What is a discontinuous spectrum

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Continuous and discontinuous spectra and the color rendering index

Most light sources, including the sun, are thermal radiators, which means that the light is generated via the detour of heat. Color temperature and brightness are directly related to the operating temperature. They emit a continuous color spectrum that only changes its red-blue weighting at different color temperatures. Depending on the temperature, the proportion of green light is between 30 and 34%.

Discharge lamps generate their light according to the principle of gas discharge: the electrons of a gas mixture are excited to glow with the help of electrical current. Depending on the composition of the gas, the lamps emit light of different spectral compositions. It is therefore possible that a lamp emits white light to the eye, but important areas are missing in the color spectrum or are represented over-dimensionally. One then speaks of a discontinuous spectrum. Uncorrected recordings under such lighting conditions often show a color cast that is difficult to predict.


Manufacturers of discharge lamps indicate a measure of color rendering quality in their data sheets: the color rendering index. A value of 100 corresponds to an absolutely neutral color rendering that can only be achieved by the continuous spectra of the temperature radiators. High-quality discharge lamps for photographic purposes achieve values ​​between 90 and 96, depending on their age.Standard fluorescent lamps, such as those used for factory hall lighting, often only achieve a color rendering index of 60. The yellow sodium vapor light that we illuminate is much lower Buildings or street lighting.

© Stefan Neudeck
www.filmtechnik-online.de December 24, 2001