Brightness

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).

In the context of imaging optics, brightness refers to the light-gathering power of an optical system and, more precisely, to how much optical power from a scene is delivered to the image plane per unit area and solid angle. It is a descriptive or qualitative term, not a single, formally defined quantitative quantity with fixed units.

Brightness is determined primarily by the aperture size of the system and its f-number (f/#). For an ideal lossless lens imaging an extended scene, the image brightness is inversely proportional to the square of the f-number: Systems with smaller f-numbers (wider apertures) produce brighter images. Importantly, for extended sources this brightness is largely independent of focal length; changing focal length alters image scale, not the brightness per unit image area, provided the f-number remains constant.

A fundamental constraint on optical brightness is the conservation of étendue (also known as throughput or optical invariant). Étendue limits how much light can be concentrated by passive optical elements, ensuring that no optical system can increase the intrinsic brightness of an extended source beyond that set by the source itself. As a result, lenses and mirrors can redistribute light spatially but cannot amplify brightness.

In practical systems, real-world factors reduce brightness relative to the ideal case. These include transmission losses from absorption and scattering in optical materials, vignetting that reduces effective aperture off-axis, and obscurations (such as secondary mirrors in reflecting telescopes). In imaging devices with image sensors, perceived or recorded brightness also depends on exposure time, detector sensitivity and quantum efficiency, though these do not change the optical brightness delivered to the image plane.