A chemical compound that creates a visual whitening or brightening effect when exposed to near ultraviolet radiation by virtue of fluorescence, i.e. the conversion of invisible ultraviolet light into visible blue light.  The fluorescent whitening agents used by the detergent industry share the further characteristic of adsorbing to fabrics during household laundering. The whiteness or brightness of the laundry is thus enhanced.  FWA is included in all purpose soap and detergent and some light duty and laundry aid products.  Its effectiveness varies with type of fabric and concentration in the wash water, which is always very low. Its effect is cumulative to a degree, so that new fabrics exhibit increased fluorescence over a period of washes.  Eventually, however, a leveling-off point is reached. In recent years an increasing number of fabrics have been pre-brightened in manufacture, i.e., have incorporated FWAs, especially the acrylic and polyester fabrics.