Light

General explanation

Light is an important agent of degradation. It is not a risk but a certainty. 50 million lux hours will spoil the colour of all ancient textiles [Tim Padfield & Sheila Landi 'The light fastness of the natural dyes', Studies in Conservation 1966 11 181-196]. That is about 30 years of permanent exhibition in artificial light. Many photographic dyes are extremely light sensitive. Identification is not practical, so all colour photographs must be protected. All organic materials are susceptible but very few inorganic materials.

Light is a form of energy which transforms into heat where it is absorbed. Artificial light releases more than just light energy into the exhibition and is a factor forcing mechanical cooling of exhibition rooms, even in cool climates where the ambient temperature is moderate.

The purpose of this standard is both to minimise photochemical damage and to minimise the energy used for lighting and consequently reduce the energy needed to remove the heat generated. In many situations, effort put to minimise lighting energy is doubly repaid in reducing, or making unnecessary, air conditioning.

Objects should be in the dark unless they are being looked at

Movement sensors can bring up lighting gently when people come into a gallery. Old fashioned curtains over display cases will also work, but waste light energy.

Direct sunlight (even through glass) must never shine on objects

Sunlight indoors is around 80,000 lux. In addition to photochemical damage it can raise the surface temperature of a dark object 40 ℃, with corresponding huge reduction in surface RH. [Tim Padfield, Henriette Berg, Nina Dahlstrøm and Anna-Grethe Rischel. 'How to protect glazed pictures from climatic insult'. Proceedings of the Rio de Janiero conference of the International Council of Museums - Committee for Conservation. ed. Roy Vontobel, London: James & James (Science Publishers) Ltd. Sept 2002. pp 80 - 85]

300 lux is enough light for anything [ref Saunders on tests run in nat gal. new extension] One could argue that fluorescent outdoor posters need a strong light, but they are not necessarily light fast.

100 lux maximum for light sensitive objects.

This applies to all coloured organic materials. It is impractical to prove that an individual object is unusually light fast.

Ultra violet (UV) radiation should be eliminated

UV filters are the nearest thing to perfection in the conservators toolkit. The 75 µW/lumen old standard was just a fix to avoid fitting UV filters to incandescent lamps. Since these are now obsolete, let us forget this number. It was a clever idea by a single manufacturer of an instrument designed for museum use, so it became a de facto standard which no-one understood. Daylight can be bounced off titanium white paint to reduce the UV component. UV absorbent film on windows has a history of inconsistent durability.

Infra red (IR) radiation should be minimised

IR doesn't do photochemical damage but it warms the object and thus reduces the surface RH. Showcase glass stops most IR, so illumination from outside the case is good. But the showcase glass will re-radiate to the interior, so this is not a sufficient protection. Note that fluorescent and LED lighting is not free of IR radiation and convective heat transmission, there is just less of it.

Short wave visible radiation should be reduced so far as possible compatible with good colour rendering

A collection of old boots hardly needs perfect colour rendering so reddish lamps can be used. The eye/brain compensates for reduced blue in the light source to an impressively large extent, so even a gallery of modern art can have blue deficient lighting, compared with daylight [ref thomson nat gal bulletin][ref michalski compendium on wavelength dependence of photodecay].

Use of accumulated light dose to limit exposure is not acceptable unless there is evidence that the institution has record keeping able to tally the dose over a hundred years

This is a clever idea that won't work in the real world. It will be used as an excuse for brightly lighting iconic works to draw in the public.

If you don't know what an object is made of - it must be assumed to be light sensitive

Light sources

Incandescent lamps should not be used for new exhibitions

The EU is phasing out incandescent lamps, eventually including tungsten-quartz-halogen varieties. LEDs are capable of giving good illumination with much less energy consumption. Note that the colour rendering index seems not to work with the characteristic spectra of fluorescent coated blue LEDs.

Physical screening to control lighting should be minimised

Absorbing light in dark screens, such as barn doors used on spotlights, wastes energy.

Light energy should be minimised

The energy released by museum lighting often has to be removed by cooling equipment, requiring full air conditioning. It is important to reduce the cost and complexity of museum environmental control. For this reason incandescent lamps should never be used in new exhibitions. Light emerging in all directions from a source should be used as fully as possible so energy is not absorbed in black screens. Lighting consultants must be persuaded of the importance of minimising waste of light, which means accurately sizing and focusing lamps for exhibitions, rather than installing over-capacity and then dimming and screening the light. However, LEDs can be dimmed without wasting energy or changing the colour temperature, so they can be installed more carelessly, then adjusted for intensity.

Note: colour rendering

Good colour rendering depends on the light source providing radiation at all visible wavelengths, reasonably smoothly distributed over the spectrum. However, the sensitivity of the eye varies with wavelength, being highest at 555 nm, which gives a yellow-green light if just this wavelength is present. The energy efficiency in lumens per watt is greatest for this monochromatic source and diminishes as the wavelength spectrum spreads to the blue and red wavelengths. Therefore good colour rendering is fundamentally incompatible with energy efficiency as registered by lumens per watt.

A related issue is what colour rendering quality is necessary. For an archaeological exhibition of wood and skin of uniform brown colour monochromatic light is good enough, since it is the variation in luminosity which gives form to the object. However, other observers of the exhibition will look strange to each other, so some compromise is necessary to avoid frightening the visitors.