Subjects matched the illuminance in the test room to the illuminance in the comparison room, indicating that illuminance effectively evaluates the overall sense of equality of illumination as it affects the appearances of the art gallery settings, despite the very different spectral compositions of the two types of lighting. The fact that the 3-band source provides a given illuminance with substantially lower irradiance on the illuminated object offers a significant benefit for conservation.
For the light sources used in the experiment, the irradiances in watts per square meter (W/m2) to provide 50 lux on the artworks were:
|MR Lamp||3-band source|
|Low CCT (2850 K)||0.22||0.13|
|Intermediate CCT (4200 K)||0.20||0.14|
It is conventional to assess the exposure of illuminated museum exhibits in terms of lux-hours per year (lx h/y so that an object lit to 50 lux and exhibited for 3,000 hours per year is exposed to 150,000 lx h/y (Rea, 1993). This measure does not distinguish between the different irradiances of light sources at the same illuminance. If the light source in this example is a regular MR lamp, then changing to a 3-band source would reduce the effective exposure to 89,000 lx h/y Looked at another way, it would take 1.7 years of exposure to the low CCT 3-band source to subject the object to the same effective exposure as would occur in one year with a regular MR lamp at the same illuminance. For the intermediate CCT sources, it would take 1.4 years.
The reported differences of appearance between the 3-band source and either of the MR sources were slight. Even so, if the requirement is to produce a low radiant power light source that is indistinguishable from a MR lamp, some increase of long wavelength visible radiation will be needed. This would have the effect of increasing the vividness of strong red surface colors and making flesh tones appear warmer.
There are two approaches to developing practical light sources that can gain the conservation advantage of the 3-band source. A new type of filter could be developed that would convert the continuous spectrum of a regular MR lamp into a 3-band spectrum. While this would achieve high radiant luminous efficacy (lm/Wr), the luminous efficacy of the lighting system in terms of lumens per watt of electrical power input would be poor. Whenever a filter is added to a lamp, it is necessary to increase the lamp wattage to maintain the illuminance. A more efficient approach would be to develop a new lamp type specifically for museum applications in which a 3-band spectrum is generated by efficient conversion of electrical power. While this latter approach offers the prospect of a superior solution, the development costs are likely to be much higher. Even so, some lamp manufacturers might be willing to enter into collaborative research.
It should not be presumed that the museum community will respond with enthusiasm to this initiative. There is a long history of museum directors, particularly art museum directors, insisting that natural light is the only true light for the museum experience. The distinctly unnatural spectral power distribution of the 3-band source favored by this study is likely to be regarded with strong suspicion, even if there is no visible difference in the lighting. A further study in a real art gallery directed towards gaining critical evaluations of museum professionals is recommended.