Penta-prisms

Penta-prisms are constant-deviation prisms.  They produce a 270° beam deviation independent of the angle of incidence at the entrance face.  This property makes them invaluable alignment aids when it is necessary to define a right angle optically.  A beam incident on one of the right angle faces is totally internally reflected at the two back faces to emerge at right angles to the direction of input. 

The accuracy of these prisms is controlled by the precision of the prism angles.  We offer two levels of accuracy having angular deviations of 3 arc minutes or 30 arc seconds.

When the input is normal to the entrance face, total internal reflection occurs.  However, in order to ensure correct operation over a wide range of incident angles it is preferable to coat the reflecting surfaces.  We offer penta-prisms either uncoated or with the following combination of coatings:

Reflecting faces — internal silver protected with black paint.

Entrance and exit faces — single layer MgF₂ anti-reflection coating.

Material used is BK7 (n_d= 1.517)

  We can supply penta-prisms in the following standard face sizes: 10, 12.7, 15, 20, 25.4, 40 mm.  Other sizes will be considered on special request.

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