Actinometer
Actinometers are instruments used to measure the heating power of radiation. They are used in meteorology to measure solar radiation as pyrheliometers.
An actinometer is a chemical system or physical device which determines the number of photons in a beam integrally or per unit time. This name is commonly applied to devices used in the ultraviolet and visible wavelength ranges. For example, solutions of iron(III) oxalate can be used as a chemical actinometer, while bolometers, thermopiles, and photodiodes are physical devices giving a reading that can be correlated to the number of photons detected.
The actinometer was invented by John Herschel in 1825; he introduced the term actinometer, the first of many uses of the prefix actin for scientific instruments, effects, and processes.
The actinograph is a related device for estimating the actinic power of lighting for photography.
Chemical actinometry involves measuring radiant flux via the yield from a chemical reaction. It requires a chemical with a known quantum yield and easily analyzed reaction products.
Potassium ferrioxalate is commonly used, as it is simple to use and sensitive over a wide range of relevant wavelengths (254 nm to 500 nm). Other actinometers include malachite green leucocyanides, vanadium(V)–iron(III) oxalate and monochloroacetic acid, however all of these undergo dark reactions, that is, they react in the absence of light. This is undesirable since it will have to be corrected for. Organic actinometers like butyrophenone or piperylene are analysed by gas chromatography. Other actinometers are more specific in terms of the range of wavelengths at which quantum yields have been determined. Reinecke’s salt K[Cr(NH3)2(NCS)4] reacts in the near-UV region although it is thermally unstable. Uranyl oxalate has been used historically but is very toxic and cumbersome to analyze.
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