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.[1]
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. This process 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 greenleucocyanides, vanadium(V)–iron(III) oxalate and monochloroacetic acid, however all of these actinometers 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 butyrophenoneorpiperylene 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.[2][3][4]Uranyl oxalate has been used historically but is very toxic and cumbersome to analyze.
Meso-diphenylhelianthrene can be used for chemical actinometry in the visible range (400–700 nm).[7] This chemical measures in the 475–610 nm range, but measurements in wider spectral ranges can be done with this chemical if the emission spectrum of the light source is known.
^Calvert, Jack G; James N Pitts (1966). Photochemistry. New York: Wiley and Sons. ISBN0-471-13091-5.
^Taylor, H. A. (1971). Analytical methods techniques for actinometry in Analytical photochemistry and photochemical analysis. New York: Marcel Dekker Inc.
^Rabek, J. F. (1982). Experimental methods in Photochemistry and Photophysics. Chicester: Wiley and Sons. ISBN0-471-90029-X.
^Anastasio, Cort; McGregor K.G. (2001). "Chemistry of fog waters in California's Central Valley: 1. In situ photoformation of hydroxyl radical and singlet molecular oxygen". Atmospheric Environment. 35 (6): 1079–1089. Bibcode:2001AtmEn..35.1079A. doi:10.1016/S1352-2310(00)00281-8.
^Chu, L; Anastasio, C. (2003). "Quantum Yields of Hydroxyl Radical and Nitrogen Dioxide from the Photolysis of Nitrate on Ice". The Journal of Physical Chemistry A. 107 (45): 9594–9602. Bibcode:2003JPCA..107.9594C. doi:10.1021/jp0349132.
^Brauer H-D; Schmidt R; Gauglitz G; Hubig S (1983). "Chemical actinometry in the visible (475-610 nm) by meso-diphenylhlianthrene". Photochemistry and Photobiology. 37 (6): 595–598. doi:10.1111/j.1751-1097.1983.tb04526.x. S2CID98387978.
