photomètre

A photometer, or photomètre, is an instrument that measures light or radiant energy. It is used to quantify luminance, irradiance, or radiant power, depending on configuration. In photography and cinematography, photometers help set exposure by indicating the amount of light reaching the film or sensor.

Photometers can be radiometric or photometric. Radiometric devices measure radiant power independently of human vision, while photometric instruments incorporate a response function that models the eye’s sensitivity. Within photography, illuminance meters measure incident light (lux) and luminance meters measure light reflected or emitted by a surface (cd/m2). Some instruments integrate light over time to provide exposure values. Modern devices use photoelectric sensors such as photodiodes or photomultiplier tubes and digital electronics to display readings.

Historically, photometers originated in the 19th century for lighting and photography, using visual comparisons with standard sources. The development of photocells and, later, solid-state sensors, improved accuracy and enabled automatic exposure control.

Standards and units: Photometry uses luminous quantities defined by the CIE. Common units include lumen, candela, and lux (lumen per square meter). Radiometry uses watts and related units. In practice, a light meter or spectrophotometer may display readings in lux or foot-candles, sometimes with spectral filters or integration capabilities.

Applications: Photography and cinematography for exposure control, architectural and horticultural lighting design, environmental monitoring, laboratories, and materials testing.