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Porro’s prism erecting systems are used to reverse the optical image. The system is direct-vision with parallel displacement between the incoming and the exiting ray. The compact version is normally the construction used in technology. In the prism system Type I, four right-angle, isosceles prisms whose hypotenuse surfaces are turned toward each other are arranged with a displacement of 90° relative to each other. The image erection is achieved by quadruple total reflection at the unaluminized short faces of the prisms.
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| Prism erecting system, type I. | Prism erecting system, type II. |
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This system, which is chiefly used in binoculars, also generates an upright, unreversed image in astronomical telescopes. It also results in a more compact design at the same time. The same optical effect is obtained with an erecting system, Type II. It consists of three prisms, two prisms reflecting by 90° and between them a prism reflecting by 180°. One of the applications of this prism system is in compensator levels. | |
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