A System for Communicating Color: Foundations and Rationale

The term “color” can be used in two different ways. One way is to describe the subjective appearance of an object or a light source. The second way is to describe the physical characteristics of the stimulus. Throughout the last century attempts have been made to bridge these two domains with some, but not complete success. Perhaps a consequence of our inability to satisfactorily make this bridge between the subjective and the objective domains has been our inability to communicate color to consumers of light sources that provide different spectral power distributions. This paper offers a system for communicating one aspect of color, the color of radiant flux generated by a light source, to consumers. The philosophy underlying the proposed system is to provide a readily understandable means of communicating light source color to consumers while utilizing existing color metrics already familiar to lighting specialists. Introduction In one important sense, color is strictly a human experience and not an inherent property of radiant power. In this sense, color is the result of phototransduction and neural computation by a human observer. Thus, it is incorrect to refer, for example, to “red” and “blue” wavelengths or “red” and “blue” regions of the spectrum. In another, more practical sense, color is the result of formal, simplifying transformations of the spectral power distributions (SPDs) of light sources agreed to by industry through international consensus. Specifically, the Commission Internationale de l’Eclairage (CIE) has created an entire system of colorimetry that has been used throughout the world for more than half a century to describe the color of light sources. Indeed, the CIE system of colorimetry has taken on a reality all its own, quite invaluable for international commerce but quite poor in representing human color experience. For example, brown and orange are distinct color experiences but both can have exactly the same color specification in the CIE system of colorimetry. Similarly, colorimetry defines colors regardless of the absolute amount of radiation, yet at low light levels color discrimination is poor or impossible. One challenge for purveyors of light sources is to preserve the highly functional and ubiquitous methods of colorimetry while providing users and consumers with meaningful information about expected perceptions of color resulting from these light sources. The industry is committed to colorimetry, however, as the only orthodox foundation for that communication. It is beyond the scope of this paper, and probably even the current science of color, to introduce a new system of colorimetry that better represents the human experience of color. Rather, this paper lays out a proposed system for communicating one aspect of light source color to consumers while staying within the constraints of colorimetry presently used by the industry. In particular, this paper only addresses communicating expected color perceptions of the radiant flux generated by different “white” light sources. Responses by consumers to the proposed color communication system described here are discussed in a companion paper.