Autocollimator – Working Principle, Types, Diagram, Advantages

In this article we are going to learn about Autocollimator, what it is used for, its working principle, how it works, different types of auto collimator, and its advantages and disadvantages.

What is Autocollimator ?

The autocollimator or autocollimating is an optical instrument which is used for the measurements of small angular differences, changes or deflections. It is also used to determine straightness, flatness, alignment, etc.

An autocollimator is a device that uses optics to measure small angular variations. This device is extremely sensitive to very small angle changes and can measure angular deviations accurately. It’s essentially a collimator and an infinity telescope combined. Autocollimators are used to align the various components of a system and measure their mechanical or optical deflections.

Parts of an Autocollimator

Six parts of Autocollimator are :-

  • Light source
  • Reflecting surface
  • Diverging lens
  • Beam splitter
  • Target graticule
  • Micrometer microscope

1. Light source

The light source is used to generate light rays so that it reaches the reflector.

2. Reflecting Surface

It is the surface that serves as a workpiece for the autocollimator. The angle of tilt of this reflecting surface is to be measured using the autocollimator. It reflects the parallel light rays that pass through the objective lens.

3. Diverging Lens

The diverging lens, also known as the objective lens, is used to parallelize the light rays that come through the beam splitter so that they reach the reflector in parallel.

4. Beam Splitter

The beam splitter is used to split the light rays and direct it towards the objective lens.

5. Target Graticule

The light rays that have been reflected reach this target graticule, and the distance between the incident and reflected rays is traced in this target graticule.

6. Micrometer Microscope

It is used to clearly see the incident and reflected ray points in the target graticule and measure the distance between them.

Autocollimator Working Principle

It incorporates two optical principles

  1. The projection and reception of a parallel beam of light by a lens,
  2. And the change in direction of a beam reflected from a plane surface with change of angle of the surface.

When a monochromatic light source’s beam of light rays falls on a beam deflector, the beam is deflected 90 degrees towards the converging lens. The converging lens parallelizes the beams and directs them to the object or reflecting surface.

To keep the light beam parallel, keep the beam deflector close to the focus of the converging lens. The parallel rays are then directed to strike a reflecting surface or an object. If there are no angular deviations on the object’s surface, the rays reflect back and proceed along the same path, in the opposite direction, eventually converge at the receiver kept at the focal distance from the converging lens. If the object is inclined at an angle, the reflected ray forms an angle with the incident beam of 2(α )degrees.

Let us understand by a example

Autocollimator Working Principle
Autocollimator diagram

Imagine, first of all, a converging lens with a point source of light O at its principal focus, as 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 the original path as in Figure (a).

When the surface is tilted at any other angle, the total angle through which the light is deflected is twice the angle through which the mirror is tilted, and is brought to a focus in the same plane as the light source but to one side of it, as in Figure (b). Obviously,

OO’ = 20f= x (say), where f is the focal length of the lens.

Thus, by measuring the linear distance QO'(x), the inclination of the reflecting surface o can be determined.

The position of the final image does not depend upon the distance of the reflector from the lens. That is, the separation x is independent of the position 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.

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 that is calibrated directly to the values of inclination Θ.

Working of Autocollimator

In autocollimators, the reflective surface is the surface whose inclination is to be measured using this device.A micrometer microscope is used to measure the distance between the source of light and the reflected ray in the focal plane.

First, the light source is illuminated, and the light line’s rays are extracted from the intersection points of the cross line target gratitude, which is placed in the objective lens’s focal plane.

After that, a ray of light reach the beam splitter and the v ray beam is the gate that 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: The reflector is perpendicular to the ray of light.

When parallel light rays reach a reflector that reflector is perpendicular to the light rays, the light rays are reflected back to their original paths.
These light rays are then brought into focus in the plane of the target graticule at the intersection of the graticule’s cross lines.
Because some of the reflected light passes straight through the beam splitter, the return image of the target crossline is visible through the eyepiece, allowing the telescope to operate as if it were focused at infinity.

