Patent ID: 7646554

Claim:
Optical element for deflecting a light beam by 90°, with a correction means for improving the relative position of the incident and emergent light beams of a single light beam comprising a combination of a pentaprism with at least one transparent glass block which is permanently attached relative to it with a planar entry and exit surfaces, dimensions of the glass block being set relative to dimensions of the pentaprism at values which will produce a rotary angle-dependent parallel offset between a single light beam which is incident and emergent on the glass block that will essentially completely compensate for an inherent rotary angle-dependent parallel offset for light beams incident on directly the pentaprism, wherein the at least one transparent glass block has a length determined in accordance with the relationship (a+b)=((a+b+c+d+e)/n), where n is the index of refraction of the at least one glass block, a is the distance from the entry surface to a point of intersection between the paths of the incident light beam and the emergent light beam, b is the distance from the exit surface to a point of intersection between the paths of the incident light beam and the emergent light beam, c is the distance from a first surface of the pentaprism from which the incident light beam is reflected to the point of intersection between the paths of the incident light beam and the emergent light beam, d is the distance between the first surface of the pentaprism and a second surface of said pentaprism from which the light beam reflected by said first surface is reflected to the exit surface of the pentaprism as the emergent light beam, and e is the distance between second surface and the exit surface of the pentaprism.