Lightfastness

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.

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