Optical brightener
Optical brighteners, optical brightening agents (OBAs), fluorescent brightening agents (FBAs) or fluorescent whitening agents (FWAs)  are dyes that absorb light in the ultraviolet and violet region (usually 340-370nm) of the electromagnetic spectrum, and re-emit light in the blue region (typically 420-470nm). Fluorescent activity is a shortterm or rapid emission response, unlike phosphorescence, which is a delayed emission. These additives are often used to enhance the appearance of color of fabric and paper, causing a perceived "whitening" effect, making materials look less yellow by increasing the overall amount of blue light reflected.
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.
There are approximately 400 brightener types listed in the Color Index, but less than 90 are actually produced commercially and only a handful are commercially important. Generically, the C.I. FBA number can be assigned to a specific molecule, however, some are duplicated, since manufacturers apply for the index number when they produce it. The global OBA production for paper, textiles and detergents is dominated by just a few di- and tetra-sulfonated triazole-stilbenes and a di-sulfonated stilbene-biphenyl. These are subject to fading when exposed long term to UV, due to the formation of optically inactive stilbene cis-isomers found at the center of the molecule. All brighteners have extended conjungation and/or aromaticity, allowing for electron movement. Some non-stilbene brighteners are used in more permanent applications such as whitening synthetic fiber.
Basic class types of brighteners include:
Brighteners can be "boosted" by the addition of certain polyols like high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be "quenched". Too much use of brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum. Besides the formation of cis isomer in stilbene-containing brighteners (only the trans isomer is optically active), continued exposure to UV-containing light will actually cleave the molecule and start the process of degradation.
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 brightness is typically measured at 457nm, well within the fluorescent activity range of brighteners. Paper used for banknotes does not contain optical brighteners, so a common method for detecting counterfeit notes is to check for fluorescence.
Care should be taken that military uniforms such as the Army Combat Uniform and Marine Corps Combat Utility Uniform (and formerly 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.