The Application of Brightness, Whiteness, and Color Measurements to Color Removal

It is recognized that brightness measurements alone are insufficient for evaluation of the effectiveness of the removal of color from pulp. Brightness measurements include only the blue portion of the visible spectrum and do not always relate well with visual assessment. Modern test instruments provide full color measurements as quickly as brightness measurements, however, color data is somewhat more complex and difficult to interpret. CIELAB is the most commonly used color system worldwide. It provides both simplicity and reasonably good agreement with visual color assessments. CMC color difference mathematics may be employed where closer agreement between instrumental measurement and visual assessments is critical. Whiteness measurement, which includes the entire visible spectrum, offers several advantages over the traditional measurement of paper brightness. The very different concepts of whiteness and brightness are often misunderstood and the measurement of these quantities is often misapplied. The use of Absorption Coefficients provides the bleaching chemist with an objective means of measuring the amount of colorant remaining tier each stage of color removal to assess the effectiveness of the process. Absorption values provide a direct measure of residual colorant concentrations. In addition to the bleaching chemistry, other factors such as natural fluorescence, optical brighteners, and residual ink in recycled fibers will influence color removal measurements. Various optical measurements may be applied to separate these variables to provide a better understanding of the decolorization process.