Case 2: The reflector is tilted at some angle.

If the reflector is tilted at an angle, the parallel light rays reflect at an angle twice the angle of tilt.
After the reflection, light rays are focused in the plane of the target graticule but linearly displaced from the intersection of crosslines by a distance of 2 * (angle of tilt) *. (focal length of the objective lens).
Depending on whether a visual or digital autocollimator is used, the linear displacement of the graticule image is measured using an eyepiece graticule and a micrometer microscope or an electronic detector system.

Most autocollimators are calibrated so that the distance measured does not need to be converted into the angle of inclination. This is converted in the autocollimator, and the angle of inclination can be read directly there.

The focal length and effective aperture of an autocollimator are the factors that determine its basic sensitivity and angular measuring range.

Types of Autocollimator

There are mainly two types of Autocollimator :

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.

A. Electronic Autocollimator

The electronic autocollimator is a high-end, high-precision angle measurement device that does not include an optical eyepiece. This device can measure small angular deviations to fractions of an arc-second accuracy. Measuring with an electronic autocollimator is quick, simple, and accurate, and it is usually the least expensive method.

These highly sensitive devices are widely used in workshops, tool rooms, inspection departments, and quality control laboratories around the world to compute small angular displacements, squareness, twist, and parallelism with extreme precision.

B. Laser Autocollimator

Today, With the advent of new technology, autocollimation equipment can now be improved to allow for direct measurements of reflected laser beams. This one-of-a-kind feature enables the alignment of lenses, mirrors, and lasers all at the same time.

This technology fusion of century-old autocollimation technology and recent laser technology results in a very versatile instrument capable of measuring inter-alignment between multiple line of sight, laser in relation to mechanical datum line, alignment of different holes and cavities, measurement of multiple rollers parallelism in rolling industries, laser divergence angle, and spatial stability.

C. Hybrid Autocollimator

An autocollimator was invented as an optical instrument several decades ago for precise, non-contact angle measurements. Since its inception, it has been used extensively in the alignment of angles and optical elements. Recent advances in photonics have necessitated optics and laser alignment and measurement, which the new hybrid technology addresses.

Furthermore, by focusing on the area to be studied and performing alignment and deviations from alignment measurements in microns, modern motorized focusing technology adds a new measurement dimension. This multi-function-multi-process hybrid optical equipment will be used to measure highly integrated systems during assembly as well as final testing and examination.

Hybrid technologies can meet the needs of a wide range of technologies, such as laser alignment, spatial characterization, and beam profiling of multiple single emitters. This analysis procedure generates angle-dependent spatial resolution patterns of light sources in relation to a mechanical datum plane, which is an excellent solution for accurately and quickly testing VCSEL lasers.

Applications of Autocollimator

  1. Direct testing of machine tool slides.
  2. Measuring very small angles.
  3. Checking for similarities.
  4. Checking the base of the column.
  5. Checking the flatness of bed plates and surface tables.
  6. Measuring very small displacement.
  7. Checking for smaller linear displacement.

Advantages of Autocollimator

  • It has very high accuracy.
  • It can measure a wide range of angles.
  • It is very easy to install and operate.
  • Calibration surpassing international standards.
  • It can be used to visually or electronically view the result, i.e., on the computer screen.
  • A wide range of available accessories and levels.

Disadvantages of Autocollimator

  • Maintenance is required regularly.
  • It is time-consuming.
  • It requires sample cutting and processing for tracing by the detector.

We have tried to cover all the topics related to Autocollimator, from definition to advantages, disadvantages, types and working principle. If you liked the article , Please share it with your friends.

Frequently Asked Questions

What is Autocollimation method?

In autocollimation, a collimated beam (of parallel light rays) leaves an optical system and is reflected back into it by a plane mirror. It’s used to determine the minor tilting angles of the mirror.

Who invented the autocollimator?

The first Nikon autocollimator-built in 1942

In 1942, Nikon (then known as Nippon Kogaku K.K.) completed work on an autocollimator that gave readings accurate to one minute of arc.

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