The importance of chromaticity in the evaluation of whiteness
The factor of paramount importance in the visual evaluation of whiteness of laundered goods is yellowness. This yellowness has in practice been neutralized in various ways, first by the use of bleaches and/or blueing dyes and more recently by means of fluorescent dyes. Any such additive provides at best however an imperfect solution to the problem of obtaining maximum whiteness. Bleaches may cause fiber damage, and blueing dyes are undesirable because their absorption prevents attainment of maximum whiteness; fluorescent dyes are not permanent because of their limited light-fastness and, more fundamentally, because a sample containing sufficient fluorescent dye to be neutral under a given illumination will necessarily appear colored when viewed under another light source containing a different ratio of ultraviolet to visible light. It is therefore essential, in the search for the best detergents, to evaluate them first in the absence of any additives and to employ bleaches, blueing, or fluorescent dyes only to minimize their limitations.
The formulation of suitable soiling procedures and the application of reliable methods for measuring the washed cloth samples represent two important steps in the development of satisfactory laboratory tests for evaluating detergents. These steps are necessarily related to one another in a fundamental way. The procedures used in the past for detergency evaluation, involving the measurement of reflectance alone, have been appropriate with the soiling methods relying on carbon black as primary soiling agent but are not useful if realistic soiling techniques are employed and the degree of whiteness is to be determined.
While the colorimetric principles for an adequate objective rating of whiteness have been developed, the actual working methods should be based on a modern re-evaluation of the relative importance of the various colorimetric and psychological factors related to visual estimates of the whiteness of textiles. Such a reevaluation, coupled with more appropriate soiling and measuring techniques, will lead to improved procedures for the laboratory evaluation of detergents.
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