The present paper is the first of a series relating to the selection and use of such (reflex) reflectors as give night-time visibility to highway signs and act as warning signals on trucks, etc. The optical function common to the various types of such reflectors—<i>retro-directive</i> reflection—is described, and the manner of its accomplishment by each of the two general types is explained. Although descriptions of the inherent characteristics of all retro-directive reflectors indicate the conditions of use for best results, variations between reflectors of different types and different makes create need for quantitative descriptions of performance in terms suitable for specifications. Chief among the terms proposed for this purpose is <i>specific intensity</i>, which designates the candlepower of the reflector per footcandle illumination under specified conditions of illumination and viewing. Other special terms required for describing these conditions are proposed. There are brief preliminary discussions of test methods and interpretation of test results.
G. A. VAN LEAR, JR., "I. Fundamental Optical Properties," J. Opt. Soc. Am. 30, 462-471 (1940)