The candela (cd) is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction. The units of luminous intensity based on flame or incandescent filament standards in use in various countries before 1948 were replaced initially by the “new candle” based on the luminance of a Planck radiator (a black body) at the temperature of freezing platinum.

This modification had been prepared by the International Commission on Illumination (CIE) and by the CIPM before 1937, and the decision was promulgated by the CIPM in 1946. It was then ratified in 1948 by the 9th CGPM which adopted a new international name for this unit, the candela, symbol cd; in 1967 the 13th CGPM gave an amended version of this definition.

In 1979, because of the difficulties in realizing a Planck radiator at high temperatures, and the new possibilities offered by radiometry, i.e. the measurement of optical radiation power, the 16th CGPM (1979) adopted a new definition of the candela:

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency (540 cdot 10^{12}) Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

It follows that the spectral luminous efficacy for monochromatic radiation of frequency of \(540 \cdot 10^{12}\) hertz is exactly \(683\) lumens per watt, \(K(\lambda_{555}) = 683\) lm/W = \(683\) cd sr/W (the wavelength \(\lambda\) of radiation of this frequency is about 555 nm).

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