The luminous intensity would still be one candela in those directions that are not obscured. If the source were partly covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux-only 2π lumens. If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, the total luminous flux emitted into that angle is one lumen (1 cd♱ sr = 1 lm).Īlternatively, an isotropic one-candela light-source emits a total luminous flux of exactly 4π lumens. The lumen is defined in relation to the candela asĪ full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux ofġ cd × 4π sr = 4π cd⋅sr ≈ 12.57 lm. Luminous flux differs from power ( radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a ' luminosity function') of the human eye's sensitivity to various wavelengths, this weighting is standardized by the CIE and ISO. The lumen (symbol: lm) is the unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time, in the International System of Units (SI).
For other uses, see Lumen (disambiguation).
