Wollaston Prisms

A Wollaston prism acts as a beamsplitter to separate or shear a polarized beam of light into two coherent components that pass through and interact with slightly different areas of a specimen.

A Wollaston prism is composed of two geometrically identical wedges of quartz or calcite (birefringent, or doubly-refracting materials), cut in a way that their optical axes are oriented perpendicular when they are cemented together to form the prism. The polarizer (positioned beneath the Wollaston prism in a microscope) is oriented so that plane-polarized light enters the prism at a 45-degree angle with respect to the axes of the two birefringent prism halves. Light entering the prism is split into two beams (separated by a small angle) that have polarization vectors mutually perpendicular to each other.

In a microscope, the Wollaston prism is placed in the front focal plane of the substage condenser, directly above a polarizer whose plane of polarization is oriented in a 45-degree angle to the prism axis. Light exiting the field diaphragm aperture first passes through the polarizer and then through the condenser aperture diaphragm and Wollaston prism, where all spatial frequencies are sheared into two beams oriented in a mutually perpendicular direction. The degree of shear for a particular Wollaston prism is set by the manufacturer and must coincide with that of a matching beam combiner prism, located at the (effective) objective rear focal plane. Each sheared pair of light rays will pass through the condenser and be refracted by the lens elements so that the two beams travel parallel to each other as they leave the condenser and pass through the specimen. The beams are actually separated to a very small degree that is beneath the resolving power of the objective (the beam separation distance, which usually ranges between 0.15 and 0.6 micrometers, is greatly exaggerated in the tutorial).

Because the individual light beams are derived from the same source prior to being sheared by the Wollaston prism, they are coherent and capable of interference. After leaving the condenser, the sheared light beams pass through closely adjacent regions of the specimen, which often induces an optical path difference between the two beams due to localized refractive index and thickness variations. Light beams exiting the specimen are captured by the objective and brought into focus at the rear focal plane, where a second Wollaston prism is strategically placed to recombine the beams into a common path. The paired light beams, still polarized and oriented with vibration directions that are perpendicular, next pass through a second polarizer (the analyzer). The analyzer has a polarization plane that is crossed with respect to the first polarizer and is oriented at a 45-degree angle to the beams exiting the second Wollaston prism. Components of the light beams vibrating in the polarization plane of the analyzer are able to recombine and interfere to form the image observed in the microscope eyepieces or captured by a traditional or CCD camera system.