AATCC fading unit (AFU)

a specific amount of exposure made under the conditions specified in various test methods where one AFU is one-twentieth (1/20) of the light-on exposure required to produce a color change equal to Step 4 on the Gray Scale for Color Change or 1.7 ± 0.3 CITELAB units of color difference on AATCC Blue Wool Lightfastness Standard L4.