Luminous flux is a quantitative expression of the brilliance of a source of visible light, which is electromagnetic energy within the wavelength range of approximately 390 nanometer s (nm) to 770 nm. This quantity is measured in terms of the power emitted per unit solid angle from an isotropic radiator, a theoretical point source that radiates equally in all directions in three-dimensional space.
The standard unit of luminous flux is the lumen (lm). Reduced to base units in the International System of Units (SI), 1 lm is equivalent to 1 candela steradian (cd · sr). This is the same as 1.46 milliwatt (1.46 x 10 -3 W) of radiant power at a wavelength of 555 nm, which lies in the middle of the visible spectrum.