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Penta Prisms are designed to deviate a beam of light by a constant 90 degrees and due to this property can be used to define the right angles in optical systems. By definition, they are five sided prisms and are unaffected by slight movements. The perpendicular deviation is independent and insensitive of the precise alignment of the prism with respect to the incident beam or image. The two opposite transmitting faces are often coated with an antireflection coating to reduce spurious reflections