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Right Angle Prisms

The right-angle prism is one of the simplest and most versatile prisms. It's name derives from the size of it's apex angle: 90 . Because the legs of a standard right angle prism are of equal length, its cross-sectional view displays the form of an isosceles right triangle.

Designers use the right-angle prism in one of two orientations. The first orientation can be called "90 Deflection". The second orientation can be called "180 Deflection" . For both orientations, incoming light must travel parallel to the plane that includes the right-angle vertex.

90 Deflection

180 Deflection

In the " 90 Deflection" orientation, the prism acts like a single mirror. Light enters the prism through one of its legs, reflects off its hypotenuse by total internal reflection, and then exits through its second leg. The centerline of the incoming light must be perpendicular to the entrance face. As is the case with a flat mirror at 45 to the incoming light, the prism in this orientation inverts the image while deflecting its direction of propagation by 90.

In the " 180 Deflection" orientation, the prism acts like two mirrors. Light enters the prism through its hypotenuse, reflects at its first and second legs by total internal reflection, and then exits back through its hypotenuse.

In the " 180 Deflection" orientation, as long as the incoming light remains parallel to the plane that contains the vertex angle, the alignment of the prism within that plane is not critical; exact retroreflection will still occur.

The dimension that controls the accuracy of the retroreflection is the right angle at the vertex of the prism. The outgoing beam will be inclined to the incoming beam by an amount equal to twice the deviation of the vertex angle from 90. For example, if a right-angle prism were manufactured with a tolerance of 1 minute of arc, then the incoming and outgoing beams could cross each other with an inclination of no more than 2 minutes of arc.

The retroreflective capability of a right-angle prism is limited to action in the plane that includes its right-angle vertex. If retroreflective action is required for randomly oriented light, then the designer must use a corner cube retroreflector.

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