Light exposure and fading calculator

Fading depends on the total amount of light an object receives, not just how bright the room is. Enter how much light falls on something and for how long, and see roughly how long a sensitive, moderately sensitive or robust colour would last before a change becomes visible.

Exposure

Illuminance on the object (lux)Hours lit per dayDays per year

438,000 lux-hours a year

Estimated time to a just noticeable fade
Sensitivity	Blue wool	Fastest	Slowest
High sensitivityMany dyes in historic textiles, watercolours with fugitive colourants, felt-tip and many inks, colour photographs with unstable dyes.	ISO blue wool 1–3	6 months	7 years
Medium sensitivityMany oil paints and more stable dyes; many modern pigmented materials.	ISO blue wool 4–6	7 years	240 years
Low sensitivityMost mineral pigments and the most lightfast modern colourants.	ISO blue wool 7–8 and above	100 years	2400 years

A “just noticeable fade” is the first change a careful observer would see beside an unfaded reference. Damage accumulates: the same dose delivered faster or slower does roughly the same harm, and it does not reverse when the light is switched off.

How this works

Annual dose is illuminance × hours per day × days per year, in lux-hours.
Years to a just noticeable fade are scaled from the Canadian Conservation Institute's published table, which assumes 3,000 hours of display a year, using reciprocity: the same total dose does roughly the same damage.
Each sensitivity category spans several ISO blue wool grades, which is why every answer is a range.

Limitations

Ultraviolet makes things worse and is not included: the calculation assumes UV has been filtered out, as museum guidance requires. Unfiltered daylight fades faster than these figures.
It says nothing about a specific object. Only testing or a conservator's assessment can place a real object in a category.
Reciprocity is an approximation. Heat, humidity and pollution also affect the rate, and some materials darken or yellow rather than fade.
Measure illuminance where the object is, not in the middle of the room. A phone light-meter app gives a rough figure; a calibrated lux meter a better one.

438,000 lux-hours a year

Estimated time to a just noticeable fade
Sensitivity	Blue wool	Fastest	Slowest
High sensitivityMany dyes in historic textiles, watercolours with fugitive colourants, felt-tip and many inks, colour photographs with unstable dyes.	ISO blue wool 1–3	6 months	7 years
Medium sensitivityMany oil paints and more stable dyes; many modern pigmented materials.	ISO blue wool 4–6	7 years	240 years
Low sensitivityMost mineral pigments and the most lightfast modern colourants.	ISO blue wool 7–8 and above	100 years	2400 years

How this works

Annual dose is illuminance × hours per day × days per year, in lux-hours.
Years to a just noticeable fade are scaled from the Canadian Conservation Institute's published table, which assumes 3,000 hours of display a year, using reciprocity: the same total dose does roughly the same damage.
Each sensitivity category spans several ISO blue wool grades, which is why every answer is a range.

Limitations

Ultraviolet makes things worse and is not included: the calculation assumes UV has been filtered out, as museum guidance requires. Unfiltered daylight fades faster than these figures.
It says nothing about a specific object. Only testing or a conservator's assessment can place a real object in a category.
Reciprocity is an approximation. Heat, humidity and pollution also affect the rate, and some materials darken or yellow rather than fade.
Measure illuminance where the object is, not in the middle of the room. A phone light-meter app gives a rough figure; a calibrated lux meter a better one.

Light exposure and fading calculator

438,000 lux-hours a year

How this works

Limitations

Read next

Light exposure and fading calculator

438,000 lux-hours a year

How this works

Limitations

Read next