Measurement and assessment of substrates containing fluorescent whitening agents

White, as a color sensation, can be determined by colorimetric methods of measurement. With other than colorimetric methods, no satisfactory correlation between measured value and visual assessment can be expected. The contribution of hue, saturation and luminosity to the degree of whiteness as found by six formulas widely used in industry was analyzed and the results were plotted in the form of a graph. An easy-to-use nomogram for whiteness determination was developed, based on the CIE 1931 2°-color system. Another nomogram is presented which provides an easy means to determine the hue of a sample treated with fluorescent whiteining agents (FWAs). The new whiteness determination is based on the measurement of true tristimulus values for fluorescent samples using the new International Commission on Illumination standard illuminant D 65. The magnitude of the just perceptible difference in whiteness was determined in a panel test using 11 samples and 22 observers. The results were evaluated statistically. Pending standardization of an illuminant, not only for colorimetric measurements, but also for visual assessment of white samples containing FWAs, it would scarcely seem possible to arrive at a more accurate determination of degrees of whiteness. All visual rankings of fluorescent white substrates are widely scattered around a mean value for the different phases of natural daylight. The relative energy emitted by artificial daylight lamps in the near UV region is insufficient in many cases. Xenon lamps are quite satisfactory for instrumental evaluation. In an internationally approved system, with a known hue preference, it would be possible to compare an objective measured value with each subjective result obtained by visual evaluation.