Autocollimator is an optical instrument that is used to measure small angular differences.
This device is very sensitive to change in angles and provides a very accurate measurement of angular differences. It is basically a combination of an infinity telescope and a collimator.
Autocollimators are mainly used to align optical components and measure optical and mechanical deflections.
When a beam of light rays coming out of a monochromatic light source is made to fall on a diverging lens and made parallel. To make the light ray parallel, the light source should be kept at the focus of the diverging lens.
After that, if these parallel rays are made to fall on reflecting surface. then the rays reflect back and travel in the same path but in the opposite direction and converges at the light source by passing through the converging lens.
Now, if the reflecting surface is tilted by an angle theta ( θ ) then the reflected ray makes an angle 2 theta ( 2θ ) with the incident ray.
Parts Of Autocollimator:
Light Source is used to generate the light rays so that it reaches the reflector.
It is the surface that acts as a workpiece for the autocollimator. Using the autocollimator, the angle of tilt of this reflecting surface is to be measured. It reflects the parallel light rays coming through objective lens.
The diverging lens which is also known as objective lens is used to paralellize the light rays coming through the beam splitter so that parallel light rays reach the reflector.
The beam splitter is used to split the light rays and direct it towards the objective lens.
The light rays after getting reflected reach this target graticule and the distance between incident and reflected ray is traced in this target graticule.
It is used to see the points of incident and reflected ray in the target graticule clearly and measure the distance between them.
Working of Autocollimator:
In autocollimator, the reflecting surface is the surface whose inclination is to be measured using this device. The distance between the source of light and the reflected ray in the focal plane is measured using a micrometer microscope.
At first, the light source is illuminated and the rays of light diverge from the intersection point of the cross line target graticule which is placed at focal plane of the objective lens.
After that, the ray of light reach the beam splitter and this beam splitter will direct the light rays towards the objective lens.
The objective lens will parallelize the light rays and the light rays will move towards the reflector. Now there can be two cases :-
Case 1: Reflector is perpendicular to ray of light.
When the parallel light rays reach the reflector and the reflector is perpendicular to light rays then the light rays will be reflected back to their original paths.
These light rays are then brought to focus in the plane of the target graticule coincident with the intersection of cross lines of the graticule.
Some portion of reflected light passes straight through the beam splitter and hence the return image of the target crossline is therefore visible through the eyepiece thus operating as telescope focussed at infinity.
Case 2: Reflector is tilted at some angle
If the reflector is tilted at some angle, the parallel light rays reflect from the reflector at an angle double the angle of tilt.
After the reflection, the light rays are focused in the plane of the target graticule but linearly displaced from the intersection of crossline by a distance of 2 * (angle of tilt) * (focal length of the objective lens).
The linear displacement of the graticule image is measured by an eyepiece graticule and a micrometer microscope or an electronic detector system based on whether visual or digital autocollimator used.
Most autocollimators are calibrated so that there is no need to convert the distance measured into the angle of inclination. This is converted in the autocollimator itself and the angle of inclination can be read directly in the autocollimator.
The factors which are responsible for the basic sensitivity and angular measuring range of autocollimator are focal length and the effective aperture.
Types of Autocollimators:
1 Visual Autocollimator:
In visual autocollimator, the angle of tilt of the reflecting surface is measured by viewing a graduated scale through an eyepiece. As the focal length of the visual autocollimator increases, the angular resolution increases and the field of view decreases.
2 Digital Autocollimator:
In digital autocollimator, the micrometer adjustment is provided for the setting but the coincidence of setting graticule and the target image is detected photo-electrically.
This autocollimator is used in the lab. It has very high precision, provides real-time measurements and is very user friendly.
Applications of Autocollimator:
1 Checking straightness of machine tool slideways.
2 Measuring very small angles.
3 Checking parallelism.
4 Checking squareness of column to base.
5 Checking flatness of bed plates and surface tables.
6 Measuring very small displacements.
7 Checking small linear displacements.
Advantages of Autocollimators:
1) It has very high accuracy.
2) It can measure wide range angle.
3) It is very easy to set up and operate.
4) Calibration traceable to international standards.
5) It Can be used to see the result visually or electronically i.e in a computer screen.
6) Wide range of accessories and levels available.
Disadvantages of Autocollimator:
1) Maintenance is required regularly.
2) It is time consuming.
3) It requires sample cutting and processing for tracing by the detector.