Photometrie

Photometrie (photometry) is the branch of optical measurement that quantifies light in a way that corresponds to human visual response. It treats luminous flux and related quantities as perceived brightness rather than radiant energy, and is distinct from radiometry. The field covers measurements of light from natural sources, artificial illumination, and astronomical objects, using a weighting function known as the photopic luminous efficiency function V(λ) to reflect the eye’s sensitivity at different wavelengths.

Key photometric quantities include luminous flux (Φv, measured in lumens), illuminance (Ev, lux), luminance (cd/m^2), and luminous intensity (candela). These differ from radiometric quantities such as radiant flux (Φe, watts). The photometric flux is obtained by weighting the spectral radiant power by V(λ): Φv = Km ∫ Φe(λ) V(λ) dλ, with Km defined as 683 lm/W for 555 nm. Units and zero points depend on standard sources and instruments.

Instrumentation includes photometers, luminance meters, and radiometers; detectors such as photodiodes, photomultiplier tubes, and CCD arrays. Measurements rely on standardized accessories such as optical filters and calibration lamps or standard stars. Calibration defines photometric zero points and passband responses, ensuring comparability across instruments and sites.

In astronomy, photometry measures apparent magnitudes and, with distance information, absolute magnitudes. Light curves track brightness over time; standard photometric systems (Johnson UBVRI, Sloan, and others) and AB magnitudes define consistent flux scales. In lighting and color science, photometry informs illuminance design, color rendering, and colorimetric quantities such as CIE xyY and correlated color temperature.