Radiance

Radiance is radiant power traveling in a given direction per unit projected area and solid angle. It describes the directional distribution of light or other electromagnetic radiation.

Radiant flux: Radiant flux is the rate at which electromagnetic radiation carries energy, measured in watts. Radiance divides this total power by projected area and solid angle.
Radiometry: Radiometry is the measurement of electromagnetic radiation in terms of physical power and energy. Radiance is one of radiometry’s directional quantities.
Black-body radiation: Black-body radiation is the electromagnetic spectrum emitted by an ideal absorber in thermal equilibrium. Its spectral radiance follows Planck’s law at a given temperature.
Exitance: Radiant exitance is radiant power leaving a surface per unit area, summed over outgoing directions. Unlike radiance, it integrates over direction rather than preserving directional detail.
Solid angle: Solid angle measures the apparent area of an object on a sphere as viewed from a point. It specifies the directional spread over which radiance is measured.
Irradiance: Irradiance is radiant power arriving at a surface per unit area. Integrating incoming radiance over directions gives irradiance.
Planck's law: Planck’s law gives the spectral radiance of a black body as a function of wavelength and temperature. It provides a standard radiance prediction for thermal sources.
Radiant energy: Radiant energy is energy carried by electromagnetic radiation, measured in joules. Radiance is a directional power density, not an amount of energy.
Projected area: Projected area is the area of a surface as seen along a specified direction. Radiance uses area projected perpendicular to the direction of propagation.
Radiant intensity: Radiant intensity is radiant power emitted per unit solid angle in a specified direction. It shares radiance’s directional measure but does not divide by source area.

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