Lightfastness is a property of a colourant such as dye or pigment that describes how resistant to fading it is when exposed to light. Dyes and pigments are used for example for dyeing of fabrics, plastics or other materials and manufacturing paints or printing inks.
The bleaching of the color is caused by the impact of electromagnetic radiation in the chemical structure of the molecules giving the color of the subject. The part of a molecule responsible for its color is called chromophore.
Light encountering a painted surface can either alter or break the chemical bonds of the pigment, resulting the colors to bleach or change, in a process known as photodegradation. Materials that resist this effect are said to be lightfast. The electromagnetic spectrum of the sun contains wavelengths from gamma waves to radio waves. Particulary the high energy of the ultraviolet radiation accelerates the fading of the dye.
The photon energy of UVA-radiation which is not absorbed by the athmospheric ozone is exceeding the disassociation energy of carbon to carbon single bond, which is resulting the cleavage of the bond and fading of the color. Inorganic colourants are considered to be more lightfast than organic colourants. Black colourants are usually considered the most lightfast.
During the fading colourant molecules undergo various chemical processes which result in fading.
When a UV-photon is reacting with a molecule acting as colourant, the molecule excites from ground state to excited state. Excited molecule is highly reactive and unstable. During the quenching of the molecule from excited state to ground state, athmospheric triplet oxygen reacts with the colourant molecule to form singlet oxygen and superoxide oxygen radical. The oxygen atom and the superoxide radical resulting from the reaction are both highly reactive and capable of destroying the colourants.
Photolysis ie. photochemical decomposition is a chemical reaction where the compound is broken down by the photons. Hight energy photon is encountering a colourant molecule bond with suitable dissociation energy. Reaction causes homolytic cleavage in the chromophoric system resulting the fading of the colourant.
Photo-oxidation ie. photochemical oxidation. Excited colourant molecule undergoes oxidation process. In the process the chromophoric system of the colourant molecule reacts with the athmospheric oxygen dorming nonchromophoric system which results in fading. Especially colourants which contain carbonyl group as chromophore are vulnurable to oxidation.
Photoreduction ie. photochemical reduction. Colourant molecule with unsaturated double bond typical to alkenes or triple bond typical to alkynes acting as a chromophore undergoes reduction in presence of hydrogen forming saturated chromophoric system. Saturation causes the length of the chromophoric system decreasing resulting the fading of the colourant.
Photosensitization ie. photochemical sensitization. Exposing cellulosic dyed material to sunlight allows dyes to abstract hydrogen from the cellulose which results photo reduction on the cellulosic substrate. Simultaneously colourant will undergo oxidation in presence of the atmospheric oxygen resulting photo-oxidation of the colourant. These processes result both in fading of the colourant and strength loss of the substrate.
Phototendering ie. photochemical tendering. As a result of UV-light material supplies hydrogen to the colourant molecules which causes reduction of the colourant molecule. As the hydrogen is abstracted from the maeterial it undergoes oxidation.
Standards and measure scales
Some organizations publish standards for rating the lightfastness of pigments and materials. Testing is typically done by controlled exposure to sunlight, or to artificial light generated by a xenon arc lamp. Watercolors, inks, pastels, and colored pencils are particularly susceptible to fading over time, so choosing lightfast pigments is especially important in these media.
The most well known scales measuring the lightfastness are the Blue Wool scale, Grey scale and the scale defined by ASTM (American Standard Test Measure). On the Blue Wool scale the lightfastness is rated between 1-8. 1 being very poor and 8 being excellent lightfastness. In grey scale the lightfastness is rated between 1-5. 1 being very poor and 5 being excellent lightfastness.  On ASTM scale the lightfastness is rated between I-V. I is excellent lightfastness and it corresponds to ratings 7-8 on Blue Wool scale. V is very poor lightfastness and it corresponds to Blue Wool scale rating 1.
The actual lightfastness is dependant on the strength of the radiation of the sun, so lightfastness is relative to geographic location, season, and exposure direction. The following table is listing suggestive relations of the lightfastness ratings on different measure scales and the relation relative to time in direct sunlight and normal conditions of display: away from a window, under indirect sunlight and properly framed behind a UV protective glass.
|Description||Measure scales||Direct exposure||Normal conditions of display|
|Blue Wool rating||ASTM rating||Summer||Winter|
|Very poor lightfastness||1||V||less than 2 years|
|Poor lightfastness||2||IV||2–15 years|
|3||4–8 days||2–4 weeks|
|Fair lightfastness||4||III||2–3 weeks||2–3 months||15–50 years|
|5||3–5 weeks||4–5 months|
|Very good lightfastness||6||II||6–8 weeks||5–6 months||50–100 years|
|Excellent lightfastness||7||I||3–4 months||7–9 months||over 100 years|
|8||over 1.5 years|
Relative amount of fading can be measured and studied by using standard test stripes. In the workflow of Blue Wool test one reference stripe set shall be stored protected from any exposure of light. Simultaneously with samples another equivalent test stripe set is exposed under the light source defined in the standard. For example if the lightfastness of the colourant is indicated to be 5 on Blue Wool scale, it can be expected to fade similar amount as the strip number 5 in Blue Wool test strip set. The accompisment of the test can be confirmed by comparing the test stripe set with the reference set stored protected from the light.
In graphical industry
In printing inks mainly organic pigments are used, so shifting or bleaching of the color of a printing product due to presence of the UV light is usually just a matter of time. The usage of the organic pigments is justified especially by their inexpensive price comparing to inorganic pigments. The particle size of the inorganic pigments is often larger than on organic pigments thus all inorganic pigments are not suitable to be used in the offset printing.
In the screen printing the particle size of the pigment is not limiting factor. Thus it is preferred printing method for printing print jobs requiring extreme lightfastness. Thickness of the ink layer effects to the lightfastness bt the amount of pigment laid on the substrate. Ink layer printed by the screen printing is thicker than layer printed on offset printing. In other words it contains more pigment per area. This leads to better lightfastness even though the used printing ink in both methods would be based on the same pigment. 
When mixing the printing inks the lightfastness of the ink being weaker by its lightfastness defines the lightfastness of the whole color. The fading of one of the pigments leads to tone shift towards to component with better lightfastness. If it is required that there will be something visible from the printing even though its dominant pigment would fade, small amount of pigment with excellent lightfastness can be mixed with it.
- Blue Wool Scale - a measure of dye permanence
- Color fastness - resistance to fading of textile colors
- Fugitive pigment - pigments that are susceptible to fading or altering over time
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