Blue scale (colour fastness)
The blue scale is a standard comparative scale from 1 to 8 (1=very poor, 8=excellent) which is used for assessing the light fastness of printed or dyed fabrics and textiles (1 = very poor, 8 = excellent). It consists of 8 standards blue dyeing on smooth wool fabric with varying degrees of sensitivity to the action of daylight or artificial light.
In order to eliminate uncertainties in the evaluation of exposed blue standards and a test specimen, attempts have been made to obtain more precise ratings by colourimetric measurements. It has been found that to achieve the same degree of colour difference from daylight exposure amounts of light are necessary depending on the season of the year.
Use of the present blue scale as a standard comparative scale for lightfastness tests has been subject to increasing criticism in recent years, particularly in the USA, where a polyester fabric dyed with a violet dispersed to dye has been recommended as a substitute.
Prof. Dr. rer. nat. Hans-Karl Rouette Encyclopedia of Textile Finishing