Blue Wool References are not correlated against actual daylight exposure. This is because real conditions vary.


It is not just the light that needs taking into consideration, but the temperature and humidity also.


As an example, we can say that if the irradiance is controlled at 42W/m² (300 – 400nm):

  • Blue Wool 5 will fade to a grey scale 4 in approximately 80 hours
  • Blue Wool 6 will fade to a grey scale 4 in approximately 160 hours


Blue Wool References are designed to fade in a geometric progression, each successive reference taking twice as long to fade to the same grade as its predecessor, e.g. a fabric with a colour fastness to light of 6 is twice as good as a fabric which has colour fastness to light of 5.


A PDF version of this FAQ is available for reference.