Short answer
Lightfastness is tested by exposing a sample to intense, filtered light alongside reference materials — classically the eight blue wool standards — and judging when the sample has changed as much as a given reference. Exposure uses xenon-arc lamps filtered to resemble daylight through glass, or real daylight. Museums add microfading, which fades a spot half a millimetre wide on the object itself. All accelerated methods assume that more light for less time gives the same result, which is only approximately true.
In the blue wool method, strips of the sample and of the eight graded blue wool references are exposed together under the same light, with part of each covered as an unexposed control. The exposure continues until the sample shows a set amount of change, judged against the grey scale, and the sample is rated by the blue wool strip that has changed by the same amount. Because the references share the sample's exposure, the method corrects itself for variations in lamp intensity, temperature and duration: what matters is which reference faded alongside it, not how many hours passed. That is also why the test can be accelerated with intense lamps, and why a rating such as Blue Wool 5 means 'fades about as fast as reference 5', not a number of years.
Laboratory tests use xenon-arc lamps, whose spectrum can be filtered to resemble daylight through window glass for indoor products or full sunlight for outdoor ones, and fluorescent UV lamps for some coating tests. Artists' paints in North America are commonly rated on the ASTM I–V scale (the ASTM D4303 lightfastness methods for artists' colourants), in which colour change is measured after a defined exposure. Textiles use the ISO 105 series with blue wool references; coatings use weathering standards that add water and temperature cycles. Acceleration relies on reciprocity — that the same total dose does the same damage whether delivered fast or slowly. Conservation guidance treats this as a working approximation; heat from intense lamps, different spectra and missing humidity or pollutant effects can make accelerated and real ageing diverge.
Traditional lightfastness tests need samples, which is impossible for a unique artwork. Microfading testing, widely used in conservation over roughly the last two decades, focuses intense light onto a spot about half a millimetre across on the object and measures its colour continuously with a sensitive spectrometer, stopping long before a change would be visible. The rate of change, compared with blue wool references tested the same way, places the object in a sensitivity category in minutes. Institutions use it to survey collections before exhibitions and find unusually sensitive works. Debate continues about possible local harm, heating, and differences between the test lamp and gallery lighting, so it is used with care and within set limits.
Test results are comparisons under defined conditions, and three cautions follow. First, they rank materials rather than predict life on a wall; converting a rating into years needs an assumed light level and daily hours, which is exactly what the Canadian Conservation Institute's sensitivity table does. Second, a result belongs to the tested product, concentration and substrate: a pigment's full-strength rating does not guarantee its tints. Third, accelerated tests can mislead in both directions — a comparison of natural and artificial weathering of coatings found some standard tests far harsher than outdoor exposure. When a decision matters, prefer ratings that state the standard used, and where possible combine accelerated data with real-time exposure evidence.
| Method | What is exposed | Speed | Strength | Limitation |
|---|---|---|---|---|
| Blue wool comparison | Sample strip with blue wool references | Days to weeks (accelerated) | Self-calibrating ranking | Needs a sample; gives a rank, not years |
| Instrumental accelerated test (e.g. xenon arc) | Sample panels | Hours to weeks | Measured colour change; controlled spectrum | Reciprocity and heat assumptions |
| Natural exposure | Panels behind glass or outdoors | Months to years | Real conditions | Slow; varies with site and season |
| Microfading | A ~0.5 mm spot on the object itself | Minutes | Tests unique objects | Small area; debate over spectrum and possible harm |
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.
FactModerate evidence
In blue wool testing, the sample is exposed alongside a set of standard blue wool strips and rated by the strip that has faded to the same extent, which allows testing to be accelerated under intense artificial illumination.
Source: Blue Wool Scale; Lightfastness
FactModerate evidence
Microfading testing performs accelerated photoageing on a spot of approximately 0.5 mm, detects colour change before it is perceptible, and is used to survey objects' light sensitivity before display, although concerns about possible damage and spectral differences from gallery lighting remain.
FactStrong evidence
CCI's light-sensitivity figures come from controlled fading experiments using a light source simulating daylight through glass, and treat damage as proportional to illuminance multiplied by time.
Caveat: Reciprocity is an approximation.
Source: Agent of deterioration: light, ultraviolet and infrared
FactModerate evidence
Standard accelerated tests can produce degradation much greater than natural exposure for some coatings, while tailored cyclic tests can better match outdoor results.
Caveat: Shown for waterborne acrylic coatings; other materials differ.
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.