Luxometry

Luxometry is a branch of photometry that focuses on measuring illuminance, the amount of visible light falling on a surface as perceived by the human eye. The primary unit is the lux (lx), defined as one lumen per square meter. Luminous flux (lumens) is weighted by the photopic luminosity function V(λ), so lux represents radiant power as perceived by the eye.

Instruments for luxometry are called illuminance meters or lux meters. They typically use a photodetector with an optical filter or sensor matched to V(λ) to approximate the eye’s response. Some meters include cosine corrections to account for the angle of incidence, and many modern devices are handheld, data-logging, or smartphone-enabled. Measurement is performed under standardized geometric conditions and is reported with units of lux; non-SI units such as foot-candles are still seen in some contexts.

Standards and calibration: measurement practices are specified by international bodies such as the CIE and ISO. Instruments are calibrated against traceable references, with response functions and spectral content specified to ensure comparability across instruments and laboratories.

Applications include designing and assessing workplace lighting, architectural lighting design, camera exposure control, horticulture, and safety lighting. Typical indoor illuminance ranges from a few hundred lux in offices to several thousand lux on bright task surfaces; outdoor daylight can exceed 10,000 lux and in bright sun over 100,000 lux.

Luxometry distinguishes illuminance from luminance (candela per square meter) and from irradiance (watts per square meter). It is a practical field for engineers, designers, and researchers working with human-centric lighting and illumination strategies.