Optical square

A pentaprism

The optical square uses a pentaprism to reflect and refract a beam or sighting 90 degrees, it is used in pairs in surveying and in a singular block in metrology.

In optical square A Horizon glass is placed at an angle of 120° with the horizon sight.

The Index glass is placed at an angle of 105° with the Index sight.

Angle between Index glass and Horizon glass is 45°.