A textile sample is exposed to artificial daylight with up to 8 different strips of blue wool reference material under specified conditions.

The blue wool reference material is divided into 8 different light fastness types – depending on the dye. Light fastness type 1 has a very low light fastness, while light fastness type 8 has a very high light fastness.

Method 3 is used to check minimum requirements, because of course different requirements are placed on light fastness depending on the intended use. For example, covers for garden furniture should have a high light fastness because they are exposed to high light intensity during the use phase. For other textiles that are only used indoors, for example, an average light fastness is sufficient.

Depending on the minimum requirement, there are various exposure levels to choose from, which differ in the duration of exposure.

Depending on the exposure level, the required light fastness types are attached to one another on a cardboard together with a textile sample. Then half of the cardboard is covered so that only part of the sample is exposed to the artificial light (xenon arc lamp).

The sample is exposed until the minimum requirement (light fastness type) shows a colour change corresponding to grade 4 of the grey scale.

Finally, the textile sample is compared with the blue light fastness types and graded with the number of the light fastness type that comes closest to the degree of fading of the textile sample.