Actinometrie

Actinometry is the science of measuring radiant energy, especially light, by means that relate the exposure to a quantifiable chemical, physical, or biological response. It is a branch of radiometry focused on actinic energy, the portion of the spectrum capable of producing photochemical changes. Actinometry provides a way to determine radiant flux, irradiance, or photon flux by using standard actinometers that convert light into a measurable change.

Historically, chemical actinometers rely on photochemical reactions whose extent is proportional to the amount of incident light. The classical example is the ferrioxalate actinometer, introduced in the 19th century by Bunsen and Roscoe, where irradiation converts ferric oxalate to ferrous ions; the Fe2+ concentration, measured spectrophotometrically, yields the number of photons delivered, after calibration. Other chemical actinometers include permanganate-based and uranyl oxalate systems. In parallel, physical actinometers use radiation-to-signal transducers such as thermopiles, photodiodes, or calorimetric elements to relate energy to an electrical or thermal signal. Modern instrumentation often uses pyranometers for diffuse solar irradiance and pyrheliometers for direct solar irradiance; these devices are calibrated against traceable actinometer standards.

Applications include meteorology, climate research, solar energy engineering, and photobiology. Actinometry is essential for calibrating radiometric measurements, validating radiative transfer models, and assessing exposure in materials testing and biological experiments.