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

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.

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A feasibility study carried out by Christopher Cuttle (J. Am. Inst. for Conservation, 39, 229-244, 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.

Component One, directed by Carl W. Dirk, University of Texas at El Paso, involves the creation of software tools that calculate a spectral wavelength distribution while maximizing color rendering and optimizing various transmission and energy efficiencies. Three questions then arise in designing such filters:

  • Is it possible to get acceptable color rendering at 50 lux (5 footcandles) with >50 percent less total energy to the surface?
  • If so, will this result in a significant and demonstrable reduction in light damage?
  • Is this new light source stable and sustainable in the long-term?

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.

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".

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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 28, 2010 to September 12, 2010. Minor modifications to lamp fixtures may be required.

Last updated: July 2010