Textile fibres do not appear perfectly white due to the presence of certain coloured impurities. During chemical bleaching, coloured impurities are either destroyed or decoloured by oxidation or reduction. Over-bleaching may reduce the fibre strength. Even well bleached fabrics possess a slight yellowish appearance. This yellowish hue of the materials can be eliminated by whitening with optical brighteners or fluorescent brightening agents (OBA or FBA). Sometimes blueing agents are also used. In 'blueing' the initial yellowish shade of the textiles is covered by the blue dye and a bluish white results. However, the corresponding blue dye itself absorbs in visible light and thus the total amount of reflected light is smaller than in the case of unblued material, so that the blued material is less bright i.e., it is dull, or greyish.
The optical brighteners counteract the yellowness of the fabric by increasing the reflection of blue light rays. They convert invisible short-wave ultraviolet rays of sunlight into visible blue light and has a degree of whiteness which is comparatively more intense.
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