Colour Fastness to Light Testing

Light fastness testing measures the resistance of textile dyes and pigments to fading under exposure to daylight or artificial light sources (xenon arc, carbon arc, UV fluorescent) — the critical fastness property for window furnishings, automotive upholstery, outdoor textiles, and apparel worn in high UV environments. ISO 105-B02 (xenon arc light fastness, SDL Atlas Xenotest, Atlas CI4000, Q-Sun Xe-3) is the international standard using a water-cooled xenon arc lamp (6,500 W or 1,700 W) with daylight filter simulating solar spectral irradiance — test exposure quantified in AFU (arbitrary fading units, 1 AFU ≈ 1 hour xenon arc under standard conditions) or kJ/m² of irradiance (calibrated by radiometer at 340 nm, 0.51 W/m²/nm standard irradiance). Assessment: ISO 105-A02 grey scale comparison of exposed versus unexposed specimen half (covered half protected from light by black card mask) — grade 1 = severe fading, grade 8 = no change (Wool Reference Standards 1–8, Blue Wool Standards L2–L9 defining grades 2–8 by reference dyed wool exposure). Light fastness specification by end-use: budget fashion: grade 4 minimum; standard apparel: grade 4-5; premium outdoor: grade 6-7; contract furnishing: grade 5-6; automotive fabric: grade 5-6 (1,000 kJ/m² at 340 nm); outdoor awning/shade: grade 7-8. Dye class light fastness: vat dyes on cotton 5–8 (excellent); reactive dyes 4–6 (moderate); direct dyes 2–4 (poor); disperse dyes on polyester 5–7; acid dyes on wool 4–7 (variable).