actinometry

Actinometry is the measurement of radiant energy, especially in the actinic (photosensitive) portion of the spectrum, by chemical or physical means. It provides a way to quantify light exposure in terms of energy or photon flux rather than brightness.

Two main approaches exist: chemical actinometry, which uses a chemical system with a known quantum yield to convert absorbed photons into a measurable chemical change, and physical actinometry, which relies on physical transducers such as chemical dosimeters or radiometers. In chemical actinometry a solution containing a photoactive compound is irradiated, and the amount of product formed (or reactant consumed) is measured. The quantum yield phi, defined as moles of product formed per mole of photons absorbed, is used to calculate the incident radiant energy or photon flux from the observed chemical change. Because phi depends on wavelength, actinometers are often calibrated for specific spectral ranges.

Ferrioxalate actinometry is among the most widely used chemical actinometers. It uses the photoreduction of ferrioxalate to ferroin, with the produced Fe2+ measured colorimetrically. Other actinometers include Reinecke’s salt and uranyl oxalate systems, useful in different spectral regions. Physical actinometry includes radiometric methods such as calibrated photodiodes or chemical dosimeters that change properties in response to light.

Common units include the einstein, the energy of one mole of photons, and joules per square meter for fluence. Actinometry is employed to calibrate light sources, assess solar simulators, and monitor photochemical reactors, ensuring accurate dose delivery in research and industrial contexts. Limitations include spectral dependence of quantum yield, temperature sensitivity, and potential side reactions that complicate interpretation.