Blue Wool Lightfastness Rating from Exposure

The doubling rule is an idealisation of a scale whose real intervals are not exactly a factor of two and are not uniform across it - the gap between standards 6 and 7 is wider in practice than the gap between 2 and 3 - so a fractional rating computed this way is an interpolation for comparing samples rather than a reportable result. What gets reported is always the whole standard the sample matches, which is why the rounded figure is given beside it. The anchor matters more than anything else here: hours for blue wool 1 depend on the cabinet, the lamp, the filter, the irradiance setpoint and the black panel temperature, so the figure must come from the blue wool standards exposed ALONGSIDE the specimen in the same run, never from a table. That is the whole reason the standards are exposed together rather than a time being specified. Humidity is the variable most often left uncontrolled and it moves lightfastness substantially for many dye classes, which is why the effective humidity control standard is exposed as well. The AFU conversion is a convenience for translating between conventions and is approximate; ISO and AATCC do not define the endpoint identically, so a sample at the boundary can be reported differently under the two methods with nothing wrong in either.