Three-Band Filtered Light Source: Thin-Film, Multi-Coatings

heoretical 3-band light filter
Thin-film, dialectric, multi-coating technology is at the heart of most high performance filter designs used today. Layers less than the wavelength of light in thickness can be added consecutively to glass substrates to achieve highly tailored spectral modifying results. Cut-off filters, narrow and wide band-pass filters, anti-reflective camera lens coatings, most telescope and binocular filters, and anti-reflective glass coatings used for framing artworks and on eye glasses, employ these methods.

A study of use of a three-band filtered light source carried out by Christopher Cuttle (2000) showed that the continuous spectra of a blackbody radiator, such as incandescent museum lighting (3500 K), can theoretically be replaced by a light source with three peaks (3-band), that roughly correspond to the wavelength sensitivities of the color rods in the human eye (see adjacent graph). Thus the "valleys" between the peaks represent energy savings.

Reducing Radiant Energy Transfer to Works of Art

For the Museum Lighting Reseach project, Carl W. Dirk of the University of Texas at El Paso (UTEP) and UTEP team members created software tools that calculate a spectral wavelength distribution while maximizing color rendering and optimizing various transmission and energy efficiencies.

Using data from Cuttle's study and the work undertaken at UTEP, the Museum Lighting Research project undertook the design of three-band filtered light sources for works on paper. Old master drawings were initially selected for this study, in part, because they have a limited-range color gamut (simplifying the filter design and manufacturing) but also because they tend to be made of stable materials
Three questions arose in designing such filters:
  1. Is it possible to get acceptable color rendering at 50 lux (5 footcandles) with >50 percent less total energy to the surface?
  2. If so, will this result in a significant and demonstrable reduction in light damage?
  3. Is this new light source stable and sustainable in the long-term?
In theory, and to differing degrees, most light-sensitive artifacts could probably profit from a limited set of filter designs. Therefore it may not be necessary to specify a large number of filter designs to span most museum collections. One drawback is that established fading standards and dosimeters will probably not behave in the same manner as unfiltered light sources and thus need their own unique "calibrations".

MR-16 lamps were the illumination source selected for this study. By January 2006, the first and second prototype filters were manufactured using Magnatron sputtering. The first prototype represents a combined ultraviolet/infrared filter; the second filter challenges limits on how far short and long wavelengths can be pulled inward towards the phototropic curve. By late 2009, the first set of three hundred filters had been manufactured and in 2010 installed in the Georgia O'Keeffe Museum in Santa Fe, New Mexico, as a demonstration project in time for the exhibition Georgia O'Keeffe: Abstraction running May–September 2010. Minor modifications to lamp fixtures may be required.