Quantification of light

Abstract

The topic of light quantification seems confusing, not only to the layman and the student, but also to the expert. Some reasons for this confusion are as follows: (i) The amount or intensity of light is often regarded as something that can be completely described by a number. Such a view disregards the following facts, (a) Light consists of components with different wavelength. A full description of the light would thus give information about the ‘amount’ of light of each wavelength, (b) Light has direction. The simplest case is that all the light being considered has the same direction, i.e. the light is collimated (the rays are all parallel). Another case is that light is isotropic, i.e. all directions are equally represented. Between these extremes there is an infinite number of possible distributions of directional components. (c) Light may be polarized, either circularly or plane polarized (Chapter 7.2). In the rest of this chapter this complication will be disregarded, but one should always be aware of the fact that a device such as a photocell may be differentially sensitive to components of different polarization, and polarization may be introduced by part of an experimental setup, such as a monochromator or a reflecting surface, (d) Light may be more or less coherent, i.e. the photons more or less ‘in step’. This complication, which is important in some experiments, e.g. with laser light, will also be neglected, (e) Finally, people often disregard, neglect or confuse the time factor. It should be decided whether to express an instantaneous or a time-integrated quantity, e.g. fluence rate or fluence, power or energy. Power means energy per unit time.