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.