actinometers

Actinometers are instruments designed to measure the actinic component of radiant energy, the portion of light capable of causing photochemical reactions. They are used to quantify light exposure in terms of energy flux or photon flux, and to calibrate light sources, radiometers, and photochemical experiments. The term derives from actinic rays, the part of the spectrum that induces chemical changes.

Principle and operation: In chemical actinometers, a reaction with a known quantum yield converts light exposure into a measurable chemical change. After a defined exposure, the amount of reaction product is determined, and the incident radiant energy is inferred using the reaction’s quantum efficiency, with corrections for spectral response, geometry, and exposure time. This approach allows estimation of actinic irradiance or radiant energy density at specific wavelengths or over a range.

Common examples: The ferrioxalate actinometer is among the most widely used chemical actinometers. It relies on the photoreduction of a ferrioxalate complex to Fe2+, which can be quantified spectrophotometrically after a colorimetric step. Other actinometers have used different chemical systems, such as uranyl oxalate or various iodide-based schemes, each with distinct spectral sensitivities and calibration requirements.

Applications and limitations: Actinometers play a key role in photochemistry, photobiology, and radiometric calibration, providing a traceable means to determine light intensity for UV and visible ranges. They are used to calibrate solar simulators, UV lamps, and spectroradiometers, and to assess spectral actinic irradiance. Limitations include spectral dependence, the need for careful calibration, potential temperature effects, and the requirement to account for geometry and scattering in real measurements.