Although machine vision systems are highly "mechanized," the operator's visual quantification of the characteristics of the input and output images often forms the basis of the required processing tasks. Thus, the ability of the eye to resolve and discriminate the space-varying (spatial) and time-varying (temporal) features of the reflected light from the object is relevant. While the absorption characteristics of the rods and cones are of fundamental importance, their relationships to the sensitivity of the whole retina dictate the behavior of the human vision system; this behavioral description is given in terms of retinal illuminance, brightness of the target object (or image), and the level of illumination around the object (or contrast embedded within the image). This chapter provides a rudimentary account of the nature of light sources and the visual process within the human eye, and defines some of the visual parameters related to image representation and discrimination.

2.1 Sources of Light

The light that stimulates the vision system is a form of energy that can be conceptualized in two different ways: (1) as an electromagnetic radiation spectrum (electromagnetic wave theory), and (2) as particles of energy (particle or quantum theory). The particle theory describes light as radiant energy emitted from a source in the form of individual quanta. The wave theory, in contrast, treats light as a form of wave motion to describe its propagation through optical media. In either case, light is considered to be a form of electromagnetic energy produced by reaction at the atomic level. The electromagnetic radiation spectrum in Fig. 2.1 contains energy with wavelengths varying from several thousand km to very small fractions of a meter. Only a very small part of this spectrum (λ = 390 to 790 nm) is sensitive to the human eye. The electromagnetic waves that fall within this visible band are defined as light. The approximate average value of 555 nm is frequently taken to represent monochrome white light.