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          ISUAL assessment of light fastness of colored    substrates …….against the Standard Blue Scale is considerably not accurate due to the difficulties in the differentiation between small color changes either at lower or higher shades. In this work, nylon fabric and polyamide films (PA) samples , and cellulose diacetate (CDA) films were  dyed with different disperse dye concentrations, also wool, viscose and unmodified and modified cotton samples were dyed with reactive dye at same concentrations. The dyed samples were exposed to artificial day light for various periods (20-160 hours) were examined. Color strength, the light fastness and the fading rate constant (K) of these samples were evaluated. It was concluded that for a series of dye concentrations, same light fastness rating were observed and/or with an appreciable change. Since 'K' is a concentration dependent, it decreases by increasing the dye concentration and at each dye concentration a definite 'K' value was obtained. Also, direct relation was found to exist between light fastness rating values and 'K' values. For each light fastness rating of the Standard Blue Scale, there exist a range of 'K' values. Thus, low 'K' values showed high light fastness of the order (7). K values are precise and more accurate and reflect the real change in the light fastness of colored textiles     .
 
Keywords: Colored textile, Blue scales, Light fastness, Fading rate constant, Reactive dye, Disperse dye and Films.