Rachel Schwen and Roy Berns Fall, 2011 Design of LED Clusters for Optimal Museum Lighting Introduction Museum lighting has historically been a field of trade offs. Museum environments have set guidelines for the lighting of fine art that aim to minimize degradation. This is accomplished by eliminating high-energy wavelengths, particularly those in the UV range below 400nm (as defined by CIE 157:2004) or is limited to 75 µW/lumen of ultra-violet radiation. There are also specifications for illuminance. For medium and high responsivity materials, total illuminance is suggested to be at or less than 50 lx (CIE 157:2004). For less responsive pigments, illuminance is suggested to be between 150 and 200 lx (CIE 157:2004, CIBS), and for those materials classified as irresponsive no limit on illumination is specified (NOTE: There are other suggested specifications for lighting in museum environments such as CIBSE, and IES which will not be discussed here). Curators are most interested in preserving artwork for future generations while displaying it in such as way as to allow current viewers to be able to enjoy the full impact of the art. The simple act of meeting these specifications creates the first level of obvious trade offs for curators. I. How much damage is too much for art? Color change due to fading is a major issue when assessing light damage done by a light source. Richardson and Saunders 2007 conducted a study, which indicates that damage as small as 2 ΔEab units is noticeable and up to 4 ΔEab indicates a reasonable amount of damage over a 50 to 100 year period. This study only examined a small number of museum staff and utilized computer-generated images to simulate fading to specific degrees. More extensive work on actual paintings with a wider variety of museum professions would help clarify this study. However, this study indicates the importance when evaluating a light source for museum lighting design to ensure that the source has accurate color rendering characteristics so that it is possible to evaluate changes due to light damage. Saunders and Kirby 2008 conducted a study comparing the damage created by several different light sources. They found that “Compared to the presence of absence of ultraviolet radiation, the difference between the two ‘daylight’ sources, or that between the ‘museum’ fluorescent and tungsten lamps, was very small.” This study also makes the important point that the “potential deleterious effect on museum objects is not the most important criterion for choosing one over the other.” Due to the fact that many LED combinations have very poor color rendering qualities. Druzik and Eshøj 2007 summarized the many trade-offs with museum lighting. Including the need for a communal environment for sharing ideas about conservation techniques, where concerned persons could discuss improving and understanding color rendering metrics, evaluate new lighting technologies and energy conservation standards, and compare damage spectra, as well as asses aesthetic and visual performance of a museum lighting design. II. What total illuminance level should be used to minimize damage while allowing viewers to appreciate the art? Reducing the illuminance of a given exhibit may make painting or drawings difficult to see, or colors difficult to distinguish making the display undesirable to visitors (Crawford 1973). Crawford’s study suggests 30 lx is the minimum for color discrimination. Therefore, it is likely that at 50 lx the discrimination of colors would also be difficult. To this end, many studies have been carried out which examine a quantitative metric for the degradation of artwork to justify the use of such a low illuminance level for the specification for museum lighting design. As a result, L S Harrison created a metric called the damage function, which has been adapted by others to reflect annual damage exposer. Discussions pertaining to damage by optical radiation are well summarized in Cuttle 1988. Loe et al 1982 state clearly that “The current recommendations, particularly those related to lighting, have been set rather arbitrarily, in that the illuminance recommendations have been set as low as was considered reasonably possible without firm evidence that they were necessary.” The Loe et al. study indicates that illuminance below 200 lux show a decreased discrimination and quality of the viewed image including both oil and watercolor paintings. III.What correlated color temperature should be used to reduce damage and create a pleasing lighting environment? Many studies have been performed with varying results as to which correlated color temperature (CCT) is preferred when observing fine art. Two articles by Scuello et al 2004 performed a series of studies that found 3600K to 3700K were ideal correlated color temperatures for museum lighting conditions. One experiment was performed using printed postcard reproductions. These postcards were largely blue or red depending on the scene they depicted, as these tend to be problem areas of color rendering for light sources