Lightfastness

Lightfastness is a property of a colourant such as dye or pigment that describes its resistance to fading when exposed to light.

[4][5] Light encountering a painted surface can either alter or break the chemical bonds of the pigment, causing the colors to bleach or change in a process known as photodegradation.

The oxygen atom and the superoxide radical resulting from the reaction are both highly reactive and capable of destroying the colourants.

The reaction causes homolytic cleavage in the chromophoric system resulting in the fading of the colourant.

A colorant molecule, when excited by a photon of sufficient energy, undergoes an oxidation process.

A colorant molecule with an unsaturated double bond (typical to alkenes) or triple bond (typical to alkynes) acting as a chromophore undergoes reduction in the presence of hydrogen and photons of sufficient energy, forming a saturated chromophoric system.

Simultaneously, the colorant will undergo oxidation in the presence of the atmospheric oxygen, resulting in photo-oxidation of the colourant.

Testing is typically done by controlled exposure to sunlight, or to artificial light generated by a xenon arc lamp.

[11] Watercolors, inks, pastels, and colored pencils are particularly susceptible to fading over time, so choosing lightfast pigments is especially important in these media.

In the workflow of the Blue Wool test, one reference strip set shall be stored protected from any exposure to light.

Simultaneously, another equivalent test strip set is exposed under a light source defined in the standard.

The thickness of the ink layer affects the lightfastness by the amount of pigment laid on the substrate.