Abstract
The paper discusses the problems that are likely to confront the illuminating engineer who is concerned with measuring the subjective effects of light. In the first section a condensed account is given of the information available regarding the reaction of the eye when stimulated by light; this includes data derived directly from physiological experiments and by inference from sensory observations. Two processes, an electrical and a chemical effect, are referred to in detail in so far as they control the sensitivity of the eye and govern the magnitude of the visual response. In the second section an outline is given of the conditions necessary to reproduce a standard scale of subjective brightnesses and of the methods, by memory or by binocular matching, with which an unknown brightness may be compared to the standard. In the third section the subdivision of the standard brightness scale into a series of contrast steps is discussed and the means by which an unknown contrast may be referred to the standard scale are considered.
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