Autocollimator – working Principle and Application

Autocollimators are optical instruments that measure angular displacements with high sensitivity. They are used to align optical components and measure optical and mechanical deflections.

Autocollimation Principle : 

The two main principles used in an autocollimator are
(a) the projection and the refraction of a parallel beam of light by a lens, and
(b) the change in direction of a reflected angle on a plane reflecting surface with change in angle of incidence.

Autocollimator - working Principle and Application
Autocollimator – working Principle and Application

To understand this, let us imagine a converging lens with a point source of light O at its principle focus, as shown in Figure  a.  When a beam of light strikes a flat reflecting surface, a part of the beam is absorbed and the other part is reflected back. If the angle of incidence is zero, i.e. incident rays fall perpendicular to the reflecting surface, the reflected rays retrace original path. When the reflecting plane is tilted at certain angle, the total angle through which the light is deflected is twice the angle through which the mirror is tilted. Thus, alternately, if the incident rays are not at right angle to the reflecting surface they can be brought to the focal plane of the light sources by tilting the reflecting plane at an angle half the angle of reflection as shown in Figure b.

Now, from the diagram, OO’ = 2Θ × f = x, where f is the focal length of the lens.
Thus, by measuring the linear distance x, the inclination of the reflecting surface Θ can be determined. The position of the final image does not depend upon the distance of the reflector from the lens. If, however, the reflector is moved too long, the reflected ray will then completely miss the lens and no image will be formed.

Working Of Autocollimator:
In actual practice, the work surface whose inclination is to be obtained forms the reflecting surface and the displacement x is measured by a precision microscope which is calibrated directly to the values of inclinationΘ.
The optical system of an autocollimator is shown in Figure The target wires are illuminated by the electric bulb and act as a source of light since it is not convenient to visualize the reflected image of a point and then to measure the displacement x precisely. The image of the illuminated wire after being reflected from the surface being measured is formed in the same plane as the wire itself. The eyepiece system containing the micrometer microscope mechanism has a pair of setting lines which may be used to measure the displacement of the image by setting to the original cross lines and then moving over to those of the image.

autocollimator construction diagram
autocollimator construction diagram

Generally, a calibration is supplied with the instrument. Thus, the angle of inclination of the reflecting surface per division of the micrometer scale can be directly read.
Autocollimators are quite accurate and can read up to 0.1 seconds, and may be used for distance up to 30 meters.

Application Of Autocollimator : 

Autocollimators are used by the optical industry and mechanical engineers in a variety of applications. Their specific functions include precision alignment, the detection of angular movement, the verification of angle standards, and angular monitoring over long periods.

Testing Application

Autocollimators can be used in the testing of

  • parallelism with a collimator and telescope
  • opaque wedges and plane parallel plates
  • transparent wedges
  • angles in glass prisms
  • the accuracy of rotary tables and index tables
  • camera objectives

These optical instruments are also used in the flatness testing of reflecting surfaces, the control of wedges and plane parallel plates, and parallel measurement of cylindrical bores.

Measurement Applications
In addition to testing applications, autocollimators can be used to measure the

  • radius of concave and convex spherical surfaces
  • back focal of lenses and optical systems
  • centration error of spherical surfaces
  • centration error of lenses in transmission
  • parallelism or perpendicularity of two surfaces

Additional applications include the

  • radius measurement of concave and convex spherical surfaces
  • relative measurement of the angular error of prisms
  • pitch and yaw measurement of slides
  • parallel setting of rolls

Finally, autocollimators can be used to measure straightness, flatness, roll angle, and the squareness between a vertical angle and a machine bed.

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