Quantum Yields of Photochemical Reactions Actinometry

The definition of the quantum yield of a photochemical reaction has been considered in section 4.1 as the ratio of the number of molecules transformed to the number of photons absorbed 

In practice a sample is irradiated with monochromatic radiation of wavelength Ar during a time t. The concentrations of reactants and products are determined by usual analytical methods. The second problem concerns the measurement of the number of photons absorbed. This is calculated from the number of photons incident on the sample, multiplied by the fraction of light absorbed x9 

In practice both the incident light intensity I0 and the fraction of light absorbed /a vary with time, the former because of the instability of the output of the light source, and the latter because of the change in the absorbance of the sample at Ar in the course of irradiation. Although it is possible in principle to measure and integrate the light intensity by means of electronic detectors, the technique of chemical actinometry is still widely used in practice. 

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