Optical brightener

From Freepedia

An optical brightener is an additive designed to enhance the appearance of colours in fabrics and on papers. It may also enhance or modify the appearance of white items.

The molecules that make up these additives are fluorescent dyes such as umbelliferone, which absorb energy in the UV portion of the spectrum. This energy is then re-emitted in the blue portion of the visible spectrum. A white surface treated with an optical brightener emits more visible light than shines on it, making it appear brighter. The blue emitted hides yellow and brown tones, making treated materials appear whiter. Optical brighteners have replaced bluing which was formerly used to produce the same effect.

The additives are commonly used in washing powders, detergents, and in paper manufacture, with the result that white paper and clothing show up as strongly fluorescent under UV illumination. Paper used for banknotes does not contain optical brighteners, so a common method for detecting forged notes is to check for fluorescence.