Radiant intensity can be defined as the amount of power emitted by a source in a particular direction per unit solid angle. In simple terms, it is the amount of the intensity of electromagnetic radiation.
Radiant intensity is represented by Ie and expressed in W/Sr-1. Consider the power (P) being emitted from a source which is directed in a particular direction along the center of a cone that encompasses the solid angle, the radiant intensity is equal to the emitted power. The solid angle represents the size of the cone intensity emitted by the light source.
Radiant intensity can be visualized by imagining an infinitely small point source of light that is emitting a radiant flux in all directions. The radiant intensity of the source can be determined by considering the amount of radiant flux that is emitted when represented in the form of a ray of light contained in a cone. Radiant intensity is sometimes referred to as radiation intensity in RF engineering studies.