Our skin, for example, is sensitive to light across the solar spectrum, from UV to infrared, as is almost every other biological system on Earth. Quantification of light is not only important for visual perception of sources. These include illuminance, measured in lux, which describes how bright a surface illuminated by a source will be, and luminous flux, measured in lumens, which quantifies the amount of visible light emitted by a source.

While the candela describes how bright a source of light will be when looking directly at it from a particular direction, other photometric quantities are sometimes more useful.

The candela, however, is not the only unit that a photometry and radiometry laboratory realises and disseminates. The relative response of the eye to visible wavelengths, also known as the photopic response curve, is a tabulated function adopted by the Commission Internationale de l’Eclairage (CIE) in 1924. Together, the contributions of the CIPM and CIE allow the luminous intensity of any source to be uniquely specified. “The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.” The definition of the standard source is given by the International Committee for Weights and Measures (CIPM): Its definition therefore requires knowledge of the spectral sensitivity of the human visual system, as well as a standard reference light source. It allows us to measure the effectiveness of lighting systems made for the human eye. The candela is the only SI base unit intended to quantify the human perception of an objective physical reality. The SI unit for luminous intensity (the brightness of a source of visible light) is the candela (cd), which gets its name from the fact that, for much of history, ‘standard candles' were used to measure the intensity of light.

Luminous intensity unit