Actinometer

An actinometer is an instrument or chemical system used to measure radiant energy, especially light, by producing a measurable signal that is proportional to the absorbed photons. Actinometers are employed to quantify light exposure in terms of energy, photon flux, or dose, and they underpin calibration of photometric instruments and photochemical experiments.

There are several main forms of actinometry. Chemical actinometers rely on light-induced chemical reactions whose extent can be quantified to infer the amount of incident radiation. The ferrioxalate actinometer is among the most widely used; irradiation converts ferrioxalate to ferrous ion, and the produced Fe2+ is measured colorimetrically (for example, after complexation with 1,10-phenanthroline). Other chemical actinometers include iodide/iodate systems and uranyl oxalate, each with known quantum yields under defined conditions. Physical actinometers, sometimes called radiometers or pyranometers, measure irradiance directly through photothermal, photovoltaic, or other transductions in solid-state or liquid sensors. Biological actinometers use living organisms or biological responses as the dose detector (for example, light-driven biological processes or growth responses).

Applications span calibration of light sources, measurement of solar radiation, and control of photochemical experiments. They help determine spectral response and dose in laboratory photochemistry, photobiology, and atmospheric studies. Limitations include spectral dependence of quantum yields, geometry of illumination, and the need for careful standardization. While modern radiometers and spectroradiometers often supplant traditional actinometers for routine measurements, actinometry remains a foundational concept for linking radiant energy to quantifiable chemical or physical changes.