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Photometry and Radiometry: Photometric units

Definition agreed by the 26th CGPM (November 2018), implemented 20 May 2019:


    The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 x 1012 Hz, Kcd, to be 683 when expressed in the unit lm W–1, which is equal to cd sr W–1, or cd sr kg–1 m–2 s3, where the kilogram, metre and second are defined in terms of h, c and DeltanuCs.

    This definition implies the exact relation Kcd = 683 cd sr kg–1 m–2 s3 for monochromatic radiation of frequency nu = 540 x 1012 Hz. Inverting this relation gives an exact expression for the candela in terms of the defining constants Kcd, h and DeltanuCs:

    which is equal to

    The effect of this definition is that one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 Hz and has a radiant intensity in that direction of (1/683) W/sr.

    The units of luminous intensity, which were 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 Planckian 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 1954 the 10th CGPM established the candela as a base unit; In 1967 the 13th CGPM amended 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 adopted a new definition of the candela.

    The present definition of the candela uses a fixed numerical value for the luminous efficacy of monochromatic radiation of frequency 540 ×1012 Hz, Kcd, adopted in Resolution 1 of the 26th CGPM (2018).


See more: History of the SI