Autocollimation, (from the Greek autos — self and collimo (a corruption of the correct Latin collineo) — I direct straight), the path of light rays in which they leave as a parallel beam the collimator that forms part of an optical system, are reflected from a plane mirror and pass through the system in the reverse direction. If the mirror is perpendicular to the optical axis of the system, a radiating point lying in the focal plane on that axis coincides with its image in the reflected rays; turning the mirror displaces the image. Autocollimation is used in optical instruments to check the parallelism of optical components (for example, of the mirrors in optical quantum generators), to check the parallelism of displacements, and so on.