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Instrumental and Operational Impacts on Spectrophotometer Color Measurements
J. Rodgers, D. Thibobeaux, X. Cui, V. Martin, M. Watson, and J. Knowlton
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Color measurements for the classing of U.S. cottons are normally performed on the Uster® High Volume Instrument (HVI), which yields two color parameters specific to cotton—Rd (reflectance) and +b (yellowness). Since Rd and +b do not readily relate to other well known and globally recognized color systems (e.g., L*a*b* or CIELAB), a program was implemented to evaluate and validate the relationships of Rd and +b to a globally recognized color system (L*a*b*) and to investigate the impacts of key instrumental and operational variables on the color results. The tile and cotton fiber batt samples analyzed were measured on 7 color spectrophotometers (bench-top and portable). Strong L*↔Rd and b*↔+b correlations were observed on all color units, and very similar linear relationships were obtained for L*↔Rd and b*↔+b regardless of the bench-top unit used. Thus, the use of L*a*b* for relating globally recognized color parameters from a color spectrophotometer to the HVI’s Rd and +b color parameters was validated. The primary variable that impacted the color agreement between units was the use of HVI glass in front of the sample. L* was the color parameter that was most impacted by the use of glass in the tile and cotton fiber color measurements. The use of glass leads to a "shift" in the sample’s reflectance spectrum (%R). These shifts in %R, and the use of only 2 color filters in the HVI unit, lead to the observed differences in L*↔Rd when glass is used in the measurement.