Luminous, Illuminated and Incandescent Body
   A luminous body emits light of its own. An incandescent body emits light when it is heated. And an illuminated body is visible because of reflecting light. The sun is luminous and the moon is illuminated by the sunlight. The fine tungsten wire in the electric bulb is incandescent.

Luminous Flux
   Radiant flux is the energy radiated in unit
time by an electromagnetic wave source. The
part of the radiant flux producing illumination
is called the luminous flux, P,  with
a unit called lumen (lm).
 
   Illuminance E is the luminous flux on a unit
area measured in lm/m2, or lux.
   A 100-W incandescent light bulb emits 1750 lm,
falling on the spherical surface surrounding it.
   Since the surface area of a sphere is 4pr2, the
illumination by the light bulb on the surrounding
spherical surface of radius 1 m is 1750 / 4p lux.