Radiant intensity is a measure of the power emitted by a light source in a particular direction per unit solid angle. It is an important concept in photometry and radiometry, which deal with the measurement of optical radiation (light). Radiant intensity is quantified in watts per steradian (W/sr) and is used to characterize how light is distributed in space.
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Radiant intensity describes how a source distributes its radiant flux over directions. A small cone of solid angle carries approximately when varies little across it; integrating over all emitting directions recovers the total radiant flux. For a source with isotropy emitting over a full sphere, and a cone carries . A planar source satisfying Lambert's cosine law instead has direction-dependent radiant intensity, even though its radiance is direction-independent.