Theory

Autocollimators are optical instruments used to measure small angular differences, and also used to measure straightness, flatness, and alignment. They are highly sensitive to small angular changes. It works as a collimator and an infinity telescope together. These are mostly used to measure deflections in a surface.

Basic principle:

Autocollimator works on the principle of reflection of light. It uses a monochromatic light source emitting a beam of light rays directed towards a beam reflector placed on the surface to be tested. Upon hitting the reflector, the reflected beam is converged to a plane that can be viewed. If the surface is flat and devoid of angular deviations, the rays reflect back along their original path, converging at a plane at the focal distance from the converging lens as shown in figure 1.

Figure 1:- Reflector is at 90 ° with the direction of rays

However, when the object is tilted, reflected rays create an angle with incident beam, deviating from the original path by a certain number of degrees (as shown in figure 2) shifting the point of focus in the focal plane.

Figure 2:- Reflector is not at right angles to the direction of the rays

Working of Autocollimator

Autocollimator mainly consists of three main parts i.e. micrometer, lightning unit and collimating lens. Figure 3 shows line diagram of auto collimator. A target graticule is located perpendicular to the optical axis. When lamp illuminates the target graticule, light rays diverges out and is directed via a beam splitter to reach the objective lens. The light rays projected from the objective lens are parallel to the reflector (right side of Figure 3).

Figure 3:- Line diagram of auto collimator

A flat reflector is placed in front of the objective, perfectly normal to the optical axis, and redirects light rays back to their original paths. These rays meet precisely at the junction of the target graticule. A fraction of the returning light travels via the beam splitter and becomes visible through the eyepiece. If the reflector is tilted, the reflected beam (deviated) is directed to the target graticule which is displaced linearly from the actual target. This displacement is also visible through the eyepiece. The displacement is equal to twice the angle of tilt (2 θ) multiplied by the focal length (f). Linear displacement can be measured by adjusting the knob of micrometer. The projected rays and reflected rays is made to coincide the displacement of the micrometer with the help of the eyepiece. This displacement is measured and directly correlates with the angle of the reflector. Autocollimator proves especially useful for verifying angular deviation, and checking small linear displacements accurately.