Blue Wool Reference Standards and Grey Scale Assessment

Blue Wool Standards (BWS, ISO 105-B01, SDC, WFK) are eight specially dyed wool reference fabrics (L2–L9) with calibrated light fastness grades 2–9 exposed concurrently with test specimens in xenon arc chambers to verify correct irradiance delivery and confirm exposure validity. BWS composition: L2 (Acid Blue 104 on wool, extremely sensitive — fades rapidly to grade 4 at 1 AFU exposure); L4 (Acid Blue 109, fades to grade 4 at 20 AFU); L6 (Acid Blue 83, grade 4 at 80 AFU); L8 (Acid Blue 121, grade 4 at 320 AFU); L9 (Mordant Blue 1, most stable — fades to grade 4 at 640+ AFU). Usage in testing: BWS L4 and L6 placed in same xenon arc exposure simultaneously as test specimens — when L4 reaches grade 4 boundary (visually assessed against grey scale), exposure equivalent to 20 AFU is confirmed regardless of clock time or kJ/m² reading, providing instrument-independent exposure verification. Grey Scale for Colour Change (ISO 105-A02, SDC): 5-step visual scale (grades 1–5 plus half-steps to 9 increments total) consisting of pairs of grey chips with defined CIE L*a*b* colour differences: grade 5 ΔE*00 = 0 (no difference), grade 4 ΔE*00 = 1.7, grade 3 ΔE*00 = 3.4, grade 2 ΔE*00 = 6.8, grade 1 ΔE*00 = 13.6. Grey scale replacement interval: SDC grey scales have ISO-certified reference values at manufacture — UV degradation yellows scale chips, altering ΔE* values after 2–3 years of laboratory use — annual calibration check against standard reference values and replacement at >0.5 ΔE*00 deviation from certified value. Instrumental grey scale grading: Datacolor 600, X-Rite Ci7800 (D65/10° illuminant/observer, SCE specular component excluded geometry, 30 mm aperture) measures CIE L*a*b* of exposed and unexposed specimen halves, calculates ΔE*00, converts to grey scale grade using ISO 105-A02 conversion table — instrumental grading inter-laboratory CV% = 0.15 grade versus visual CV% = 0.35 grade (>2× more reproducible).