Fotometria

Fotometría (photometry) is the branch of science that measures light as perceived by the human eye. It quantifies visible light using photometric quantities that correlate with visual perception, such as luminous flux, illuminance, and luminance, rather than total radiant power. In photometry, radiant power is weighted by the human eye sensitivity curve, known as the photopic luminous efficiency function V(λ). This makes photometric measurements observer-dependent.

Common quantities include luminous flux (Φv, in lumens), illuminance (Ev, in lux), luminance (Lv, in cd/m^2), and luminous intensity (Iv, in candela). The lumen represents radiant flux weighted by V(λ) over a source's emission; lux is lumen per square meter; candela defines luminous intensity in a given direction.

Measurements rely on detectors whose response approximates V(λ), such as photometers and calibrated photodiodes, often with spectral filters or integrating spheres. Photometry distinguishes absolute photometry (using a standard source with a known luminous flux) from relative photometry (comparing a test source to a reference).

Applications span lighting design and energy efficiency, architectural lighting, photography and cinema, display calibration, and astronomical brightness measurements. In astronomy, photometry characterizes the brightness of celestial objects and supports magnitude scales and color indices.

Standardization is governed by bodies such as the CIE and ISO, which define photometric quantities, units, and reference spectra. Photometric systems frequently use filter sets (for example UBVRI) and consider the V(λ) curve to ensure consistency across instruments.