Action Spectrum and Overlap Integral

The term action spectrum has been used in two rather different ways. The first usage is strictly incorrect in that it relates to the overlap integral of the spectra for the 

The second usage of the term action spectrum is the more correct, according to photochemists. The action spectrum is obtained by measuring the radiation dose required to evoke the same chemical or biological response at different wavelengths. It will usually coincide with the absorption spectrum of the compound when the irradiation source variation with wavelength is corrected. To parallel the absorption spectrum, an action spectrum should meet the following conditions  

Absorption of radiation by inactive chromophores and radiation scattering is negligible. 

Only a small fraction of the incident radiation is absorbed by the sample in the wavelength range of interest. 

The exposure time is inversely proportional to the fluence rate for the same effect. 

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