The SI Series of shearing interferometers can be used to determine qualitatively if a coherent beam of light is collimated. The design consists of a wedged optical flat mounted at 45° and a diffuser plate with a ruled reference line down the middle. The diffuser plate is used to view the interference fringes created by a coherent beam of light incident at an angle of 45° with respect to the normal of the optical flat (see the images below). If the beam is collimated, the resulting fringe pattern will be parallel to the ruled reference line. In addition to the degree of collimation, the fringes will also be sensitive to spherical aberration, coma, and astigmatism. For each member of the SI series, the thickness and degree to which the optical flat is wedged is chosen such that 4-6 fringes will be seen on the diffuser plate.

The base is constructed from anodized aluminum, and the plate with the wedged optical flat is held in place magnetically so that it can be easily swapped out for a plate with a different wedged optical flat. Behind the wedged optical flat, a hole is bored through the base to allow the transmitted beam to pass through the optical flat unimpeded. The shearing interferometer has one 8-32 and one M4 threaded mounting hole on the bottom of the base as well as on each of the two sides (i.e. 6 holes total of which 3 are imperial and 3 are metric). The Specs Tab summarizes the compatibility between bases and plates and also lists the thread type associated with each item in the series.

For small beam diameters, the corresponding fringe pattern will also be small, which can make it difficult to see. For these situations, the SIVS accessory can be purchased to replace the standard diffuser plate viewing screen, thereby increasing the size of the fringes on the diffuser plate. The SIVS consists of a mounted diverging lens and diffuser plate and can be used for beam diameters ranging from 1-10 mm.