This project examines a proposal to reduce the exposure of museum exhibits to incident radiant power without reducing the level of visual satisfaction for museum visitors. Many types of museum exhibits are susceptible to damage caused by exposure to light. Current recommendations for museum lighting include avoidance of non-visible radiant power (UV and IR), limitation of light level, and restriction of exposure duration. However, these recommendations do not distinguish between the potential of incident radiant power in the visible spectrum to stimulate the sense of brightness (illuminance) and its potential to cause damage (irradiance).
Generally, current practice for museum display lighting utilizes incandescent filament light sources, such as MR lamps. The spectral power distribution for this type of illumination is characterized by a continuous, smooth curve throughout the visible spectrum, increasing towards the long wavelength end. This project investigates the notion that the visual satisfaction provided by incandescent sources could be matched with significantly less irradiance by illumination composed of three spectral bands corresponding to red, green and blue light. The theoretical basis for this 3-band source is described in the introduction to the report.
Pairs of identical artworks were displayed in two adjacent simulated art galleries, and 16 subjects adjusted the illumination in the test gallery to match the appearance of the similar display in the comparison gallery. The artworks included chromatic and achromatic samples, and the light source in the comparison gallery was an MR display spotlight of either low (2850 K) or intermediate (4200 K) color temperature. The light source in the test gallery was either an MR spotlight identical to the lamp in the comparison gallery, or it was an experimental 3-band source for which the color temperature had been matched to that in the comparison gallery. For the same illuminance, the irradiance due to the 3-band source was less by 41% than that for the MR lamp at 2850K, and was less by 31% at 4200K.
It was found that subjects set for equal illuminance in the two galleries in all cases. They were questioned on any differences of appearance that they noticed, and generally they reported only slight differences. These differences of appearance are discussed, and it is concluded that a practical light source for museums could be developed that would equal the visual satisfaction provided by incandescent lamps at equal illuminances, while exposing exhibits to significantly less irradiance. This would have the effect of reducing the rate of degradation of museum objects on display.