Koshel, R. John Photon Engineering, LLC, Tucson, Arizona.
Last reviewed:November 2019
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The antiquated term for candela (1 cd = 1 lumen/ steradian), which is the unit of measure for luminous intensity. Candlepower (cp) is derived from comparison to the light produced from a standard candle of one-sixth pound (∼75.6 g), pure spermaceti burning at a rate of 120 grains per hour (∼7.776 g/h or 2.16 mg/s). The candela is a fundamental unit of the International System of Units (abbreviated SI due to the French form of Système International d'Unités), and it has the following definition: The candela [candlepower] is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of 540 × 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian. See also: Luminous intensity; Metric system; Physical measurement; Units of measurement
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