[1][2] The American Association of Textile Chemists and Colorists pioneered the work on these test methods and together with the ASTM D13 committee.
A rating between 0 and 8 is awarded by identifying which one of the eight strips on the bluewool standard card has faded to the same extent as the sample under test.
[4][5] Zero denotes extremely poor colour fastness whilst a rating of eight is deemed not to have altered from the original and thus credited as being lightfast and permanent.
Relative values of fading, comparing the sample with the standard blue test strip, will depend less on intensity.
For example, if a pigment is rated as "BW5" it can be expected to fade to the same degree as strip number 5 on a bluewool test card, for any specific light exposure.