Autocollimation is an optical setup where a collimated beam (of parallel light rays) leaves an optical system and is reflected back into the same system by a plane mirror. ... Light from an origin point O is collimated (made parallel) by a high quality objective lens.

What is the purpose of autocollimator?

An autocollimator is an optical instrument for non-contact measurement of angles. They are typically used to align components and measure deflections in optical or mechanical systems.

How do you measure focal length in Autocollimation?

By sliding the lens-mirror combination back and forth along the axis a sharp image of the bright spot is obtained on the white card. The distance between the card (object-image screen) and the lens is the focal length.