Meaning
A standardized set of reference fabrics dyed with specific blue dyes measures the light sensitivity of other materials. Professionals use the blue wool scale to assign a rating from one to eight based on how much a sample fades compared to the reference fabrics. Lower numbers indicate high sensitivity to light while the highest number represents exceptional resistance to fading.
This system originated in the textile industry but now applies to plastics and printing inks.
Comparative Standard
Eight distinct wool strips receive varying amounts of dye to create a predictable sequence of light sensitivity. During a test, the blue wool scale strips sit alongside the target material under a light source for a designated period. Inspectors look for the first sign of fading on both the test material and the reference strips to find a match.
The rating depends on which wool strip shows the same degree of change as the specimen. Every evaluation requires a visual comparison under standardized viewing conditions to ensure accuracy. Results are recorded as a single integer to simplify communication between suppliers and buyers.
Sensitivity Grade
Each level on the scale represents approximately double the lightfastness of the previous level. Using the blue wool scale allows a contract to specify a minimum light resistance for retail packaging or outdoor displays. Level six provides sufficient stability for most indoor commercial environments over several years.
Manufacturers of low cost goods might only require level four for short term seasonal promotions.
Contractual Reference
Purchase agreements often mandate a specific grade to ensure that product branding does not deteriorate on store shelves. Reliance on the blue wool scale protects the distributor from losses associated with faded inventory.
