The lumen is the luminous flux emitted
into a solid angle of 1 steradian from a source which
has an intensity of 1 candela. In other words, 1 candela
= 1 lumen/steradian.
The lumen is a weighted value of watts. The luminous flux
of a source is calculated by integrating the product of
the spectral flux (power per unit wavlength band) and
the CIE V-lambda curve. The integrated value, or area
under the curve represents the total lumens from this
source.