fotometrie

Fotometrie (also called photometry) is the branch of optical metrology that deals with measuring light as perceived by the human eye. It is distinguished from radiometrie by weighting radiant power with the eye’s sensitivity, so photometric quantities reflect perceived brightness rather than total energy. The primary quantities are luminous flux (Φv, in lumens), illuminance (E, in lux), and luminance (L, in candela per square meter). Radiant flux (Φe, in watts) and irradiance are the radiometric counterparts; conversions rely on the luminous efficiency function V(λ) defined by the CIE for standard observers (2° and 10°).

The luminous flux is obtained by integrating the spectral radiant flux times V(λ): Φv = ∫ Φe(λ) V(λ) dλ. The lumen scales luminous power so that a source with a given spectrum has a lumen value proportional to its perceived brightness, depending on the observer and its adaptation state.

In practice, fotometrie uses calibrated detectors, photometers, or photodiodes with photometric responses and sometimes standard filters. In lighting engineering, photometric data underpin luminaire design, daylighting assessments, and color rendering metrics. In astronomy, fotometrie involves measuring celestial sources’ brightness through standard filter systems (for example, UBVRI or gri), deriving apparent and absolute magnitudes, and calibrating against standard stars while correcting for atmospheric extinction.

Standardization is maintained by bodies such as the CIE and IEC. Photometrie is distinct from radiometrie: it weights light by the eye’s response, aligning measurements with human perception, while radiometrie sums energy across wavelengths.