Penta Prism is a five-sided prism containing two reflecting surfaces at 45° to each other, and two refracting faces perpendicular to the entering and emerging beams. The important properties of Penta Prism are:

1. The image is neither inverted nor reversed, just deviated by 90°.
2. With the prism, a constant deviation angle applies to all rays transmitted by the useful aperture, regardless of the angles between these rays and the optical axis (or entrance and exit face normal).

Only two of the inclined sides are used to reflect light; manufacturers create the fifth side, or face, by removing a corner to reduce the weight and size of the prism.

Engineers specify a penta prism rather than a double mirror because of several advantages found in the prism’s construction. One of those advantages is stability. Glass has a low coefficient of expansion, and alignment of the reflecting surfaces of the prism will remain constant over a long time even when exposed to environmental changes and shocks.

A second advantage is in simplicity and ease of assembly. A penta prism can be placed into a single mount, whereas two mirrors would require two separate mountings and a more complex procedure for assembly.

Still another advantage to the prism lies in its size. A penta prism can be easily manufactured with small dimensions and then simply placed into an assembly. A miniature mirror system might require special design for mountings and special tools for assembly.

The geometry of reflection inherent in a penta prism deserves special note. The 90° deviation, which is imparted to the incoming beam, remains constant for different angles of incidence. This invariance means that alignment of a penta prism is not critical in terms of the deviation of a centerline through 90°. Although this invariance to alignment is shared by other prisms oriented for two coplanar reflections, the penta prism is unique in its ability to deviate the centerline by precisely 90°. The manufacturing tolerance of the 45° angle between the two reflecting faces determines the accuracy of the centerline’s 90° deviation.