Actinometry is a field of optical metrology (or more generally radiation metrology) which is similar to radiometry; the difference, however, is that it does not refer to energies or powers but rather to the number of radiation quanta (photons in case of optics). For example, the actinometric quantity corresponding to the radiometric quantity radiant flux is the photon flux, which is the number of photons per second. Similarly, one can define a photon flux density (photons per second and unit area) corresponding to the radiometric quantity irradiance, which is also called flux density.
Historic Evolution of the Term Actinometry
The history of actinometry is somewhat confusing. The term actinometer has been introduced by John Herschel in 1825, long before the photon nature of light was discovered. Essentially, that instrument was made for measuring the thermal energy of sunlight, and not a photon flux. Herschel himself called it a “thermometer of great delicacy”.
Later on, however, chemical actinometers have been invented, where light induces photochemical reactions and afterwards one registers the amount of chemically converted material. For example, one may use a liquid solution of oxalic acid containing uranyl sulfate, where the oxalic acid is decomposed into carbon dioxide, carbon monoxide and water. The amount of decomposed oxalic acid can later be measured by titration with potassium permanganate. Because one can usually assume that each converted molecule is related to the absorption of a single photon, irrespective of its precise energy, a chemical actinometer is indeed suitable for measuring photon flux rather than light energy. It only must be ensured that the absorption efficiency does not substantially depend on the photon energy – for example, by using a thick enough layer of the substance to obtain near-complete absorption.
At some point, the original definition of actinometry was modified to obtain the modern meaning, referring to radiation quanta. The historical details are not known to the author.
Actinometric measurements can also be done with photodetectors such as photodiodes, if their quantum efficiency is approximately constant in the relevant spectral region. That method is of course far more convenient then using a chemical actinometer.
For the measurement of high energy radiation (gamma rays, alpha radiation etc.), one could use a cloud chamber with a photodetector for measuring the rate of registered particle events. For some kinds of radiation, a simple Geiger counter serves the same purpose.