Monochromatic LEDs create the blue glow across the gallery fabric and floor at the Science Museum’s Atmosphere climate science gallery
What’s not to love about LEDs? Compared with tungsten halogen lamps, they save energy and don’t cost as much to maintain. LED efficacies and lives are better than those of halogen lamps, and the sources are rugged, ideal when luminaires are constantly being repositioned luminaire repositioning as exhibitions are changed.
Perhaps best of all is the limited ultra-violet and infra-red emissions that do so much harm to museum exhibits lit with conventional sources.
But the solid state source has been slow to catch on in the critical display areas of museums. Questions have been asked about their their spectral power distribution (SPD), which affects human colour discrimination and some light sensitive materials.
In 2007, researchers tested LED sources for use in museum lighting. They measured the effect of the sources on natural dyes and blue scale standards (the index of light colour fastness for dyed fabrics) using sources that represented several LED technologies. An International Commission on
Illumination (CIE) standard sets out an accumulated exposure time of 150,000 lux-hours.
The conclusion was that single phosphor LEDs with a colour temperature over 4000K, particularly those with abrupt peaks in the spectrum around 400-500nm, fade yellow dyes more than museum fluorescent lamps.
Also, the non-continuous spectrum of some LED sources can affect the accurate perception of art.
Moving right along
But LED makers insist that things have moved on. The quality of light from LED modules, particularly those with a separated and tuned phosphor, not only overcome these shortcomings but rival halogen lamps in the quality of their output.
For example, they have a high colour rendering index across all 15 of the test colours set out by the CIE in its standards, ensuring accurate perception of museum exhibits. Also, the light output between 400 and 500nm is not significantly higher than that from halogen lamps. Finally, the SPD is generally free from spikes.
One of the best
Xicato has gone to considerable lengths to demonstrate how its Artist Series modules – using its corrected cold phosphor technology – compare with more conventional museum light sources, both in colour rendering and spectral power distribution. It says the results show that LEDs can better typical compact metal halide or compact fluorescent sources, and are on a par with halogen sources.
Anyway, it’s time to judge for yourselves. We’ve assembled five case studies, one of which doesn’t use LEDs. See if you can spot which one it is…
What’s the damage?
Xicato and Art Preservation Services have carried out relative damage potential calculations using a number of sources including tungsten halogen, fluorescents, LEDs and, of course, daylight.
The table below shows the results. Clearly, LED sources have a lot to offer the curators of museums and galleries.
The calculations are based on reference dyes and reference media, and weight short wavelengths as causing significantly more damage.
Sperone Westwater Gallery, New York
The Sperone Westwater Gallery in the Bowery area of New York has a red ‘moving room’ – a lift that carries visitors between the gallery floors – which is partially obscured by the milled glass façade.
By studying the effect of the milled glazing, Buro Happold was able to bring the building to life at night. When the room is at its highest position, 60 feet of shaft wall are exposed, so a high output dimmable linear LED fixture was specified with a six-degree beam capable of grazing down the surfaces from less than nine inches away.
When the room is at the lowest position, light output from the fittings on the top of the moving room is at its peak. Linear asymmetric LED fixtures wash the rear surface, while two rows of wide beam symmetric LED fixtures provide ambient light to the ceiling. Analysis of the original milled glass using Snell’s Law showed that it would only appear to glow when the viewer was directly in front of the gallery.
Buro Happold recommended a much shallower profile, expanding the glowing effect to each end of the block.
Victoria and Albert Museum, London
Two types of LED moduless and halogen spotlights were engaged in a ‘shootout’ to decide which would illuminate the Asian Gallery of the Victorian and Albert Museum in London.
One LED module had conventional phosphor-on-chip LEDs and one had Xicato Spot modules. Jonathan Howard of DHA Designs and the museum were happiest with the Xicato LED modules, which had the right light quality while staying within energy consumption targets.
Thirty-six Xicato 1,000lm XSM modules were used, fitted in Mike Stoane Track Type X luminaires, the first track spot from the company to use the Xicato Module. The purpose of the refurbishment was to save energy and improve the lit appearance. The previous installation of fluorescent and halogen luminaires was replaced with fluorescent downlighters and LED spots.
Science Museum, London
Museum lighting specialist DHA Design was under pressure to keep energy use to a minimum when lighting the Atmosphere climate science gallery at the Science Museum. To create the blue glow across the gallery fabric and floor, DHA’s Jonathan Howard used monochromatic LEDs rather than theatrical gels, which often transmit only two per cent of the energy of the lamp. The LED fixtures use 18W each, but produce at least as much useful blue light as a 100W theatrical profile. Low voltage tungsten halogen lamps were replaced with LED fixtures and metal halide sources for spotlighting.
Object lighting throughout the displays is from LED sources. A typical display case, with six 3W LED fixtures, cuts energy use by about 80 per cent.
Catholic Museum, Nagasaki
The curators of the Catholic Museum in Nagasaki, Japan, wanted high – but varying – levels of illumination for their displays. Good colour rendition was essential and any artificial light source chosen could not damage the delicate artefacts.
The Getty Conservation Institute carried out blue wool testing to demonstrate that the Xicato LED module was less destructive than tungsten or fluorescent light sources.
With the source decided, AlphaLED’s Kamra 15-degree spotlight was specified, equipped with glare filters. The fitting tilts and rotates to spotlight exhibits.
The sources were individually addressable under Dali control. The final installation has a colour rendering index of 98.
California Academy of Sciences, San Francisco
Arup’s lighting designers specified Sill Lighting’s range of 003 metal halide projectors for the California Academy of Sciences in San Francisco. The high power projectors are fitted above the glass ‘bolla’, the home of the rainforest exhibit.
The fittings supplement the daylight from the skylights in the concrete structure of the outer, living roof, and are squeezed into the space between the concrete roof and the 90-foot diameter glass dome. Arup specified 140 projectors rated at 1 and 2kW and fitted with Osram HQI-TS lamps, which emulate daylight. They are installed outside the glass dome to keep the temperature of the tree canopies below 38°C.
The fixtures have been arranged in clusters at the crossover of the roof beams with the power supply arranged over two separate circuits.