Displaying some of the most famous - and delicate - paintings in the world is a tricky business, particularly when you have to decide between quality of light and energy conservation. The National Gallery thinks it has found the answer: LEDs. Andy Pearson reports

London’s National Gallery is pioneering the use of LEDs to illuminate its priceless collection of paintings. Although the technology is not new, it is rarely used for installations where the quality of light is so critical. Now scientists at the gallery are convinced that the technology has advanced to a point where its application will not only save the gallery energy but will actually improve the quality of light on the pictures too.

The gallery has committed to using LEDs to replace its existing lighting installations on all of its pictures and throughout the adjoining Sainsbury Wing over the next two years. The move is expected to cut the gallery’s lighting energy consumption by 85%, which will reduce its carbon emissions by up to 400 tonnes a year.

We were looking to maintain or improve the lighting’s aesthetic quality while ensuring the safety of the paintings

“There were a number of issues behind the move to LEDs,” says Joseph Padfield, conservation scientist in the scientific department of the National Gallery. “We were looking to improve our lighting-related energy efficiency but also to maintain or improve the lighting’s aesthetic quality while ensuring the safety of the paintings.”  

Currently, track-mounted tungsten halogen spotlights illuminate the artworks. These units are similar to those used to light retail displays and shop windows. In the galleries, the lamps are used to supplement daylight that enters through glazed roof-lights. A system of external roof-light blinds adjusts automatically according to the amount and angle of sunlight to ensure only diffused light enters each room. Even so, the glazing is fitted with a filter to remove the ultraviolet (UV) component of sunlight, which would damage the precious paintings.  

“We measure the levels of light on the paintings continuously,” explains Padfield. “If there is insufficient daylight then we add artificial light.” The tungsten halogen lights are switched on and off under the control of light sensors in response to changes in light levels. Although the system does have a time lag to prevent it switching on and off rapidly as clouds pass over the sun, Padfield says the lamps do switch on or off abruptly rather than fading up and down, which is distracting for visitors. The new LED system has been designed
to prevent this distraction.

The LED solution is the culmination of three years of work by Padfield and other scientists and engineers at the National Gallery. Although the technology has been around for many years, it is only in the last few years that it has evolved to the point where it has become viable for specialist applications such as lighting galleries and museums. “I’ve been testing LEDs for a number of years to see how good they have become,” says Padfield.

In the past, one of the biggest criticisms of LED technology has been that the lamps tend to make objects look bluer than they would under halogen lamps. However, improvements in the phosphor coatings used in the LEDs’ manufacture mean that this is no longer an issue. White LED lamps are now manufactured in hundreds of different shades of white to enable them to be used for different applications. The actual shade of white is defined by a lamp’s temperature; temperatures over 5,000 Kelvin (K) appear bluish white and are called cool colours, while lower temperatures of 2,700- 3,000K have a higher yellow and red component and are called warm colours. The “colour temperature” of a lamp is what Padfield describes as “aesthetics”. “Different museums and galleries will have different preferences,” he explains.  

With the LEDs, it becomes a lot easier to tune the lamps to maintain light levels, and because they can be dimmed it is a lot less intrusive than the current system

How do you know the correct light conditions under which a painting should be viewed; for example could the artist have selected the paints by the light of an oil lamp? Padfield says it boils down to what is historically known about the object. “Some modern art might have been created to be viewed under a particular model of fluorescent lamp but older paintings could only have been viewed under daylight or flame,” he says. “From a National Gallery perspective we try to use daylight and then fill in with artificial when that’s insufficient.”

So what makes the perfect lamp for lighting priceless paintings? Padfield says he tests lamps from a number of distributors for a number of museums and galleries. “There is no real definition of what is a perfect light as far its colour component goes, you aim for an aesthetic based on the types of object you’re displaying and the preferences of the people responsible for the display.” As a result of his tests, the LEDs that the gallery has selected are a fixed white light with a colour temperature of 3,200K, according to the manufacturer, although Padfield says his tests have measured them to be a little over 3,050K.

national gallery

The tungsten halogen lamps used to light artworks have a cooler colour temperature of about 2,700K. However, these lamps have colour correction filters fitted to remove some of the yellow component of the light, to make it appear more like daylight. They also have filters to eradicate the lamp’s high levels of harmful ultraviolet. To get a correct colour balance, two or more lamps are needed for every area of coverage, one with a strong UV filter and the other with a blue filter to correct the overall colour, making it closer to daylight.
LEDs do not produce harmful UV light. “The LEDs chosen are actually slightly warmer and redder than the blue filtered tungsten lamps we were using, so the colour of the LEDs selected falls somewhere between the colours of the light that they will replace,” Padfield explains.

