Lighting system (optics)

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Illumination system of a slide projector :
left: spherical mirror to double the amount of light;
right: condenser (2 lenses, heat protection filter in between); Dia (not shown; its holder is usually directly on the right condenser lens)

An illumination system is used in optical devices for bundling of a light source derived luminous flux and for the uniform illumination of objects to be imaged.

It is part of such imaging optical systems, essentially a projection device or a microscope , the imaging optical system, in addition to a light source, being preceded by illumination optics. The task of the lighting optics is

  • to be able to use as much light as possible from a relatively small luminous area for the imaging process and
  • to illuminate the object as evenly as possible.

In addition to the lamp ( illuminant ), the condenser is an essential part of a lighting system, which is therefore often only referred to as a condenser .

Light sources are z. B. a carbon arc lamp (out of date), a halogen lamp or a high pressure gas discharge lamp . If the lamp shines in the entire solid angle, there is often a concave mirror on the back , with the help of which the light output can be roughly doubled. The light source is mapped onto itself. In the case of lamps with incandescent filaments , the mirror image is offset a little laterally so that the filament wires of the original and mirror image do not lie on top of one another. The light spot becomes more evenly bright.

A so-called interwoven beam path is created between the illumination and the imaging optics, which is the common characteristic of optical imaging devices with an artificial light source: In an interwoven beam path, the hatches of the illumination beam path coincide with the pupils of the imaging beam path .

In the case of a projector , for example, the edge of the luminous surface of the lamp is already the limit for the light beam (small aperture angle) with which the original ( slide ) is displayed. Because the illuminated area is very small, the projection would be too faint. It is therefore imaged in the projection lens with the aid of a condenser as a field lens , where it acts as an enlarged entrance pupil. It is the exit hatch for the light optics with condenser.

In order to reduce the thermal load on the illuminated objects, additional heat protection filters can be installed in the beam path, which if necessary can also be cooled by an air stream. Projectors are increasingly being built with light-emitting diodes as a light source, which are characterized by relatively low thermal radiation .

Koehler illumination beam path

See also the section Köhler illumination in the article Brightfield microscopy

In the case of a microscope, Koehler's illumination device is mostly used, two illuminating and one imaging beam path being interwoven . A collector lens is also connected upstream of the condenser. The light source is further away from the object to be imaged, which means that it is less heated. The collector images the light source in front of the condenser in its focal plane. There is an adjustable aperture diaphragm with which the illumination angle of the object to be imaged can be set and adapted to the numerical aperture of the objective. There is another adjustable aperture at the focal length behind the collector. It is called the field diaphragm and is imaged in the object plane with the help of the condenser. As a result, the illuminated area there can be limited to the image field of the lens and thus disruptive surrounding brightness can be kept away (scattered light). The field diaphragm on the collector is the entrance pupil of the illumination system and the entrance hatch of the imaging system.

Individual evidence

  1. ↑ Heinz Haferkorn: Optics - physical-technical basics and applications , Barth, Leipzig, 1994, ISBN 3-335-00363-2 , p. 618
  2. ^ Dieter Meschede: Gerthsen Physics . Springer DE, 2003, ISBN 3-540-02622-3 , pp. 496 ( limited preview in Google Book search).
  3. ↑ Ludwig Bergmann , Clemens Schaefer : Textbook of Experimental Physics , Volume III: Optics , Chapter I, 12: Radiation Optics - The Eye and Some Optical Instruments , 7th Edition, Verlag Walter de Gruyter, Berlin / New York, 1978, page 158/159
  4. ↑ Dietrich Kühlke: Optics - Basics and Applications , Harri Deutsch, Frankfurt / Main, 2011, ISBN 978-3-8171-1878-6 , p. 151
  5. ↑ Dietrich Kühlke: Optics . Harri Deutsch Verlag, 2004, ISBN 978-3-8171-1741-3 , pp. 154 ( limited preview in Google Book search).
  6. ↑ a b Dietrich Kühlke: Optics . Harri Deutsch Verlag, 2004, ISBN 978-3-8171-1741-3 , pp. 92 ( limited preview in Google Book search).
  7. ↑ Dietrich Kühlke: Optics - Basics and Applications , Harri Deutsch, Frankfurt / Main, 2011, ISBN 978-3-8171-1878-6 , p. 160
  8. ↑ Gottfried Schröder: Technical optics , Chapter 6.2.1: Overview of lighting systems , page 114, 5th edition, Vogel-Buchverlag, Würzburg, 1986
  9. ↑ LED beamer: Competition for LCD and DLP projectors , chip.de, accessed online on October 15, 2012
  10. ↑ Dietrich Kühlke: Optics . Harri Deutsch Verlag, 2004, ISBN 978-3-8171-1741-3 , pp. 145 ( limited preview in Google Book search).