Of electromagnetic radiation measurements

The various terms that are used for the description of the emission of electromagnetic radiation from a radiant source or for the receipt of electromagnetic radiation by a specified surface element are summarized in Tab. 3-9. The terminology of electromagnetic radiation measurement is divided into radiometry and the subset of photometry (Fig. 3-18). The former is the science that involves the energy measurement of electromagnetic radiation in general. The latter is applied for the same purpose when visible radiation is to be described or measured in relation to the human eye s response. Important photometric quantities are for example luminous flux, luminous intensity, illuminance and luminance (McCluney, 1994). Every photometric quantity has its counterpart in radiometry, and vice versa. 

Photomultipliers, charge coupled devices (CCDs) and avalanche diode detectors are able to detect single photons over the visible to near-IR range with efficiencies approaching unity. The arrival of photons at a detector is not correlated, due to the quantum nature of electromagnetic radiation. Measurements of intensity as the averaged sum of photon events has a well-defined stochastic variance associated with a Poisson distribution. This variance scales as the square root of the number of photons. 

---