Optical Brighteners are colorless dyes. Sometimes, it is added to cleaning chemicals to enhance the visual appearance of the substrate to be cleaned. When applied, optical brighteners absorb invisible, ultra-violet radiation, such as in sunlight and emit visible blue-white light. This phenomenon counteracts the natural yellowness of the substrate. Cleaners contain optical brightener (fluorescent whitening agents) to provide brighter and cleaner looking substrates relative to traditional cleaners.