In photometry, intensity is a measure of the amount of radiant energy received per unit solid angle per unit time per unit area of a surface element perpendicular to the direction of propagation of the radiation. Also known as specific intensity.


In visual perception, intensity is one of the three basic parameters (along with hue and saturation) that may be used to describe the physical perception of color. Intensity is a measure of the energy of the spectral distribution, at a given point in an image or scene, weighted by the spectral response of the visual system. Luminance is the energy of the physical spectrum, but not weighted by the visual response. Brightness sometimes is used synonymously with either term.


In radio astronomy, 'intensity' is a term often used much more loosely to mean either the flux density of an unresolved radio source or the surface brightness of an extended source.