[5][6] It measures luminous power per unit solid angle emitted by a light source in a particular direction.

Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the luminous efficiency function, the model of the sensitivity of the human eye to different wavelengths, standardized by the CIE and ISO.

[7][4][8] A common wax candle emits light with a luminous intensity of roughly one candela.

[9] The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018.

[10][11] The new definition, which took effect on 20 May 2019, is: The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×1012 Hz,[a] Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.

[12]The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres.

The human eye, when adapted for bright conditions, is most sensitive near this frequency.

One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour.

Germany, Austria and Scandinavia used the Hefnerkerze, a unit based on the output of a Hefner lamp.

The resulting unit of intensity, called the "violle", was roughly equal to 60 English candlepower.

Platinum was convenient for this purpose because it had a high enough melting point, was not prone to oxidation, and could be obtained in pure form.

In practice, realizing a standard based on Violle's proposal turned out to be more difficult than expected.

Over the following half century various scientists tried to make a practical intensity standard based on incandescent platinum.

The successful approach was to suspend a hollow shell of thorium dioxide with a small hole in it in a bath of molten platinum.

The shell (cavity) serves as a black body, producing black-body radiation that depends on the temperature and is not sensitive to details of how the device is constructed.

In 1937, the Commission Internationale de l'Éclairage (International Commission on Illumination) and the CIPM proposed a "new candle" based on this concept, with value chosen to make it similar to the earlier unit candlepower.

In 1967 the 13th CGPM removed the term "new candle" and gave an amended version of the candela definition, specifying the atmospheric pressure applied to the freezing platinum: The candela is the luminous intensity, in the perpendicular direction, of a surface of 1 / 600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre.

[17]In 1979, because of the difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by radiometry, the 16th CGPM adopted a new definition of the candela:[18][19] The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of ⁠1/683⁠ watt per steradian.

An appendix to the SI Brochure[20] makes it clear that the luminous efficiency function is not uniquely specified, but must be selected to fully define the candela.

If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4π: a uniform 1 candela source emits 4π lumens (approximately 12.566 lumens).

If ri is the position of the ith source of uniform intensity Ii, and â is the unit vector normal to the illuminated elemental opaque area dA being measured, and provided that all light sources lie in the same half-space divided by the plane of this area,

In the case of a single point light source of intensity Iv, at a distance r and normally incident, this reduces to