Resolution 3 of the 16th CGPM (1979)

SI unit of luminous intensity (candela)

The 16th Conférence Générale des Poids et Mesures (CGPM),

considering

• that despite the notable efforts of some laboratories there remain excessive divergences between the results of realizations of the candela based upon the present black body primary standard,
• that radiometric techniques are developing rapidly, allowing precisions that are already equivalent to those of photometry and that these techniques are already in use in national laboratories to realize the candela without having to construct a black body,
• that the relation between luminous quantities of photometry and radiometric quantities, namely the value of 683 lumens per watt for the spectral luminous efficacy of monochromatic radiation of frequency 540 x 10¹² hertz, has been adopted by the Comité International des Poids et Mesures (CIPM) in 1977,
• that this value has been accepted as being sufficiently accurate for the system of luminous photopic quantities, that it implies a change of only about 3 % for the system of luminous scotopic quantities, and that it therefore ensures satisfactory continuity,
• that the time has come to give the candela a definition that will allow an improvement in both the ease of realization and the precision of photometric standards, and that applies to both photopic and scotopic photometric quantities and to quantities yet to be defined in the mesopic field,

decides

1. 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.
2. The definition of the candela (at the time called new candle) adopted by the CIPM in 1946 by reason of the powers conferred by the 8th CGPM in 1933, ratified by the 9th CGPM in 1948, then amended by the 13th CGPM in 1967, is abrogated.

DOI : 10.59161/CGPM1979RES3E

NOTE

Photopic vision is detected by the cones on the retina of the eye, which are sensitive to a high level of luminance (L > ca. 10 cd/m²) and are used in daytime vision. Scotopic vision is detected by the rods of the retina, which are sensitive to low level luminance (L < ca. 10^–3 cd/m²), used in night vision. In the domain between these levels of luminance both cones and rods are used, and this is described as mesopic vision.

This definition of the candela was abrogated in 2018 by the 26th CGPM (Resolution 1), which became effective in 2019.