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The candela is the SI unit of luminous intensity. It is defined since 1979 as 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]

An earlier definition, adopted in 1967, clarifying a definition adopted in 1946, was the luminous intensity, in the perpendicular direction, of a surface of 1⁄600,000th square meter of a blackbody at the temperature of freezing platinum under a pressure of 101,325 newtons per square meter [2042 K].

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[edit] References

  1. International Bureau of Weights and Measures From the website of the International Bureau of Weights and Measures.
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