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glossary

Photometry: field of science studying the energetic characteristics of visible light radiation as perceived by the human eye
Radiometry : field of science studying the energetic characteristics of radiation, including radiation in the optical spectral range
Radiance ( radiometry ): for a light source or a diffusing/reflecting surface, power emitted per unit of solid angle and per unit of apparent area
Luminance ( photometry ): for a light source or a diffusing/reflecting surface, emitted luminous power per unit solid angle and unit area expressed in photometric unit
Wavelength ( of a light wave ): distance between two consecutive identical states of the electric ( or magnetic ) field