4. Color fastness to light
Taking GB/T 8427-2019 method 3 as an example, in the rating of the light fastness test, a sample and a target blue wool standard sample and a blue wool standard sample one level and two levels lower than the target blue wool standard sample are exposed together until the end of the exposure, and then the exposed sample and the blue wool standard sample are compared for grade determination. During the evaluation, each stage of the sample corresponds to the blue wool standard sample. If the grades obtained at different stages are different, the arithmetic average of the grades at different stages can be used to evaluate the grade; when the average color fastness to light is not a whole level or half level, the nearest higher half level or whole level is taken. (The American standard is to evaluate the sample after sun exposure and the original sample according to the color change gray card)
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