Blue Wool Scale.

A set of wool cloths dyed blue to known lightfastness, numbered from 1 for very low to 8 for very high, exposed to light beside a sample so its fading can be rated by comparison; the method sits in ISO 105-B02. PermaJet publishes one such certificate for an inkjet paper, run under a xenon arc lamp against references 2 to 6, and states that Blue Wool 6 is the accepted standard for limited edition inkjet prints; the Fine Art Trade Guild asks for 6 or more in all areas of the print. A rating is a rank against the references, not a number of years on a wall; the same PermaJet certificate equates its 6 to an 85-year display life, but that conversion is the laboratory's assumption about light levels, not part of the scale, and sellers quote the years as if the scale had produced them. It also belongs to the ink and paper pair that was tested, so a result earned on one stock does not travel to a cheaper one bought later. Claim it on the wrong pair and the claim fails at the first faded print sent back from a sunny room.