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The unit of luminous intensity in SI, one of the base units. Roughly speaking, it is used to express how bright a beam of light is. Symbol, cd. Since October 1979, the candela has been “the luminous intensity, in a given direction, of a [light] 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.”¹ The candela is the only SI unit based on human perception, namely stimulation of the human eye. It is also the least certain, having an uncertainty of about half a per cent.

See also luminance.

History of the candela

In the 19th century units of luminous intensity were based on the brightness of particular artificial light sources (candle, Hefner unit, carcel, bougie de l'étoile, etc.). Measurements made in this units are not very reproducible. Much depends on the apparatus, the way it is manipulated, the composition of the fuel, and even such factors as the temperature and humidity in the room.

Scientists wished to define a unit that was more purely related to unvarying natural phenomena. The French physicist Violle was the leader in this work, basing his standard on the brightness of the surface of molten platinum at the temperature at which it freezes. In 1889 the Second International Electrical Congress based its definition of a new unit, the decimal candle, on Violle's standard (1 decimal candle = ¹⁄₂₀th of a violle). But just 7 years later the International Electrotechnical Congress redefined the decimal candle as equal to the Hefner unit, a unit based on an oil-burning lamp. Molten platinum is very difficult to work with.

By 1937 the International Commission on Illumination (CIE) and the CIPM had framed a definition of a new unit to be called the “new candle” (in French, bougie nouvelle), based on the brightness of freezing platinum. Because of World War II the CIPM did not promulgate its definition until 1946 (“the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimeter”²).

In 1948 this definition was ratified by the Ninth CGPM, but it changed the name “new candle” to “candela” to avoid confusion with the wealth of preexisting candles.³ Thus the preceding definition became the first definition of the candela.

In 1967 the 13th CGPM altered the definition to correct possible ambiguities, and the candela became “the luminous intensity, in the perpendicular direction, of a surface of 1⁄600,000th square meter of a black-body at the temperature of freezing platinum [2042°F] under a pressure of 101,325 newtons per square meter.”⁴

The 1967 definition was eventually discarded because it was too hard to realize in practice. While the technology of photometry was struggling with molten platinum at 2042°F, great advances were being made in radiometry. In 1977, the CIPM adopted a definition relating photometric and radiometric quantities: the spectral luminous efficacy of monochromatic radiation of frequency 540 × 1012 hertz would, by definition, be 683 lumens per watt.

In October 1979, noting that “despite the notable efforts of some laboratories there remain excessive divergences between the results of realizations of the candela based upon the present blackbody primary standard” (i.e., some people's candelas were bigger than other people's), the 16th CGPM (Resolution 3) turned the 1977 equivalence around, adopting the definition of the candela first given above.

1. Resolution 3 of the 16th CGPM, 1979, (Comptes Rendu, page 100).

2. CIPM, 1946 (Proces-Verbaux vol. 20, pages 119-122.

3. 9th CGPM, 1948 (CR, page 54). The 13th CGPM (resolution 7) also formally deleted the name “new candle” from the list of approved unit names in resolution 7 of the 9th CGPM.

4. Resolution 5 of the 13th CGPM, 1967-1968 (CR, page 104) 

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