Radiometry and photometry are similar terms in that both deal with the laws governing the transfer of optical radiation from one location to another—with or without the aid of optical systems. Traditionally, these subjects have included the description of the geometrical laws of radiation transfer, the behavior of radiant energy when transferred through and detected by optical systems, and the nature of radiation sources. Discussion of sources is deferred to Chapter 4.
Specifically, radiometry treats electromagnetic radiant energy of any wavelength, whereas photometry restricts itself to describing the laws of radiation transfer in the visible region of the spectrum, taking into account the physiological sensation that such radiation produces in an average human eye. If the human eye is regarded as merely a detector of electromagnetic radiation with a given spectral response, then photometry can be considered as just a special case of radiometry in which the characteristics of the detector are taken into account in computing the effect of the incident radiation. It is only natural, because of the spectral response of the human eye and its ready availability as a photodetector, that the development of the field of photometry preceded that of radiometry. As a result, however, two different sets of units are in use today, one describing the more general radiometric quantities and the other describing photometric quantities, which are applicable to visible optics only.
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