Candela
The candela (cd) is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of a frequency 540·1012 hertz and has a radiant intensity in that direction of 1/683 watt per steradian.
The measurement of light per se needs no special units since light is nothing more than electromagnetic radiation and consequently can be expressed in terms of units already known. However, given that the human sense of sight carries such a great importance, there is nevertheless general agreement that the subjective effect of electromagnetic radiation on the human visual organ should have its own units, and, for historical reasons, its own base unit. Photometric units measure light not only for its physical nature, but also in terms of the sensitivity of the human visual organ.
The sensitivity to brightness of the human eye depends on the wavelength, or colour, of the light, and demonstrates in this slight differences from individual to individual. If we want to measure light with physical instruments, as is common today in photometry, we require a measure of the subjective perception of brightness of the individual colour components. This measurement scale gives rise to the so-called spectral luminous efficiency function V(λ) , whose values for the wavelengths from 360 to 830 nm have been determined on an international basis from measurements on a large number of test persons.
At METAS, the candela is represented by a group of calibrated illuminance meters. These instruments consist of silicon detectors with 100 % quantum efficiency, and a colour filter for adaptation to V(λ) .