There is a special class of dyes which are similar to direct cotton dyes in that they may be applied like direct dyes. But these are colorless compounds having affinity to cotton. These dyes are called ‘White dyes’.
These have the property of fluorescence, by virtue of which they absorb ultraviolet light and convert it into visible light and brighten the textile fabric; therefore these are called ‘Fluorescent brightening agents’.
Since this whitening is an optical effect, they are also called ‘Optical brightening agents’.
Examples- Fluorescent purple 2G, Leucophor B, Glow white etc.