Penta prisms are five-sided reflective prisms which are used to deviate light by a constant 90-deg. The reflections inside the Penta prism are not due to total internal reflection but due to the reflective coatings on two surfaces. The two opposite transmitting sides can be coated with an AR coating upon customers' request. The light entering the Penta prism reflects inside the prism twice allowing the transmission of an image through a right angle without inverting or reversing it.