Optical brighteners, also called brightening agents, fluorescent bleaches, and optical whiteners, are chemicals designed to help fabrics appear to be cleaner, brightening colors and lessening the natural yellowing of fabric over time. Some examples of optical brighteners are coumarins, naphthotriazolylstilbenes, benzoxazolyl, benzimidazoyl, naphthylimide, and diaminostilbene disulfonates (about.com). These ingredients are often shown on the labels of laundry detergents, but often are omitted.