Candela.
A unit of light intensity - luminous flux.
The technical definition of a candela is complex - it’s the luminous intensity of a light source which produces monochromatic light at a frequency of 540 terahertz (ie: a wavelength of about 550 nm; the frequency of light, sort of yellowish-green, to which the human eye is most responsive) with an intensity in one direction of 1/683 watt per steradian.
This last bit is to ensure that a candela is in keeping with the old definition, which referred to the light produced by a certain area of glowing molten platinum. A steradian (sr) is a metric unit for solid angular measurements. It’s equivalent to a cone originating from a central point (the light source) and covering one square metre of surface area on the interior of an imaginary sphere one metre in diameter that’s drawn around the light source. A sphere contains 4 x pi steradians or about 12.57 steradians.
Candelas themselves are not commonly used in light measurement but form the basis for other units, such as foot-candle and lumen.
cf. foot-candle, lumen.
Entry last updated 2002-04-03. Term 182 of 1487.
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