The Candela

Unit for
Luminous Intensity

"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 X 10^12 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian (16th CGPM (1979), Resolution 3)."

The International System of Units (SI)
NIST Special Publication 330, 1991 Edition

This 1991 definition replaced the earlier (1967) one in which the definition of the candela was based on a Planckian radiator at the temperature of freezing platinum (1772 degrees C, 2045 kelvins).

The frequency 540 X 10^12 Hz corresponds to a wavelength of 555 nm.

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