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)

The illumination (or illuminance) E of a surface is the luminous flux per unit area that reaches the surface:

E  =  F/A Illumination   =   luminous flux / area

lumens = 4π × cd                             cd = lumens
                                                                  4π 

lumens  =  4π  ×  (Mean Spherical Candlepower) 

fc = cd                                           cd = fc × d²
       d²

lumens = fc  4π× d²                      fc = lumens
                                                            4π× d²

Inverse Square Law:   E =   I
                                              d²

Cosine Law:                             E =  I cosq
wheree q is angle of incidence                d²