optical compensators with a movable wedge.

Polarization compensators are used in the analysis of elliptically polarized light—that is, the determination of the orientation of the axes of the ellipse of polarization and the ratio of their magnitudes. The effect used in such optical compensators is the ability of birefringent crystals to divide an incident light beam into two beams that are polarized perpendicularly to each other. The velocities of the two beams within the crystal, and hence their optical path lengths, differ; therefore, during their passage through the crystal, they acquire a path difference whose magnitude depends on the thickness of the crystal. The simplest optical compensator of this type is called a quarter-wavelength plate because of the path difference it introduces.

Polarization optical compensators change the polarization characteristics of the light transmitted through them. For example, they convert an elliptically polarized light into plane-polarized or circularly polarized light. Such compensators can have an accuracy of measurement of up to 10⁻⁵ · 2π.

Optical compensators are widely used in range finders, in studies of stress distribution in transparent objects by means of polarized light, in studies of the structure of organic substances, in saccharimetry, and particularly in crystal optics, where the optical compensator is the main auxiliary instrument used in conjunction with polarization microscopes.