Porro prism

The Porro prism, named after its inventor Ignazio Porro, it was patented in 1854, is the classic inverting prism in a traditional binoculars with Keplerian telescopes. Each of the built- in every part of the telescope porro prism is usually made ​​up of two simple rectangular reflection prisms ( so-called half -prisms ), which happens to be from the beam path after the other and each causing a 180 ° rotation of the image. The correct side, but upside-down picture produced by the Kepler telescopes is raised in this way.

The beam path is in a half cube prism twice, so even, reflected, so that the image remains basically correct side. He will be deflected 180 °, which is utilized with the aid of the first of the two prisms to erect the inverted image, it is turned upside down. The unintended 180 ° deflection - one would observe with the telescope to the rear - is compensated by the second prism. For the image about a vertical axis is reversed, and the telescope remains geradsichtig. A raising of the upside-down image analogous process is omitted, because the observer participates the second turn of the beam path. He does not follow the first turning point, because he is not even on the head. The condition to be met is in physical- optical formulation as follows: The picture is completely reversed when the reflections take place in two different main sections of a prism system.

There is also the Porro prism type 2, which also deflects the beam four times 90 °. A double- reflecting half cube prism is placed between two smaller only once each reflecting half -prisms. The main sections of the outer prisms are arranged at right angles to the central prism. Its advantage is that vertical is no displacement of the beam path. Another variant with the same function is composed of two sub- prisms Porro -Abbe prism

Beam path in a half cube prism pair ( Porro prism )

Schematic cross section through a Porro prism binoculars

Cross-section through a Porro prism binoculars

Beam path in a Porro prism type 2

Side effects

As a component in the Porro prism binoculars is now in competition with the geradsichtigen Roof pentaprism. The latter is geometrically more elaborate, but permitted a less broad construction of the binoculars ( at the expense of spatial vision, see 2 )