Optical Brightener also known as optical brightening agent, fluorescent brightening agent or fluorescent whitening agent are dyes that absorb light in the ultraviolet and violet region of electromagnetic spectrum and re-emit light in the blue region of fluorescent.

They used to make white or light colored fabric appear brighter. Mostly bleached white fabric are treated with these brighteners. Fabrics and garments that are truly prepare for dyeing should not contain brighteners. Optical brighteners can interfere with some dyes by competing for the dye sites on the fibres.

Optical brighteners have the property of absorbing ultra violet light and reemit energy in the form of weaker energy, so that the yellow color of material will appear white.

The chemical structure of optical brightening agent contain an aliphatic carbon carbon double bond which is sensitive to sunlight, oxidation, weathering etc. Therefore this compound don’t have good or standard fastness properties and tend to loose the ability to absorb ultra violet light over short period time of use. Using high concentration or improver application or cheap quality of optical brighteners could leave the material yellow instead of whitening.

Mainly optical brighteners are used in laundry detergents and textile finishing. Optical brighteners are found in domestic waste waters that have a component of laundry effluent.