Brightness

Definition: a term mostly used in a qualitative way, related to the output power and beam quality of a laser; quantitatively: often used instead of radiance or luminance

German: Helligkeit, Leuchtdichte

Category: general optics

Author: Dr. Rüdiger Paschotta

Cite the article using its DOI: https://doi.org/10.61835/csr

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The term brightness should be used only for non-quantitative references, e.g. in the context of physiological sensations; that is recommended by the U.S. Federal Standard 1037C, for example. For actual quantitative references, one should usually use one of the following terms:

The radiance is defined as optical power (radiant flux) per unit area and solid angle; its units are W cm⁻² sr⁻¹. This quantity is used in radiometry, where the physical properties of light and not its visual perception are relevant.
The luminance is the luminous flux per unit area and solid angle, with units of candela per square meter (cd/m²). This is a quantity of photometry, where the spectral response of the human eye is taken into account.

Unfortunately, the term brightness is often somewhat inaccurately used instead of radiance or luminance.

In laser technology, the term brightness is often said to be higher for one type of laser than for another, for example, but rarely with an actual quantitative specification (some numbers and units). What is usually meant is the clearly defined term radiance. That implies e.g. that the brightness of a laser is increased if its beam quality is improved for a fixed output power level.

The term also occurs as part of composite terms:

Brightness converters are essentially devices which receive optical radiation and emit radiation with a higher radiance. That is not possible with passive optical elements, but e.g. with certain optically pumped lasers.
High brightness laser diodes are laser diodes which are optimized for a particularly high radiance (brightness).

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