Candela
Candela (cd)
Candela (pronounced /kænˈdiːlə/; symbol: cd) is the SI base unit of luminous intensity. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr−1 kg−1 m2 s3, where the kilogram, meter and second are defined in terms of h, c and ΔνCs.
Etymology
The term candela means candle in Latin and Spanish. The unit was formerly defined as the light emitted by one candle of specific size and constituents. In 1948, the 9th CGPM redefined the unit in terms of the luminous intensity of a black body, and again in 1979 in terms of a specified frequency of monochromatic light.
Related Terms
- Lumen: The lumen is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time. Lumen is related to candela as 1 lm = 1 cd⋅sr.
- Lux: Lux is the SI derived unit of illuminance, measuring luminous flux per unit area. It is equal to one lumen per square metre.
- Foot-candle: A non-SI unit of illuminance. One foot-candle is approximately 10.764 lux, or the light emitted by one candela over a square foot.
External links
- Medical encyclopedia article on Candela
- Wikipedia's article - Candela
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