ABSTRACT
Optical prisms are multifaced objects that are designed to perform a variety of functions. They are made from transparent material selected for good transmit tance in the spectral region where they are to be used. Usually at least one surface is a polished reflecting surface and it may have a reflecting coating or be left uncoated, relying on total internal reflection. Coated surfaces are pro tected because the reflecting surface of the coating is adjacent to the prism material. An external coating of black paint is often applied for further protection against scratches and abrasions, and to prevent external light from leaking through a sightly transmitting coating. Totally internally reflecting surfaces are not covered with protective paint as this would reduce the reflectance. The reflecting surface must be protected by careful handling.
