Although radiant luminous efficacy provides a useful basis for comparing alternative light sources, it is far from being the whole story. A critical concern in museum lighting is how the illumination affects the appearances of colored materials, and for this reason the emphasis of this study is upon relating the spectral power distribution of lighting to the responses of subjects viewing art works in simulated art gallery settings.
The color rendering properties of light sources are specified by reference to a black body, which is a theoretical substance for which its temperature defines the spectral distribution of radiant power emission. The correlated color temperature (CCT) of a light source is the temperature in degrees Kelvin (K) of a black body that most closely matches the color appearance of the source. At a low CCT (<3000 K) the appearance is a warm, yellowish light reminiscent of sunlight or a candle flame; at an intermediate color temperature (~4000 K) the color appearance is a more neutral or a more white light; and at a high color temperature (>5000 K) the appearance is a cool, bluish-white light reminiscent of sky light.
The color rendering index of a lamp is defined by a procedure that compares color metrics for a set of reference color samples illuminated by the lamp, with the color metrics for the same samples illuminated by a black body source having the same CCT as the lamp. (A different type of comparison source is used where CCT>5000 K.) If all of the samples match perfectly under both sources, the lamp is accorded a CRI of 100. Any departures from a perfect match reduce the CRI. This procedure assumes that, for low and intermediate color temperatures, a black body is the ideal color rendering source.
The black body source is luminous because it is incandescent, and its relative spectral power distribution almost exactly matches that of an electric incandescent lamp at the same CCT. Incandescent lamps are quoted to have CRI values of 99 or 100, and are widely perceived to be perfect color rendering lamps, but this perception needs to be qualified. They are perfect only in that the color appearances of illuminated surfaces match the appearances that they would have if illuminated by a black body of the same, low color temperature.
It is well understood by lamp manufacturers that it is not necessary to match the spectral power distribution of a black body to achieve a high CRI value. Tri-phosphor fluorescent lamps concentrate their radiant power emission into three spectral bands, and they achieve high CRI values. Thornton’s research (1992) has identified three optimal wavelengths for matching the lamplight from incandescent sources and achieving this with high radiant luminous efficacy. The band center wavelengths are approximately 450nm, 530nm and 610nm, and the light of these wavelengths have the characteristic colors of blue, green and red respectively.
These findings are highly relevant to this study, as they indicate that that there is scope to significantly reduce the irradiance of exhibits without reducing illuminance or sacrificing color rendering. If the irradiance is reduced, the rate of damage will be correspondingly reduced.