If we are given an optical imaging system, one thing we would like to know is how much light will be available to our sensors. Because all sensors have limited operating ranges, controlling the amount of light irradiating the sensors is very important to obtain the best usage of the sensors and for the best quality of our images. In order to study the energy flow of light through the various stages of image formation, we have to carefully define the concepts and terms that we are going to use. The study and measurement of optical energy flow are the subject of radiometry.

Over the years several nomenclature systems have been proposed for light measurement and although there is still some debate on the subject, the units and terms proposed by the CIE have gained general acceptance. These units and terms are described in detail in the CIE publication International Lighting Vocabulary [187]. They have also been adopted by the American National Standards Institute (ANSI Z7.1–1967) and recommended by the publications of the (US) National Bureau of Standards [700, p. 8]. We will describe the radiometric concepts using CIE units and terms.

Concepts and definitions

The concepts and measurements of optical energy flow in radiometry are traditionally defined using geometrical optics. For example, optical rays are used to define the cone associated with a light beam and the path by which a ray is transmitted from one medium to another is determined by Snell's law. As a consequence of this idealization, many concepts lose their meanings when the spatial dimension is reduced to an infinitely small distance.