The unit of luminous intensity I is the candela (cd) also known as the international candle. The intensity of a light source is commonly referred to as its candlepower.
The unit of luminous flux F is the lumen (lm). One lumen is equal to the luminous flux which falls on each square meter (m2) of a sphere one meter (1m) in radius when a 1-candela isotropic light source (one that radiates equally in all directions) is at the center of the sphere. Since the area of a sphere of radius r is 4πr2, a sphere whose radius is 1m has 4πm2 of area, and the total luminous flux emitted by a 1-cd source is therefore 4π1m.
Thus the luminous flux emitted by an isotropic light source of intensity I is given by:
F = 4πI where
Luminous flux (lm) = 4π × luminous intensity (cd)