Q: How many hours of light fastness testing do zippers need?
Light fastness is assessed by exposing the specimen alongside blue wool references and comparing the amount of fading. The result is expressed as a blue wool grade from 1 to 8, and the exposure can be measured in hours under a xenon arc lamp or, more reliably, as a measured dose of radiation. Reporting only hours makes results hard to compare between laboratories.
The method, the filter system, the black-panel temperature, the humidity and the irradiance all affect the outcome, so quote them with the grade. For a zipper, the exposed part is usually the tape on an outer placket, and the contrast between tape and teeth is what customers notice. Test the darkest and the brightest shade, since fading shows differently on each, and test after any finish that could affect the dye.
Acceptance is commonly a blue wool grade of 4 or better for apparel and higher for automotive or outdoor applications, but the figure should come from the customer's specification. Judge by the loss of colour against the reference rather than by eye alone, and keep the exposed and unexposed specimens together.