How are the color rendering properties of light sources defined?
Color rendering is a general term for describing the ability of a light source to provide color information to a human observer when objects are illuminated by that source. The color rendering properties of a light source cannot be accurately assessed by visual inspection of the light source or by a cursory examination of its spectral power distribution (SPD). Rather, a calculation procedure must be used.
Color Rendering Index (CRI) is currently the only color rendering metric recognized internationally (CIE 1986, 1995), and it is universally used by the lighting industry. Nevertheless, many other methods for quantifying the color rendering properties of electric light sources have been proposed (Guo and Houser 2004). All of these methods, including CRI, have limitations in characterizing the various aspects of color perception associated with color rendering (e.g., vividness, discriminability, naturalness). Every method utilizes the SPD of the light source. Most, but not all, incorporate one or more reference light sources to which a particular light source is compared. Most procedures also incorporate a reference set of colored objects to be illuminated.
Three color rendering metrics are discussed here, each emphasizing a slightly different aspect of color rendering: color rendering index (CRI), full-spectrum index (FSI), and color gamut area (GA). As new light sources are developed, particularly light-emitting diodes (LEDs), more than one metric will be required in order to evaluate the color rendering capabilities of a light source.