UV Protection
Dye can darken the yarn but it cannot close the gaps. Open area sets a UPF ceiling before the colour is even chosen.
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Reciprocal of total UV transmission
Decision-support only, and not a sun-protection claim. Real UPF is measured spectrophotometrically across the UVA and UVB bands and must be tested to the governing standard before any garment is labelled. Stretch, wetting and wear all open the structure and lower UPF from the value predicted here.
Fabric UPF (Ultraviolet Protection Factor) Predictor — free, with the formula and a worked example, at Textile School.