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MONDO ARC

The art of lighting

Issue 62 Aug / Sep 2011


Kit Cuttle runs through the different lighting settings for museum objects related to their visible and material attributes.

Lighting designers should recognise that a museum may be so much more than just a building in which exhibits are presented. It can provide a setting that forms an integral part of a visitor’s visual experience. However, the ways in which this may be achieved are strongly influenced by the susceptibility to light exposure of the materials on display.

The International Commission on Illumination specifies four categories of material responsiveness to light exposure (CIE 157:2004, Control of Damage to Museum Object by Optical Radiation), and in Table 1 these are indicated ranging from R0 (non-responsive) to R3 (highly responsive). The figure also shows these R categories related to four categories of lighting control, ranging from L0 (uncontrolled daylight) to L3 (minimal exposure). The rule is that the R rating of the material should not exceed the L rating of the lighting, which take account of the range of lighting parameters that contribute to damage exposure.

A display of R0 materials gives the designer the most degrees of freedom for revealing the visible attributes of displayed objects in their settings. Figure 1 shows a superb interaction of light with an object that is, at least from the conservation point of view, totally non-responsive. ‘The Kiss’ is displayed in the Rodin Museum, Paris, and at the time of my visit, the flow of light in this setting brought this classic sculpture into strikingly beautiful relief. Obviously, the illumination is daylight, but the building in which Rodin’s works are displayed was not designed to be a museum. It was, in fact, a mansion owned by an aristocrat, and the daylight flows into this space through a line of arched windows that provide views onto the surrounding gardens. It is a lovely setting and, depending on the vagaries of the weather, a visitor may experience lighting that reveals this sculpture quite beautifully. This is a fine example of an excellent visual experience in a R0/L0 situation.

If some of the materials on display are slightly responsive to light exposure, the R1 category applies and so the lighting must be L1 or higher. Figure 2 shows a daylit picture gallery suitable for oil paintings (but not watercolours) in which the architect, Heinz Tesar, has developed a modern interpretation of the classic European gallery, moulding the picture-hanging walls into the characteristic curved ceiling vault. The lantern skylight achieves sunlight and daylight control through diffusion that, in combination with the high reflectance wall surfaces, produces a soft, shadow-free illumination giving an even wash of light over the walls. Museums typically aim for an illuminance of 200 lux for R1/L1 situations of this sort, which may be achieved entirely by daylight, as in this case, or by electric lighting, or a combination of the two. Where the proportion of daylight is high, it is not practical to treat the 200 lux value as a maximum, but rather it should be treated as an average value that may be exceeded some of the time, and as the daylight level fades, the display illuminance will become increasingly due to the electric lighting. Wherever lighting has this kind of variability, conservation control shifts from limiting illuminance (lux) to limiting cumulative exposure (lux hours per year), which makes monitoring considerably more complicated. Conservators, not unnaturally, tend to prefer illuminance limits that can be more reliably maintained and are not subject to the variability of daylight, but this carries lighting practice one further step towards controlled, invariant illumination.

For moderately responsive materials in the R2 category, the limiting illuminance is 50 lux, and this calls for an altogether different illumination distribution. A diffused, ambient illumination of 50 lux would produce a dull overall appearance, and for displays to attract attention at this illuminance, the illumination needs to be focused onto the displays to establish a distinct brightness imbalance between the displays and the ambient condition. Figure 3 is an example of a R2/L2 situation, and shows a gallery space that is lit entirely by light reflected from the display walls and cabinets. Visitors who have been adapted to moderately low brightness levels before they enter this space will find themselves in a softly lit setting in which the displayed works on paper catch their attention and appear quite sufficiently illuminated, despite the illuminance limit.

The highly responsive R3 category comprises materials that are too responsive to be placed on permanent display at 50 lux. The illuminance limit is still set at 50 lux, because reducing illuminance below this level can challenge viewers’ ability to discriminate detail and colour, particularly for older people. This puts curators in the awkward position of having to either display such material substantially below the recommended illuminance levels, or needing to introduce some procedure for restricting the duration of display. Generally, a means of restricting display duration is necessary for R3 materials, but even so, L3 lighting may involve producing an illumination distribution that is designed specifically to reveal the visible attributes of the displayed objects, and is set to an intensity that is judged to be just sufficient for satisfactory viewing. It often happens that R3 materials are small scale and intricate, and need to be displayed in ways that permit close-range viewing. Figure 4 shows an example of a R3/L3 situation that provides well for this, and in which the ambient illumination is close to zero.

There is a tendency for the conservation limits to be thought of as an irritating nuisance that can sometimes get in the way of effective display, but, as can be seen from these examples, those limits largely determine opportunities for the viewing experience at all stages of a museum visit. Of course it would be possible to meet the limits by installing L2 and L3 lighting throughout, but that would mean rejecting opportunities to expose visitors to the dynamics of daylight, as well as cutting visitors off from visual contact with the outside world. There is no doubting the brilliance of the display techniques achievable with modern electric lighting, but to plan a museum layout with consideration to guiding visitors from outdoor daylight, through L0 spaces to L1, and on to L2 and L3, is to design for a variety of visual experiences. Table 2 shows a schematic outline of such a plan, which has the potential both to exploit the display opportunities of those objects that can withstand interaction with intense light, and to present attractive displays of light-responsive objects that would be damaged by such exposure. Such a plan has an added advantage. It can be arranged so that the more fragile materials, that have the most demanding environmental requirements for their conservation, are located more remotely from the more variable external conditions. In this way, the museum becomes a series of settings for housing and presenting objects in environmental conditions suited to their visible and material attributes.

http://kit-lightflow.blogspot.com

Acknowledgments
Tables 1 and 2 were first published in 2007 by Butterworth-Heinemann in the author’s book, ‘Light for Art’s Sake: Lighting for artworks and museum displays’ (ISBN: 978-0-7506-6430-1), and are reproduced here with permission.

Christopher Cuttle, MA, FCIBSE, FIESANZ, FIESNA, LC, is a lighting designer, advisor, author and educator on all aspects of lighting.

 

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