Review of the Cone-to-rod Efficiency Ratio as a Specification for Lighting Systems

John Lott Brown

Jan 1957 · Wright Air Development Division, Air Research and Development Command, United States Air Force

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"The need for systems of illumination which will afford the maximum stimulation of the eye at low luminances has led to the widespread use of red light. The theoretical basis for the advantage of red light is reviewed. Several methods for the practical specification of illumination which are based on a cone-to-rod luminous efficiency ratio are considered. These methods are analyzed and compared in terms of the actual physical significance of the numerical values which they yield. It is evident that no method of calculating a cone-to-rod ratio provides an index from which the effects on sensitivity after adaptation to a given illumination system can be predicted. The general problem of specification is discussed in relation to a number of the underlying practical problems, such as luminance level at which a given illumination system must be used, the nature of the visual tasks for which it is provided, and the nature of the visual tasks for which sensitivity must be preserved. It is concluded that a cone-to-rod luminous efficiency ratio may afford a satisfactory index for the specification of lighting systems. However, the use of such a ratio to predict the effect on eye sensitivity of exposure to different lighting systems must await a careful experimental investigation of the relations among cone-to-rod ratio, luminance, and criteria of sensitivity within the context of specific practical situations."--Abstract.