Fluorescent Whitening Agents (also called optical brightener) absorb high energy radiation in the ultraviolet to violet region (330nm-380nm) on the part of characteristic molecules and emit lower energy radiation in blue region in visible spectrum (400nm-450nm), which yields the counteracting the yellowing appearance. FWA should be transparent on the substrate and should not absorb the visible region of the spectrum. Many FWAs contain the structure of stilbene. Stilbene is an unsaturated hydrocarbon with the formula C₆H₅CH=CHC₆H₅. Other nucleus of FWAs are coumarin, imidazole, 1,3-diphenyl-2-pyrazolin, naphtalimid, benzoxazole and heteroaromatics. FWAs are used in thermoplastics, detergents, plastic films, fibers, papers, adhesives and leathers to enhance the appearance.