Short answer
Museums increasingly treat light as a budget measured in lux-hours rather than a fixed ceiling. Using the Canadian Conservation Institute's sensitivity categories and a goal such as no more than one just noticeable fade per century, a highly sensitive object may be allowed only a few tens to a few hundred hours a year at 50 lux — which is why such works are shown briefly and then rested in the dark.
The familiar guidance — about 50 lux for highly sensitive materials, 150 lux for oil paintings — sets how bright an object may be lit, but damage depends on the product of brightness and time. The Canadian Conservation Institute therefore works in cumulative exposure, megalux-hours, and gives for each sensitivity category the number of years to a just noticeable fade at 50 lux for eight hours a day, 3,000 hours a year. Converting those years into a dose gives a budget that can be spent in different ways: brighter for a short exhibition, dimmer for longer, or in rotation. Illuminance limits remain useful as a sanity check, but the decision is really about how much light an object should receive over its display life.
A budget needs a goal. CCI illustrates one: aiming for no more than one just noticeable fade in 100 years. For the most sensitive end of the high category, whose just noticeable fade comes after 1.5 years at 50 lux, that allows display only about 1.5% of the time — some 45 hours a year at 50 lux. The table extends the same arithmetic to every category and to 150 lux. Medium-sensitivity objects can be shown far longer, and low-sensitivity ones effectively continuously. The ranges are wide because each category spans several blue wool grades; a museum narrows them with knowledge of the object's colourants, and sometimes by microfading tests that measure an object's own sensitivity on a pinhead-sized spot.
Once a budget exists, practical measures follow. Sensitive works go on display for fixed periods — often around three months for textiles — and then return to dark storage. Lights on motion sensors or timers are off when nobody is looking. Blinds and filters tame daylight, which is both bright and rich in ultraviolet. Facsimiles stand in for originals in bright or long-running displays. And logging matters: CCI's textile guidance recommends recording display time, light level and conditions for each object, so that exposure can be totalled over years. The light exposure calculator on this site does the same arithmetic for any light level and schedule.
Low illuminance is only acceptable if visitors can still see. The eye adapts: the same 50 lux that looks gloomy on entering from a sunlit hall looks adequate after a few minutes. Good galleries step light levels down gradually, use dark surroundings so that objects are the brightest thing in view, avoid glare from glazing and give visitors time. CCI's textile guidance acknowledges that older viewers, fine detail, dark objects and limited viewing time each justify more light; curators often respond with brief, brighter viewing — a light on a push button, a short opening — instead of permanently higher levels. The environments pillar's gallery lighting page covers what low light does to perceived colour; this page is about spending light wisely.
| CCI sensitivity | Dose to a just noticeable fade (lx·h) | Budget per year (lx·h) | Hours a year at 50 lux (share of 3,000 h) | Hours a year at 150 lux (share of 3,000 h) |
|---|---|---|---|---|
| High sensitivity | 225,000–3,000,000 | 2,250–30,000 | 45–600 h (1.5%–20%) | 15–200 h (0.5%–6.7%) |
| Medium sensitivity | 3,000,000–105,000,000 | 30,000–1,050,000 | 600–21,000 h (20%–100%) | 200–7,000 h (6.7%–100%) |
| Low sensitivity | 45,000,000–1,050,000,000 | 450,000–10,500,000 | Continuous display within budget | Continuous display within budget |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
With a goal of no more than one just noticeable fade in 100 years, high-sensitivity colourants whose just noticeable fade takes 1.5 years at 50 lux can be on display only about 1.5% of the time.
Source: Agent of deterioration: light, ultraviolet and infrared
Colourwise analysisModerate evidence
Under the same 100-year goal, the annual light budget ranges from about 2,250 to 30,000 lux-hours for CCI's high-sensitivity category and from about 30,000 to 1,050,000 lux-hours for its medium category.
Based on: Colourwise calculation from CCI's years to a just noticeable fade at 50 lux and 3,000 h a year, converted to a dose and divided by 100 years; see the table.
Caveat: Assumes reciprocity and CCI's category ranges; institutions set their own goals.
Source: Agent of deterioration: light, ultraviolet and infrared
ConventionStrong evidence
CCI's textile guidance recommends limited display periods, such as a three-month maximum, and keeping a record of display time, light level and environmental conditions for each textile.
FactModerate evidence
Microfading testing accelerates fading on a spot of about 0.5 mm and measures colour change before it is visible, allowing objects to be surveyed for light sensitivity before display.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.