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The '''Blue Wool Scale''' measures and calibrates the permanence of colouring [[dye]]s. Traditionally this test was developed for the [[textile]]s industry but it has now been adopted by the [[printing]] industry as measure of lightfastness of ink colourants.<ref>1</ref><ref>2</ref>
Normally two identical dye samples are created. One is placed in the dark as the [[Scientific control|control]] and the other is placed in the equivalent of sunlight for a 3 month period.
The amount of fading of the sample is then measured by comparison to the original colour. A rating between 0 and 8 is awarded by identifying which of the strips on the bluewool standard card has faded to the same extent as the sample under test. <ref>3</ref><ref>4</ref>.
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. The [[Ultraviolet]] (UV) radiation in light is responsible for ink fading. As the intensity of UV radiation differs from place to place, the ink fading also depends on place. It will be more in areas with more UV radiation and vice versa.
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