Whiteness Index and Optical Brightener Measurement

Whiteness index measurement (ISO 11475, ISO 11476, AATCC TM110) quantifies the perceived whiteness of bleached, optically brightened textile substrates beyond the limits of CIE L*a*b* — since pure white in L*a*b* space is L* = 100, a* = 0, b* = 0, but optically brightened fabrics can exceed Y = 100 reflectance in blue region through fluorescence, making standard reflectance measurement insufficient. CIE Whiteness Index W (ISO 11475, D65/10°, SCI measurement mode): W = Y + 800(xn − x) + 1700(yn − y) where xn, yn are CIE D65 10° standard illuminant chromaticity coordinates (xn = 0.3138, yn = 0.3310) — W scale: 0 = reference white, typical bleached cotton without OBA (optical brightening agent) W = 50–70; bleached cotton with OBA 0.2% owf W = 90–110; maximum over-brightened W = 140 (UV fluorescence saturation). CIE Tint value Tw (ISO 11475, same formula): positive Tw = greener tint (over-blue), negative Tw = redder tint (yellower) — Tw within ±3 units indicates neutral white balance; Tw > +6 indicates excessive OBA giving blue-violet cast visible in retail under UV-rich fluorescent lighting. Fluorescence measurement (UV-excluded versus UV-included spectrophotometry, Datacolor 600 UV filter system): reflectance at 400–450 nm with UV filter included versus UV filter excluded reveals OBA contribution — OBA content ≈ (Y_UVinc − Y_UVexcl) / Y_UVexcl × 100: cotton woven fabric 0% OBA: 0% UV contribution; 0.2% OBA owf: 8–12% UV fluorescence contribution to Y. Whiteness calibration: BaSO₄ pressed powder reference (W = 100 by definition in many scales), Ceramic calibration tile (NIST or PTB traceable, assigned L*a*b* values, recertified annually) — daily calibration check using reference tile essential; inter-instrument whiteness agreement ±2 W units achievable with calibrated instruments.