Lux-hours: how light dose adds up on a wall

Lux measures how intense the light is on a surface. Hours measure how long it stays there. The Canadian Conservation Institute (CCI) defines the total exposure, or dose, of light on a surface as the product of light intensity (lux) and time (hours) [1]. NEDCC, a US conservation centre, calls the unit lux hours (lx h) [3].

CCI also notes that damage rate scales with intensity [1], so doubling the lux doubles how fast the damage accrues, and doubling the hours doubles the total (our own reading of the formula).

Museums often count in millions of lux hours, written Mlx h [1]. For a wall in a shop, office or hotel, plain lux-hours are enough.

The key idea is that light dose adds up. CCI's note on textiles gives a simple example: exposure at 100 lux for 400 hours results in the same damage as exposure at 50 lux for 800 hours [2]. Both multiply out to 40,000 lux-hours (our own arithmetic).

NEDCC states the same principle: the same expected damage occurs with brighter light over a short time as with dimmer light over a long time [3].

By this principle, dim light left on for many hours can add up to the same dose as bright light left on briefly (our own illustration of [2][3]). A single camera flash is very bright but lasts a second or less, so the museum fact sheet says the damage from one flash photo is minimal [4].

One caution applies. These sources describe the principle for planning. They do not give a fixed fade time for any specific print or ink, so treat the arithmetic as a way to compare options, not to predict a date.

NEDCC gives a worked case. An object lit for 10 hours a day at 50 lux for 100 days receives 50,000 lx h [3]. NEDCC also suggests, as an ideal, that light-sensitive materials receive an annual exposure of only 50,000 lx h, whether or not they are displayed every year [3]. That is a museum ideal for sensitive collections, not a rule for commercial walls.

To see how lux changes the total, keep the same 1,000 lit hours as the NEDCC case and change only the lux. At 50 lux the dose is 50,000 lx h [3]. At 100 lux it is 100,000 lx h. The second figure is our own arithmetic, not a source figure. Doubling the lux doubles the dose.

The method, which you can repeat for any wall, is lux x hours per day x days per year (our own working of the lux-times-hours formula), in four steps.

1. Find the lux on the wall. Use a light meter or ask the lighting supplier, because the lux on your wall depends on your own lamps, distance and daylight.
2. Count every hour the light is on, not only opening hours. Lights are often on for housekeeping when a space is closed [3], and openings or receptions add lit time [4]. Daylight is harder to count because its intensity changes with time of day, time of year and weather [4].
3. Count the days per year the light is on.
4. Multiply lux by hours per day by days per year, then repeat the sum after you change the lux or the hours and compare the two totals.

The table below shows how the same dose can be reached in different ways.

Think of the yearly dose as money. Each lux and each hour spends some of it, and nothing refunds it.

Your levers are the two factors in the formula, plus the light source itself. CCI says raising light levels only when circumstances require it, then lowering them again, keeps cumulative damage to a minimum [2]. The controls themselves are covered in the pillar guide: light, UV and fading of printed artwork on walls.

A UV filter removes only part of the problem. For colours sensitive to light, CCI puts UV at usually under half of the fading, and often about a tenth. For fading from display lighting, CCI says the one option is to reduce light exposure, and it notes UV becomes dominant only for colours durable to light [1]. In our own reading, lux-hours still matter after filtering. For glazing choices, see acrylic vs glass glazing and the guide on what UV-filtering glazing does and does not block.

Museum guidance sorts materials by sensitivity. The Museums and Galleries of NSW fact sheet puts paper (including prints), watercolours, photographs, textiles and vegetable-dyed material in its very light sensitive category, with a suggested ceiling of 50 lux [4]. The pillar guide linked above covers the sensitivity categories in more detail.

That is a museum limit for irreplaceable objects, and your walls may be lit differently, so the practical question is relative: which walls take the most dose? By the arithmetic above, walls with more lux or more lit hours spend the budget fastest. You can compare walls this way and decide where to rotate pieces. For room planning, see our wall art plan for hotel rooms. For how ink choice affects lightfastness, see dye ink vs pigment ink lightfastness for wall prints.

If your artwork will hang in a bright or long-lit space, tell us the setting and the daily lit hours when you contact us, so we can discuss it before the quote. ArtzFolio is a private-label and OEM/ODM manufacturer in Noida serving businesses and brands only. Payment is 50% advance, shipping is EXW Noida, and we reply within 1 business day. See pricing, minimums and lead times or contact us.

1. Agent of Deterioration: Light, Ultraviolet and Infrared, Canadian Conservation Institute, Government of Canada. canada.ca (read 2026-11-08)
2. Textiles and the Environment (CCI Notes), Canadian Conservation Institute, Government of Canada. canada.ca (read 2026-11-08)
3. Preservation Leaflet 02-04: Protection from Light Damage, Northeast Document Conservation Center (NEDCC). nedcc.org (read 2026-11-08)
4. Light damage and light control (fact sheet), Museums and Galleries of NSW. mgnsw.org.au (read 2026-11-08)