Roof prism

Two reflecting surfaces perpendicular to each other are called roof ridges, and the prism with roof ridges is called roof prism.
The roof prism is small in size and can make the objective lens and the eyepiece lie in a straight line, so it is often used in extremely compact binoculars.
The roof prism replaces the chord surface of the right-angle prism with two perpendicular reflection surfaces. The function of the roof surface is to increase the reflection without changing the direction of the optical axis, so that the total number of reflections of the prism changes from an odd number to an even number. The right-angle roof prism has the property of turning the light beam by 90° and forming a positive image. Therefore, this prism is widely used in telephoto systems, imaging systems, observation systems and ranging systems.