radiometrie

Radiometrie, or radiometry in English, is the science of measurement of electromagnetic radiation. It encompasses the measurement of radiant power across the electromagnetic spectrum and aims to quantify energy flow in absolute terms rather than human perception. Practical radiometry typically focuses on the ultraviolet, visible and infrared regions, where many measurements of light sources, detectors, and optical systems are required.

The principal radiometric quantities include radiant flux Phi (W), radiant intensity I (W per steradian), irradiance E (W per square meter), radiance L (W per square meter per steradian), and radiant exitance M (W per square meter). Spectral versions add wavelength dependence, using Phi_lambda, E_lambda, I_lambda, M_lambda, or L_lambda with units such as W/nm, W/(m^2 nm), and W/(m^2 sr nm).

Photometry is related but different: it measures visible light according to the eye’s response, producing luminous flux (lumen), illuminance (lux), and luminance, which are derived from radiometric quantities via the luminous efficacy function V(lambda).

Instrumentation includes radiometers, spectroradiometers and radiance meters. Detectors such as thermopiles, photodiodes and bolometers sense radiant power. Calibration and traceability rely on national metrology institutes and primary standards, often using blackbody radiators and integrating spheres to ensure uniform, well-characterized illumination.

Applications span sensors and imaging (radiometric calibration of cameras and satellites), solar and atmospheric research, astrophysics, material testing, and lighting engineering. Standards and definitions are maintained within the International System of Units and by international bodies such as the BIPM and ISO/IEC committees, ensuring consistent radiometric measurements worldwide.