POPULATION INVERSION and LUMINOUS FLUX:

POPULATION INVERSION: The process in which number of electrons (N ) in a higher2 energy state called metastable state of an active medium is increased to a value greater than the number (N ) in the ground state (i.e. N  > N ) is called population inversion.

LUMINOUS FLUX : The amount of radiant (light) energy following from a source of light per unit time is called luminous flux or light flux (Φ) . It is measured in lumens.

LUMINOUS INTENSITY : Luminous intensity of a point source of light is defined as the luminous flux emitted from it per unit solid angle.

It is otherwise called Illuminating power (I). It is measured in candela (cd).