Optical brightener

Optical brighteners, optical brightening agents, fluorescent brightening agents or fluorescent whitening agents (FWAs) are dyes that absorb light in the ultraviolet and violet region of the electromagnetic spectrum, and re-emit light in the blue region. These additives are often used to enhance the appearance of color of fabric and paper, causing a "whitening" effect, making materials look less yellow and by increasing the overall amount of light reflected to the eye.

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The most common class of chemicals with this property are the stilbenes and fluorescent dyes such as umbelliferone, which absorb energy in the UV portion of the spectrum and re-emit it 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 light emitted by the brightener hides yellow and brown tones, making treated materials appear whiter.

Brighteners are commonly added to laundry detergents to replace whitening agents removed during washing and to make the clothes appear cleaner. Optical brighteners have replaced bluing which was formerly used to produce the same effect. Some brighteners can cause allergic reactions when in contact with skin.

Brighteners are used in many papers, especially high brightness papers, resulting in their strongly fluorescent appearance 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.

Care should be taken that military uniforms such as the Army Combat Uniform, Marine Corps Combat Utility Uniform, and the Airman Battle Uniform are not washed with optical brighteners, as these will make them more visible through Night Vision Devices or under low light conditions.

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