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.