All dyed materials experience change in colour when exposed to daylight for a sufficiently long period because of the action of daylight on pigments used in them. The severity of exposure in both time and intensity of daylight determines the extent of change in colour.
The colorfastness of any fabric is denoted by the number of the light fastness standard fabric that has changed colour to the same extent as the test specimen after a specified degree of exposure. It is expressed as the numerical rating for light fastness.
The Exposure Rack is designed to test colorfastness of textile and leather materials to daylight. The test is carried out by exposing a set of light fastness standard fabrics 1 to 8 and a set of test specimens to daylight till the desired fading of the colour of the either the test specimen or light fastness standard 7 is obtained.
IS 686 – 1985 : Methods of Determination of Colour Fastness of Textile
Materials to Daylight