Radiance of a source is the power that is radiated from a surface within a solid angle and passing through a cross-sectional area. Radiance may be described as the power traversing at a point in a specified direction per unit area perpendicular to the direction of travel. Solid angle is the area projected on a unit sphere centered at that point. The symbol used for radiance is L having unit as Watt/ (m2sr1).
Radiance and spectral radiance are terms that are commonly used in radiometry to describe the emission and reflection of electromagnetic radiation. In astrophysics, radiance is a term used to describe the emission of neutrinos and other astral particles. Radiance of an optical system quantifies the amount of power emitted by an emitting or reflecting surface that will be received by the optical system for a particular angle of view. The field of view is given by the solid angle. The human eye being an optical system, radiance and luminance are good indicators of the brightness of the object.
For a real, passive optical system the output radiance is nearly equal to the input, provided there is no change in the refractive index.