Light fastness tells you how long a colour resists fading. It is one of the first questions in any outdoor or long-life application.
How it is rated
In the blue wool method (for example ISO 105-B02 for textiles), samples are exposed to filtered xenon light together with eight blue wool references that fade at known, roughly doubling rates. The rating is the reference that fades about as much as the sample: 1 is very poor, 8 is outstanding. Plastics and coatings often use a similar 1 to 8 scale or report ΔE after a set exposure.
What affects the result
- Concentration: tints with white fade faster than full shades
- Pigment class: inorganic and high-performance organic pigments outlast dyes and fluorescent colours
- Binder or polymer: the medium can protect or degrade the colorant
- UV stabilisers: help both the polymer and many colorants
Practical advice
Specify light fastness at the concentration and in the medium you will use, and add weathering tests for outdoor products, since light fastness alone does not cover heat and moisture.
