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Collimator for parallel light beam.
A pinhole diaphragm (B) is used as a point light source.

A collimator is used to generate light with an approximately parallel beam path from a divergent source. This collimation is often used to give the light a certain direction. Condensers and lighting systems often consist of a collimator followed by a lens .

In technical optics , scales are imaged at infinity with a collimator. They then overlay the image of the object and allow its dimensions to be determined.

Visible light collimator

Collimator for parallel light bundles in front of a prism spectrometer
Angle measurement with an autocollimator

When using visible light, a collimator is generally used in technical optics on the one hand, and on the other hand in particular:

  • In principle, the light from a more or less point-shaped source is converted into a parallel bundle of rays by means of a converging lens L (see figure above). The light source is arranged in the front focal plane of the lens. An example is the collimator lens connected upstream of the dispersion prism of a prism spectrometer (see adjacent figure).
  • In particular, there is an illuminated measuring scale ( e.g. a reticle ) in the front focal plane of the lens , which is mapped to infinity (∞) by means of a parallel beam path after the lens. Such an artificial target at infinite distance is suitable for angle measurements because it is insensitive to parallel displacement of the collimator. If the measuring scale is in the focal plane of a telescope lens, an auxiliary device is created for testing and adjusting optical instruments. The connection of a collimator with its own lens with a telescope in front of it allows a wide range of applications for metrological tasks, in particular for direction and angle determination. A special collimator is the autocollimator (or autocollimation telescope ), in which the light from the rotating measuring mirror is reflected back to its starting point (see adjacent figure). The sensitivity in determining the direction and angle is twice as great as with the combination of collimator and telescope.

The focal length of a lens is defined for an image from infinity. With the help of a collimator, a measuring object located at a finite distance can be imaged from infinity with an objective to be examined, whereby the outer main planes of the objective can also be determined in addition to the focal length .

The light source / aperture / measuring scale and lens are often surrounded by an internally blackened tube (tube) to keep stray light away. To reduce aberrations , either an aspherical lens or a system of several lenses can be used.

The collector lens in an illumination device for transmitted light microscopes has a similar function to the collimator . The light, which is initially parallel to the collector (collimated), is then focused with a condenser lens in the objective.

Areas of application

Collimators are used, among other things, for imaging in astronomy and medicine, e.g. B. as a multi-leaf collimator in radiation therapy . Collimators are also used in radiation detectors where a pronounced preferred direction is required. With the help of autocollimators , exact angle measurements can be made.

In the field of X-ray optics , collimators such as the collimator aperture are used to control X-ray radiation , which are not (only) based on absorption, but on grazing reflection. In medical recordings, a collimator ( bucky screen ) helps to eliminate scattered radiation. So that the structure of the collimator is not shown, it can also be moved back and forth during the exposure time, which can be heard and felt as rumbling on the pressed body.

In the military sector, collimators are used in reflex sights to make aiming firearms easier. Also, head-up displays for displaying information in the visual range of pilots and nowadays also in civilian cars contain collimators.


  • Friedrich Kohlrausch: Practical Physics, Volume 2, 24. neubearb. u. exp. Ed., 1996, ISBN 3-519-23002-X
Chapter 6.1 Geometric optics (ray optics)
Chapter 7.2 Radiation Sources, Reference Radiations
  • Bergmann, Schaefer: Textbook d. Experimentalphysik, Volume 3 Optics, 10th edition, 2004, ISBN 978-3-11-017081-8
  • Max Born: Optik, 3rd edition, 1972, ISBN 3-540-05954-7
  • OKW: Regulation D 250 - directional circle collimator 12 m - 1942

Individual evidence

  1. Dietrich Kühlke: Optics - Basics and Applications . Harri Deutsch, Frankfurt / Main 2011, ISBN 978-3-8171-1878-6 , pp. 157 .
  2. Fritz Hodam: Technical Optics . Technology, Berlin, 1967, p. 190-191 .
  3. Fritz Hodam: Technical Optics . Technology, Berlin, 1967, p. 195 .
  4. Bernd Leuschner: Determining the focal length . Ed .: Laboratory for device technology, optics and sensors, Beuth University of Applied Sciences Berlin. ( online, PDF, 134 kB ).