Radiant intensity

In radiometry, radiant intensity is the radiant flux emitted, reflected, transmitted or received, per unit solid angle, and spectral intensity is the radiant intensity per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength.

Radiant intensity is distinct from irradiance and radiant exitance, which are often called intensity in branches of physics other than radiometry.

Radiant intensity, denoted Ie,Ω ("e" for "energetic", to avoid confusion with photometric quantities, and "Ω" to indicate this is a directional quantity), is defined as[1] where In general, Ie,Ω is a function of viewing angle θ and potentially azimuth angle.

When calculating radiance received by a detector, Ω refers to the solid angle subtended by the source as viewed from that detector.

Radiant intensity is used to characterize the emission of radiation by an antenna:[2] where Unlike power density, radiant intensity does not depend on distance: because radiant intensity is defined as the power through a solid angle, the decreasing power density over distance due to the inverse-square law is offset by the increase in area with distance.