The lightfastness of a fabric is a value indicating the degree of discolouration caused by sunlight and daylight.
Intensive exposure of the fabric to daylight and sunlight will accelerate the discolouration of the fabric. Additionally, air, particularly ozone levels, and humidity also have an effect. To determine the lightfastness of a fabric, it is tested under ISO 105-B02. The result is a figure between 1 and 8, where 8 is the highest value and has almost no discolouration. Values between 4-5 are considered OK.