Photometric Quantities

a system of quantities that characterize light in the process of its emission, propagation, and conversion—for example, such processes as reflection and transmission. Photometric quantities are determined with respect to the average light-adapted human eye (seePHYSIOLOGICAL ADAPTATION). The relative spectral sensitivity of this standard light detector is regarded as a function of the spectral luminous efficiency. The function is normalized as a result of experimental statistical studies where averaging is carried out with respect to the eyes of a large number of persons with normal vision and with respect to the responses of the same eyes at different times.

The principal photometric quantities and their units in the International System of Units are given in Table 1 on page 193.

quantities that characterize optical radiation. A distinction is made between energy photometric quantities and reduced photometric quantities.

Energy photometric quantities characterize optical radiation irrespective of its effect on any radiation detector. The quantities are expressed in units formulated on the basis of energy units, such as the joule (the SI unit of energy), the erg, or the calorie.

Reduced photometric quantities characterize optical radiation in accordance with its effect on a specified selective radiation detector, such as the human eye. Various systems of reduced photometric quantities—for example, bactericidal photometric quantities and phytometric quantities—have been proposed and are used to characterize optical radiation in accordance with its effect on such selective detectors as bacteria and plants.