Until the late nineteenth century, a museum gallery was lit by whatever daylight could enter through its windows and, after dark, by gas jets or candles. The arrangement was imperfect in aesthetic terms - some parts of the gallery were always over-bright and others always gloomy - but in one respect it was accidentally benign. Daylight varies through the day and year, and at night there was hardly any illumination at all, so the cumulative exposure of a painting to light was modest. The introduction of electric lighting in the 1880s transformed galleries into spaces that could be illuminated consistently for long hours. The transformation also introduced a slow-motion crisis for conservation, the scale of which became apparent only decades later.

Light damages almost every organic material found in a museum collection. Pigments fade, fabrics weaken, papers turn yellow and brittle. The damage is cumulative: an object exposed to one hundred lux of light for five hours sustains roughly the same damage as an object exposed to fifty lux for ten. This simple rule, known as the reciprocity principle, makes museum lighting a straightforward matter of arithmetic. The more tightly an object is controlled against both intensity and duration of light, the longer it will survive in a condition close to its current one.

Different materials have different sensitivities. Oil paintings, with their reasonably stable pigments bound in a protective oil medium, tolerate relatively high illumination levels and are commonly displayed at two hundred lux. Watercolours, in which pigments sit exposed on the surface of paper, are far more sensitive; standard guidelines limit their illumination to fifty lux. Textiles, with their natural dyes and fragile fibres, are often displayed at the same low level and for restricted periods each year. The Japanese conservator Takumi Nakamura has compiled a detailed reference table of recommended levels for more than a hundred material categories, a document that has been widely adopted as a basis for exhibition planning.

Ultraviolet radiation is even more damaging than visible light. Wavelengths shorter than about 400 nanometres carry enough energy to break chemical bonds directly, and a small amount of ultraviolet causes far more damage than a larger amount of visible light. Until the 1960s, most museum lighting used tungsten-filament lamps, which emit little ultraviolet and therefore caused relatively modest damage for a given level of illumination. The enthusiastic adoption of fluorescent lighting in the 1970s, which offered lower energy costs but much higher ultraviolet output, was later recognised as a mistake, and museums have since applied ultraviolet filters to their windows and light fittings as standard practice.

The recent shift to light-emitting diode, or LED, lighting has transformed the calculation. A modern white LED produces almost no ultraviolet, very little infrared heat, and can be adjusted in colour rendering to suit the display of any given object. Its efficiency allows the same illumination level to be achieved with a small fraction of the electricity consumed by earlier technologies. LEDs also do not fade or flicker as fluorescent tubes do, reducing the visual fatigue of visitors who spend hours in a gallery. Almost every museum rebuilt since 2015 has adopted LED lighting throughout.

The transition has not been painless. Early LEDs offered poor colour reproduction, particularly in the red part of the spectrum, and paintings whose reds looked vivid under tungsten light appeared dulled when viewed under the first generation of LEDs. Manufacturers have since improved the spectrum substantially, but a sensitive colour specialist can still identify the source of illumination from the appearance of a familiar painting. Some museums have chosen to mix LED with small amounts of traditional tungsten, at least until colour rendering improves further. Nakamura has argued that conservators and curators should participate actively in lighting specifications rather than leaving them to electrical engineers, since the visual experience of the collection depends sensitively on choices that look like technical details.

A further change in modern practice is the increasing use of reduced-hour display. Extremely sensitive objects are now often shown for only a few months a year, with the gallery light extinguished for the remainder. The Japanese textile dyes of the Heian period, which were given a permanent display in the mid-twentieth century, have in recent years been rotated into storage for much of the year; similar practices are common in watercolour rooms in European museums. The approach is unpopular with some visitors, who travel to see a particular object only to find it unavailable, but the logic of the reciprocity principle is unavoidable: an object shown year-round at even a low illumination level will fade, while the same object shown for three months a year at the same level may last several times longer.

The most recent innovation is dynamic lighting, in which illumination levels are varied depending on whether a visitor is present. Motion sensors trigger the lights as a visitor approaches a case and return them to very low levels when the visitor moves on. The technique, first deployed at a museum in Sweden, reduces cumulative exposure by as much as eighty percent in practice, because objects spend most of the day at minimal illumination. The cost of retrofitting an existing gallery with dynamic lighting is still significant, but the example has attracted attention from large museums facing the twin pressures of conservation and energy economy. Nakamura has remarked that the technique represents an overdue return, in a sophisticated form, to a principle the pre-electric era understood by accident: if objects are not being looked at, they should not be strongly lit.