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Photometry and Radiometry: Photometric units
Photometric units
SI Brochure, Section

    The units of luminous intensity based on flame or incandescent filament standards in use in various countries before 1948 were replaced initially by the "new candle" based on the luminance of a Planck radiator (a black body) at the temperature of freezing platinum. This modification had been prepared by the International Commission on Illumination (CIE) and by the CIPM before 1937, and the decision was promulgated by the CIPM in 1946. It was then ratified in 1948 by the 9th CGPM which adopted a new international name for this unit, the candela, symbol cd; in 1967 the 13th CGPM (Resolution 5) gave an amended version of this definition.

    In 1979, because of the difficulties in realizing a Planck radiator at high temperatures, and the new possibilities offered by radiometry, i.e. the measurement of optical radiation power, the 16th CGPM (1979, Resolution 3) adopted a new definition of the candela:

      The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

    It follows that the spectral luminous efficacy for monochromatic radiation of frequency of 540 x 1012 hertz is exactly 683 lumens per watt, K = 683 lm/W = 683 cd sr/W.

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Decisions relating to the base units of the SI
SI Brochure, from Appendix 1

    Luminous intensity:

    CIPM, 1946 definitions of photometric units, new candle and new lumen
    13th CGPM, 1967/68 defines the candela, symbol cd, in terms of a black body
    16th CGPM, 1979 redefines the candela in terms of monochromatic radiation
    24th CGPM, 2011 on the possible future revision of the International System of Units, the SI
Photometry and Radiometry
Photometric units
Committee structure
Strategic plan
International equivalence
Calibration and measurement capabilities (CMCs)
International comparisons
At the BIPM
BIPM liaison and coordination work
BIPM contact person