A shear plate is a very simple interferometer, i.e., an optical arrangement that combines two wavefronts. Shear plates are thick, high-quality optical flats. The wavefront or beam to be tested is incident on the shear plate at a 45 . angle for maximum sensitivity. The uncoated front and rear surfaces generate reflections of almost equal intensity.
Interference occurs in the region where the wavefronts overlap as shown in the figure at right.
Wedged shear plates produce a graded path difference between the front and back surface reflections. Consequently, a parallel beam of light produces a linear fringe pattern where the reflections overlap. The fringes are equally spaced, and the fringe direction for perfect collimation can be easily marked by a reference line either on the plate itself, or on a viewing screen placed in the reflected beam path. The use of a viewing screen greatly simplifies matters when measurements have to be made using the shear plate. For this reason Melles Griot shear plates are mounted in aluminum housings that contain viewing screens.