Auto-collimation 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
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
If the reflector is tilted by an angle (in radians), the reflected beam is
deflected by twice that angle i.e. 2. 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
auto-collimator and the tilt angle of the
reflector: d = 2 f.
The tilt angle can be ascertained with the
= d / 2f
where f is the effective focal
length EFL of the auto-collimator.
Since the f is a constant of
the auto-collimator, the eyepiece reticle can be
graduated in angle units and the tilt angle can
be directly read off.