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The measurement of optical radiation has historically formed two measurement systems for infrared equipment, namely radiometric and photometric.

Radiometric is an objective measure of radiant energy based on a physical measurement system, which is not limited by the subjective vision of the human eye. Therefore, the concepts and methods of radiometrics are applicable to the entire electromagnetic band. Invisible radiation, such as infrared radiation, ultraviolet radiation, etc., must be measured by radiometric methods.

Photometric is based on the subjective perception of light by the human eye, and it is a psychophysical measurement. Therefore, the measurement method is only suitable for the narrow visible light band in the electromagnetic spectrum, which is sensitive to the human eye. The physical quantities of these two types of unit systems are different in physical concept, but the physical symbols used correspond one-to-one. For the sake of distinction, the superscript row "e" represents the radiometric physical quantity, and the superscript "v" represents the photometric physical quantity.