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Double Porro prism systems are used in small optical telescopes to re-orient an inverted image (an arrangement is known as an image erection system ), and especially in many binoculars where they both erect the image and provide a longer, folded distance between the objective lenses and the eyepieces.
Porro prism and Perger prism binoculars and roof prism binoculars using the Abbe–Koenig roof prism configuration do not use mirror coatings because these prisms reflect with 100% reflectivity using total internal reflection in the prism rather than requiring a (metallic) mirror coating.
Leica GEOVID 8×42 HD-B laser rangefinding "Perger–Porro prism system" binoculars that have a 14 mm (0.55 in) eyepiece/objective axis offset used by a hunter. A Perger prism or Perger–Porro prism system is a prism, that is used to invert (rotate by 180°) an image.
The image is rotated 180° in the process, and for this reason the prism is used as an image erection system in some binoculars, and camera viewfinders. The Porro–Abbe system reduces the lateral beam axis offset by 23% compared to a traditional double Porro prism system in binoculars.
In this way, since the 1990s, roof prism binoculars have also achieved resolution values that were previously only achievable with porro prisms. The presence of a phase-correction coating can be checked on unopened binoculars using two polarization filters.
Ignazio Porro (25 November 1801 – 8 October 1875) was an Italian inventor of optical instruments . Porro's name is most closely associated with the prism system which he invented around 1850 and which is used in the construction of Porro prism binoculars . He also developed a strip camera in 1853 for mapping, which was one of the earliest such.
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