The route from a luminous environment to the perception of that environment by a human being is long and convoluted. As formulated by Cuttle (2008),From this formulation, it should be apparent that the photometric and colorimetric quantities described in Chapter 1 and that are used to quantify the luminous environment are effectively measures of the stimulus to the visual system. The response of the visual system, in terms of the perception, is related to the stimulus received but not to the stimulus alone. This is for three reasons. First, perception depends on the state of adaptation of the visual system.