PorroPrismen

Porro prisms are a type of optical prism used to invert and laterally reverse an image. They consist of two right-angled prisms, each with an apex angle of 45 degrees. In their most common configuration, the two prisms are joined at their hypotenuses, forming a single block. Light enters one prism, reflects internally twice, and then exits through the second prism. This sequence of reflections results in a 180-degree rotation of the image, both vertically and horizontally.

The Porro prism system is widely used in binoculars and other optical instruments where an upright and laterally correct image is desired. Its primary advantage is its ability to achieve this image inversion without the need for lenses, thus simplifying the optical design and potentially reducing cost. The characteristic offset, or "broken back" appearance, of Porro prism binoculars is a direct result of this prism arrangement. While effective, the Porro prism design can be bulkier compared to other prism systems like the roof prism. The internal reflections are highly efficient, and when used with appropriate coatings, they can provide excellent image quality.