For most materials, colourfastness against artificial light is an important property that determines their suitability for many specific end uses. This characteristic is determined by exposing test specimens taken from the sample under test to a controlled intensity of artificial light under a specified humidity and temperature to obtain a specified degree of exposure. The actual degree of exposure is assessed with the help of colourfastness rating standards (blue standards), which are exposed along with the test specimens. Colourfastness rating is determined with the help of grey scale for change in colour. 

The Light Fastness Tester consists of a mercury vapour lamp, which is mounted in the centre of a cylindrical drum on which the test specimens are mounted. The drum is rotated at a constant speed to obtain an even exposure all over its inner surface. Humidity is maintained with the help of a standard saturated salt solution, which is kept in a stainless steel tray located below the drum. A fan is provided for gently stirring the air.