Short answer
A colourant fades when the molecule that absorbs light is broken or changed so that it no longer absorbs. The energy usually comes from the light itself: a colourant must absorb light to be coloured, and each absorbed photon is a chance for a reaction, most often with oxygen. Shorter wavelengths carry more energy per photon, so ultraviolet and violet drive damage out of proportion to the light they contribute, but visible light alone still fades sensitive dyes. Heat, humidity and pollutants add further routes.
A dye or pigment is coloured because a part of its molecule — the chromophore — absorbs some wavelengths. Absorbing a photon lifts the molecule into an excited state. Most of the time it simply returns to its ground state and the energy leaves as heat, which is what a stable colourant does billions of times. Occasionally the excited molecule reacts instead: it breaks a bond, passes its energy to oxygen and creates reactive singlet oxygen, or reacts with a radical formed nearby. Each such event can destroy a chromophore. Fading is the slow accumulation of these rare events, and the rarer they are per photon, the more lightfast the colourant. This is also why fading is roughly proportional to total light dose: more photons, more chances.
Photon energy rises as wavelength falls: at 340 nm a photon carries about 3.6 electronvolts, at 600 nm about 2.1. The table shows the values. Higher-energy photons can break bonds and start reactions that visible photons cannot, and they are absorbed by binders, fibres and papers that are colourless and would otherwise be untouched — so ultraviolet yellows, weakens and chalks materials as well as fading colour. The Canadian Conservation Institute notes that daylight coming through window glass carries roughly 400–500 microwatts of UV per lumen, several times the 75 µW/lm of an incandescent lamp, which is why daylit rooms are harder on collections than their lux reading suggests. Removing UV is cheap, since it contributes nothing to vision; but CCI stresses that visible light remains a primary cause of fading for sensitive colourants, so UV filtering alone does not stop it.
Not all colour loss is photochemical. Some dyes hydrolyse or oxidise slowly at room temperature in the dark — colour photographs are the classic case, with dyes that fade appreciably in storage — and the rate climbs steeply with temperature and, often, humidity. Air pollutants attack others: ozone fades many dye-based inkjet inks, and sulfur-containing gases darken lead-based pigments. Heat from lamps or sun can raise surface temperatures and speed every thermal process. Conservation guidance therefore treats light, incorrect temperature, incorrect humidity and pollutants as separate agents of deterioration, each with its own controls, rather than blaming everything on sunlight.
Fading is rarely even. A colour mixed from several colourants shifts in hue as the least stable one goes first — a green made of blue and yellow turns bluer when the yellow fades, a purple textile turns brown or grey as its red component goes. Printed images take on a cast when one ink fades faster. Pale tints fade sooner than full-strength colour because there is less colourant to lose, and the edge of a picture protected by a mount or frame rebate often shows the original colour beside the faded field. Fading is irreversible: the molecules are destroyed, not hidden. The measure conservators use is the just noticeable fade, which CCI equates with grey scale 4, a colour difference of about ΔE 1.8.
| Wavelength | Region | Energy (eV) | Energy per mole of photons (kJ/mol) | Relative to 550 nm |
|---|---|---|---|---|
| 300 nm | UV-B; largely blocked by window glass | 4.13 | 399 | 183% |
| 340 nm | UV-A; excites optical brighteners | 3.65 | 352 | 162% |
| 380 nm | Edge of visible light | 3.26 | 315 | 145% |
| 420 nm | Violet | 2.95 | 285 | 131% |
| 500 nm | Blue-green | 2.48 | 239 | 110% |
| 600 nm | Orange | 2.07 | 199 | 92% |
| 700 nm | Deep red | 1.77 | 171 | 79% |
Why: Visible light still fades sensitive dyes; the filter removed only part of the risk.
Fix: Reduce light level and hours as well, or display a copy and keep the original in the dark.
Why: The green was made of blue and yellow dyes and the yellow faded first.
Fix: Choose fabrics rated for lightfastness for sunny rooms, and use blinds or UV-filtering film at peak hours.
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.
Colourwise analysisStrong evidence
Photon energy is inversely proportional to wavelength: about 3.65 eV (352 kJ/mol) at 340 nm against about 2.07 eV (199 kJ/mol) at 600 nm.
Based on: Colourwise calculation E = hc/λ using the exact SI values of the Planck constant, speed of light, elementary charge and Avogadro constant; see the table.
Source: CODATA Value: Planck constant
FactStrong evidence
Daylight through window glass contains about 400–500 µW/lm of UV, compared with about 75 µW/lm for an ordinary incandescent lamp, the value adopted as the traditional museum UV limit.
Source: Agent of deterioration: light, ultraviolet and infrared
FactStrong evidence
Both UV and visible light cause fading; it is incorrect to assume that avoiding UV exposure will stop this form of damage, and light-sensitive materials fade mainly from visible light with a minor contribution from UV.
FactStrong evidence
Print fading is the chemical breakdown of colourants into invisible forms by heat, light (especially UV-containing light) and pollutants; unequal fading of colourants produces a colour shift.
FactStrong evidence
CCI defines a just noticeable fade as the change the industry considered just noticeable, equal to grey scale 4 or a colour difference of ΔE 1.8.
Source: Agent of deterioration: light, ultraviolet and infrared
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.