Los abrillantadores ópticos, agentes abrillantadores ópticos, agentes fluorescentes o fosforescentes abrillantadores o agentes fluorescentes o fosforescentes blanqueadores son colorantes que absorben luz en la región ultravioleta y violeta (usualmente 340-370 nm) del espectro electromagnético, y re-emiten luz en la región azul (típicamente 420-470 nm). La fluorescencia es una respuesta rápida de emisión de corta duración, a diferencia de la fosforescencia, que es una emisión retardada. Estos aditivos son usados frecuentemente para mejorar la apariencia de color de textiles y papeles, causando un efecto percibido de "blanqueamiento", haciendo que los materiales parezcan menos amarillos al incrementar la cantidad total de luz azul reflejada.

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The most common classes of compounds with this property are the stilbenes, e.g., 4,4′-diamino-2,2′-stilbenedisulfonic acid. Older, non-commercial fluorescent chemicals include as umbelliferone, which absorbs 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 can emit more visible light than that which shines on it, making it appear brighter. The blue light emitted by the brightener compensates for the diminishing blue of the treated material and changes the hue away from yellow or brown and toward white. Approximately 400 brightener types are listed in the Colour Index, but fewer than 90 are produced commercially, and only a handful are commercially important. Generically, the C.I. FBA number can be assigned to a specific substance, 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 derivatives. The stilbene derivatives are subject to fading upon prolonged exposure to UV, due to the formation of optically inactive cis-stilbenes. They are also degraded by oxygen in air, like most dye colorants. All brighteners have extended conjugation and/or aromaticity, allowing for electron movement. Some non-stilbene brighteners are used in more permanent applications such as whitening synthetic fiber. Brighteners can be "boosted" by the addition of certain polyols, such as high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be "quenched". Excess brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum.

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