with different CCTs. Also it should be noted that most artist paints are not spectrally similar to conventional printing inks particularly in regard to phthalocyanine cyan ink and ultramarine and cobalt blue artist paints. Cobalt blue and ultramarine both feature a long wavelength tail in the red region of the spectrum, which often impacts how these pigments are rendered, whereas phthalocyanine does not feature this tale and would therefore be more color constant than the equivalent artist pigment. Likewise the addition of black to printing ink adds color constancy to printed materials, which may not be present in actual artwork. The other experiment involved using a Maxwellian-view to evaluate the warm or cool quality of the preferred lighting condition for museum lighting. They concluded that observers prefer lighting that is neither noticeably warm nor cool. Where as, Pinto et al 2006 and Pinto et al 2008 found that 5100K was the ideal CCT for viewing a CRT displaying “digitized” artwork calculated with a hyperspectral and multispectral imaging system. This study noted that the initial change in average spectra between the digitization and the actual artwork began at 2%. It may be important when using LEDs to examine which CCT is ideal for viewing artwork in a gallery environment. It is also important to account for chromatic adaptation in the evaluation of this question, which was not considered in either study. Manipulating the CCT of a light source was previously difficult to examine due to the limitations of light sources which met the museum requirements. He and Zheng 2010 discussed the possibility of using LED clusters to create a combination that would be tunable to different CCT while maintaining a high color-rendering index. With the addition of broadband white LEDs and three single color LEDs they were able to create clusters with a high CRI and tunable CCT from approximately 2700K to 6100K. One of their combinations also had a high luminous efficacy. This study suggests that LEDs could certainly be used to create a “toolbox” of lighting design options for museum lighting environments. IV. What options are available for light sources? Traditionally these specifications have limited the light sources available to curators to mostly incandescent lighting. Recently advances in the science of lighting have led to the creation of new light sources that meet museum specifications (CIE 157:2004) and provide additional flexibility in lighting design and presentation. Halogen, and fluorescent blubs have become more widely used in museum lighting as they can now be manufactured with filters that cut UV radiation, the enemy of all curators of historical artifacts (Cuttle 2000). These advances have allowed curators to have greater flexibility in lighting designs and a method to change the lighting characteristics based on the goals they wish to obtain in a given exhibition. For example, Cuttle 2000 proposed the use of MR tungsten-halogen lamp and a MR1620/12 BAB/FL/40 lamp. This work aimed to compare a black body light sources to one with three-bands of radiation. Three prints of famous paintings with different colors used and from different artistic styles were used in a simulated art gallery to test observers on five major categories brightness, clarity, acceptability of overall color appearance, brightness or colorfulness of individual colors and naturalness of individual colors. This study investigated two correlated color temperatures (CCTs) one low and one intermediate. They concluded that once observers had matched for equality of illuminance observers were able to notice a difference between the two sources but they did not show a clear preference for one over the other suggesting that light sources with out a smooth spectral power distribution were acceptable to viewers. However, these new lights create environments are much different than traditional museum environments. These lights have different fundamental characteristics than incandescent blubs. For example, halogen blubs when dimmed emit a more yellow light then at full brightness. In an effort to evaluate metrics for lighting design with a goal of reducing degradation of art work and the best possible color rendering Delgado et al 2011 explored using filters to eliminate regions of the spectrum in order to ascertain if reduced damage might be possible. This work used filtered tungsten-halogens as an analogue to LED sources in an effort to create the best luminous efficacy while preserving high quality color rendering. They came to the basic conclusion that it is possible to optimize filters to reduce damage to art while maintaining high luminous efficacy as well as reasonable CRI values. They cite that their multi-band filter showed more degradation than did the broadband filter. V. What about the use of LEDs? The newest and most promising light source for museum lighting is solid-state lighting, which includes LEDs, light emitting diodes, OLEDs, organic light-emitting diodes, and PLEDs, polymer light-emitting diodes.