Definition: Source; Light source

Definition: Source; Light source | Glossary entry

Source; Light source

The origin of light is of critical importance to photographers. This is because the quality of the light is the result of how it is created. All light is emitted by a ‘source’. There are many sources of light. Most commonly light is produced by a heat source or ‘thermal’ source. One thermal source we are all familiar with is the Sun.

All light is a type of radiation. It is measured in wavelength units. Not all wavelengths are visible to the human eye, or to the camera digital image sensor. The light we can see with our eyes is known as visible light. Some organisms can see wavelengths that humans are unable to see. In particular infra red light and ultraviolet light are invisible to us. Flames appear to be light sources. In fact they contain solid particles that are glowing whilst they burn – which is in fact the source of the light (and heat).

Any light source is made up of different wavelengths of light. In the case of sunlight only about 44% of the light is visible. Incandescent light bulbs (bulbs with a filament) produce only 10% visible light. The rest of their energy is given off in the infra red range (which makes them very hot).

A visible light source has a colour according to its ‘colour temperature’ External link - opens new tab/page This is measured in a temperature scale called Kelvins. In very general terms, colour temperatures higher than 5,000 Kelins are ‘cool’ colours and tend toward blueish white. Colour temperatures in the range 2,700–3,000 Kelvins are ‘warm’ colours which exhibit yellowish white through red hue.

Light is emitted by the source in all directions. However, it is possible to direct the light so that the source is radiating only in the required direction. To do that materials are used that do not allow the light to penetrate or reflectors which redirect the light.

Every light source has five properties affecting the quality of its light. All are of interest to photographers.

  • Hard light or Soft light (or of the range in between);
  • Intensity – the light brightness;
  • Direction of travel;
  • Colour;
  • Polarisation.

Further discussion of these properties can be found in the article: The Properties of Light.

In practical terms there are a large number of different light sources. In broad terms these may be categorised into the following:

  • Astronomical sources (the sun, the stars, events like explosions)
  • Atmospheric sources (like lighting)
  • Chemical sources (chemical reactions of various kinds)
  • Bioluminescence (light created by organisms like fireflies)
  • Geological (volcanos etc)
  • Artificial lights (man made sources like electric lights, oil lamps etc)

A comprehensive list of light sources can be found on Wikipedia  External link - opens new tab/page

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