Radiant intensity is a quantitative expression of the brilliance of a source of electromagnetic (EM) energy over the entire electromagnetic spectrum . This quantity is measured in terms of the power emitted per unit solid angle from an isotropic radiator, a theoretical point source that radiates equally in all directions in three-dimensional space.
The standard unit of radiant intensity is the watt per steradian (W/sr or W ? sr -1 ). For a theoretical point source, 1 W/sr is equivalent to 4 p (approximately 12.5664) W of total radiated power at all wavelengths.