The photochemist is interested in the reactivity of substances under light excitation. The rate of a photochemical reaction can be quantified by the quantum yield, also called quantum efficiency, defined as:

unit per time absorbed photons ofnumber unit per time molecules reacted ofnumber

Actinometry allows determination of the photon flux for a system of specific

geometry and in a well defined spectral domain; the most favorable case is when the incident light is monochromatic. The term actinometer commonly indicates devices used in the UV and visible spectral range. In absolute actinometric measurements a physical device (such as a photomultiplier, a photodiode, a bolometer) converts the energy or the number of the incident photons in a quantifiable electrical signal [9101, 0401]. However, the most commonly utilized method is based on a chemical actinometer, a reference substance undergoing a photochemical reaction whose quantum yield is known, calibrated against a physical device, well studied actinometers or by calorimetric methods.