The differential threshold or the least concentration of the fluorescent brightening agents in laundering solution which affects the visual colour of the plain dyed fabrics have been investigated on the thirteen kinds of cotton broad fabrics by using the psycho-physical method of mimimal change. The light pinkish and two kinds of white fabrics,
i.e., commercial fabrics finished or unfinished with a fluorescent brightening agent, have shown an effect of fluorescent brightening agents even under lower concentration of fluorescent brightening agents than those of normal laundering solution. However, bluish and grayish fabrics have shown an effect under rather higher concentration. Other broad fabrics of eight kinds of colours have shown no visual effect even though fabrics have been treated in the solution containing a fluorescent brightening agents of 55 times concentration of normal laundering solution. In addition, the correlationship has been analysed between the results of sensory tests and the variation of colour due to absorption of fluorescent brightening agents,
i.e., intensity of fluorescent light, spectral reflectance, dominant wave length, excitation purity, or brightness (Y value) . Analyses has shown that the fabrics treated with fluorescent whitening agents have higher reflectance value in the range of short wave and, moreover, reflectance value and intensity of fluorescent light are more accelerated by a sorption of fluorescent brightening agents in the range of short wave.
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