Lightfastness is a property of a colourant such as dye or pigment that describes its resistance to fading 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 ultraviolet 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 the chromophore.
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.  Materials that resist this effect are said to be lightfast. The electromagnetic spectrum of the sun contains wavelengths from gamma waves to radio waves. The high energy of ultraviolet radiation in particular accelerates the fading of the dye.
The photon energy of UVA-radiation which is not absorbed by atmospheric ozone exceeds the dissociation energy of the carbon-carbon single bond, resulting in 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.
Lightfastness is measured by exposing a sample to a lightsource for a predefined period of time and then comparing it to an unexposed sample.