In time, all artworks will degrade. This is the bittersweet truth at the heart of the preservation efforts of art galleries and museums across the world. It's how best to slow this process, while allowing great works of art to remain on display, that preoccupies them.
"All visible light is potentially damaging to works of art, but without the light you can't see the work," points out Joseph Padfield, Senior Scientific Officer at the National Gallery, speaking recently at a press conference.
Balancing these two tugging directives - the need to preserve, yet the need to display - dictates a delicate balancing act that has spawned a complex and esoteric field of light study, enlivened by hearty debate on the best methods to stall the degradation process.
Daylight provides the ideal light conditions under which humans can view objects to their best effect. However, unfiltered daylight can be ruinous to old artefacts. Many of the objects in the Natural History Museum, for example, have been washed to a faded beige by 30 years of exposure to light streaming in from the skylights above.
In art galleries, the imperative to preserve the aesthetic beauty of artefacts is even greater. To fully understand the complexities of which lighting is appropriate for paintings hanging in The National Gallery, it's necessary to characterise the properties of light at the lowest physical level.
Three measures of light are the most important in this endeavour. The first is Lux, which in layman's terms constitutes a measure of the intensity of light. However, this can be broken variously into the electromagnetic radiation of a light source (its radiometric value), and the quantification of 'brightness', as perceived by the human eye. The study of the former is termed radiometry, the study of the latter is photometry. One value can be transposed into the other via the luminosity function, which reflects a standardised model of humans' visual brightness perception.
The luminosity function peaks at 555 nanometres - the colour 'green' to you and I - meaning this is the wavelength of light to which the human visual system is most sensitive. (By comparison, the luminosity function is zero for wavelengths that fall outside the human spectrum - UV or infrared for example.)
Lighting conditions that ensure the human visual system is sufficiently sensitised are essential to appreciating art. However, it's important to consider the radiometric values of light too, regardless of what humans are able to see. "The works of art are sensitive across a much broader range," says Padfield. "Particularly into the UV and the UVA range, which are a lot more concerning."
But Lux is only one measure that's taken into account. The second important measure to consider is colour temperature.
Colour temperature is calculated by referring to a theoretical 'black-body radiator' that emits light through a range of temperatures (otherwise known as thermal electromagnetic radiation) - spanning from infrared through the visible spectrum up to UV rays. The sun is an approximate black body, with emission peaking in the green-appearing light spectrum visible to humans, as well as emitting UV rays. Flames, to a lesser extent, are also a close approximation to this black-body radiator.
Tungsten (or incandescent) bulbs used in the past created a reasonably close approximation of the same light spectrum distribution curve, however the LED and fluorescent lighting we use today cast a much 'spikier' distribution profile. This is because Tungsten bulbs emit light primarily via thermal radiation, making them less efficient energy users, and even leading them to being phased out or prohibited by many countries. The introduction of fluorescent lighting prompted a step change in how galleries lit their artwork, with varying consequences.
The final measure employed in the pursuit of the ideal light for paintings in The National Gallery is the Colour Rendering Index (CRI). This measure allows scientists to calculate how effectively different light sources illuminate different colours compared to an ideal or natural light source.
The CRI is closely linked to a light source's spectrum. While an incandescent bulb has a continuous spectrum, a fluorescent bulb has a discrete one. Daylight (or a black-body radiator) would be given a 'perfect' score of 100.
But how do these three measures shape the process of selecting lighting for The National Gallery? "Colour temperature can be selected for aesthetic reasons - what we would like to display in our museum or gallery - and colour rendering is a measure of how well it is actually going to achieve that," says Padfield.
However, this last measure has been gathering dissidents. "In recent years, CRI has come under a lot of criticism," says Padfield, "simply because it was designed to look at Tungsten curves - smooth curves which are fairly close to the black-body curve - it wasn't designed for LEDs or fluorescent lighting."
This is one of the ways that modern lighting is disrupting the practice of painting preservation. A great deal of research is currently being carried out into the effects of different light wavelengths on the degradation of different materials. At the National Gallery, a damage function based on this research is used as a weighting function to discover the impact on different materials. "That will change based on whether it's a watercolour painting or whether it's a piece of ceramic art," says Padfield.
Because of this, art is separated by type and probably will be even more so in future. If you've paid attention, you may have noted that delicate works of art such as charcoal sketches are often stored together at lower lighting levels.
Another broadly emerging principle is that light sources containing more blue spectrum light expedites the degradation of an art work far quicker than those with more red wavelengths. "At the National Gallery, we try to light all of our objects with a filtered white light - that's our method," says Padfield. "Artificial lights are the best replacement for that, relative to daylight." This filtered light cuts out universally harmful elements like UV rays.
Whenever a decision is taken about the lighting for a particular painting, the team will measure the light source to ascertain whether it's achieving what was intended. One of the pieces of kit they use is Wave Go, a device from Wave Illumination. This small, cylindrical object measures light levels at a particular point and reveals a raft of important information about the light source including chromaticity, absolute spectrum and illuminance.
A measurement with the device may prompt an analysis and the adjustment of various elements of the light environment.
However, preserving artworks is not the gallery's only impetus - if it was, the paintings would be stored in a darkened room. Art is created to be gazed at, and another of the gallery's tasks is to decide how best to showcase the pieces in their collections. "Lighting is both an art and a science: the art side will involve how you want to perceive it and the science side is how to prevent damage to the painting," says Paul Higham, Business Manager at Wave Illumination.
But the matter of lighting aesthetics is by no means settled. "There's a lot of debate in the lighting design world within museums about how they want to recreate a piece of art," says Ian Macrae, an independent lighting designer and consultant responsible for projects such as Heathrow terminal five and Wembley Stadium. He says that while some galleries simply want to cast the colours of a painting as vividly as possible, there is also a movement for displaying art in candlelight - to recreate the conditions in which it was created.
The colour of paint is illuminated most strongly by light containing wavelengths of the same colour. This is because it reflects this colour of light, while absorbing the others. "What we need to know is the pigment in the paint and how it responds to light energy at a specific wavelength," says Higham.
This can be trickier than it seems. Red paint will reflect red light the most and absorb other colour wavelengths composed in the light source. "But actually red pigments can have other colours within them to create the richness of the red - so they can also reflect some green and yellow for instance," says Macrae. "If you punch too much of that energy onto them, they can start to degrade."
This effect can lead to unexpected discoveries. Research carried out by the University of Antwerp indicated the lighting used in galleries may be degrading the yellow tones in Van Gogh's and Cezanne's works of art. The research found that primrose and lemon shades of yellow were unstable under the ambient LED light conditions, causing them to fade to a brown or olive green shade. Because of this, researchers advised that the museums not use light spectrums packed with 'extra' blue or green wavelengths.
In most galleries, Macrae says, there will be one or two light sources on each work of art, so these can be attuned to a particular painting, accounting for both preservation and perception. However, one colour remains a problem. "How do you bring out the blues of a Monet?" asks Macrae. "You're going to limit the lifetime of the painting by lighting it."