The ability of a light source to reproduce the colours of various objects faithfully in comparison with an ideal or natural light source is called its colour rendering ability. This is measured by a lamp’s colour rendering index or CRI. Light sources with a high CRI are desirable in colour-critical applications such as art galleries, while a low CRI means the object will appear unnatural. The LEDs for this project have a high CRI of 85.

It is a subject Padfield has researched at length. “We have a website where you can see and compare the different lights tested and look at the different power distribution curves,” he says. The website explains some of the metrics and variables used to calculate things like the CRI (it can be found at http://research.ng-london.org.uk/scientific/spd).

In addition to ensuring that a painting’s colour is reproduced faithfully, Padfield also had to ensure that his choice of LED didn’t damage any of the artworks by causing them to fade or dry out.

Most of the difficulties with lighting come from changing the lighting solution as you move from one space to another

Three factors contribute to light-related damage: the photo spectrum, the illuminance on the object and the duration of irradiation. The Relative Damage Factor is the index used toassess light sources; it is a ratio of damaging radiation intensity and illuminance. Short-wave components of light are more harmful because they have higher energy levels so lamps with a high blue component are unsuitable for sensitive exhibits. However, the warm white LEDs that Padfield has selected have a Relative Damage Factor lower than the current low-voltage halogen lamps fitted with a UV filter.

As part of the switch to LEDs, engineers at the gallery have been exploring ways to improve the control of the lighting systems. With the old tungsten halogen system, it was not possible to dim the lamps without changing the lamp’s colour temperature (this is why they tended to turn on and off abruptly). The new LED lighting system can be dimmed to 20% with no change in colour temperature, which means the system can be controlled to slowly augment the natural light as needed. As a result, the National Gallery will be the first institution in the world to control LED lighting levels in conjunction with daylight. “With the LEDs it becomes a lot easier to tune the lamps to maintain light levels, and because they can be dimmed it is a lot less intrusive than the current system,” says Padfield.

The gallery previously illuminated the artwork using Erco’s Eclipse spotlights with 100W low-voltage halogen lamps. The system’s low luminous efficacy of 22 lumen per watt (lm/W) meant the original lighting had an illuminance of only 5 lux per watt (lx/W). By replacing a 100W halogen spotlight with an Erco 14W Optec LED spotlight will achieve an efficacy of 23lx/W. This five-fold increase results from both the lamp’s higher luminous efficacy at 62lm/W and the LED’s high performance lenses. The fittings are smaller too, reducing their impact on the room.

Six LED lamps will be installed behind a special lens in each track-mounted fitting. The tracked solution gives the gallery the flexibility to alter its displays. The solution will be standardised throughout the gallery: “Most of the difficulties with lighting come from changing the lighting solution as you move from one space to another,” Padfield says.

Having decided to install an LED system now, is Padfield worried that if he’d waited the technology would have improved? “You cannot sit and wait forever,” he says. He accepts that in six months there may well be a product launched that provides a better solution. “The issue is, can we make an improvement now? Are the lights safe to use with the artworks? Do LEDs give us an energy improvement while maintaining or improving the aesthetic? If the answer to these questions is yes, then it makes sense to change the lighting,” he says.

The increased life-span of LEDs is also expected to enhance the visitors’ experience and save on maintenance costs because the lamps will fail less often. LEDS can have a lifespan of up to 50,000 hours. This should equate to over 20 years’ operation once gallery opening times and daylight levels are taken into consideration, which will bring additional maintenance savings and reduce the risk to the artworks of technicians replacing lamps. And, because LEDs emit much less heat than halogen lamps, the switch is expected to reduce the amount of energy needed to cool the galleries too.

This reduction will be important when the Carbon Reduction Commitment (CRC) energy-efficiency scheme comes into effect in 2012. Under this government tax on emissions, the gallery will have to pay £12 for every tonne of CO2 emitted. So, in addition to reducing electricity costs, the initiative will save almost £5,000 in CRC payments.

The use of LEDs throughout the exhibition spaces will contribute to the gallery’s aim of an overall carbon footprint reduction of 43% by 2014 through its carbon management plan. Work commenced in July in the Sainsbury Gallery and the project is expected to be completed by spring 2013, when you will be able to jjudge the results for yourself.

LED

Erco Optec LED spotlights fitted with flood Spherolit lenses have been used to illuminate the galleries

This article was originally published under the headline ‘Light is his medium’