Autocollimation is an optical technique of projecting an illuminated reticle to infinity and receiving the reticle image after reflection on a flat mirror. The reflected image is brought to the focus of the objective lens in which the eyepiece reticle is located. Thus the reflected image of the collimator (illuminated) reticle and the eyepiece reticle can be simultaneously observed. When the collimated beam falls on a mirror which is perpendicular to beam axis, the light is reflected along the same path. Between the reflected image and the eyepiece reticle - which are seen superimposed-no displacement occures.
 
If the reflector is tilted by an angle a, the reflected beam is deflected by twice that angle i.e. 2a. The reflected image is now laterally displaced with respect to the eyepiece reticle. The amount of this displacement "d" is a function of the focal length of the autocollimator and the tilt angle of the reflector: d = 2 a ƒ. (a in radians) The tilt angle can be ascertained with the formula:
 
a = d / 2ƒ
 
where ƒ is the effective focal length EFL of the autocollimator. Since the ƒ is a constant of the autocollimator, the eyepiece reticle can be graduated in angle units and the tilt angle can be directly read off.