Penta Prisms deviate a beam by 90º without inverting or reversing the image. These prisms also have the property of constant deviation, which means that the 90º deviation angle applies to all rays transmitted by the effective aperture, regardless of their angle with respect to the optical axis.
Penta prisms are thus very useful in applications where prism orientation cannot be precisely controlled. Due to the geometry of penta prisms, the reflecting surfaces due not utilize total internal reflection and must be coated with a metal or dielectric coating.
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