A fluorescent dye used to make white fibers appear brighter; often called just "optical" in dye industry; also called Fluorescent Brighteners or Fluorescent Brightening Agents (FBA). Optical brighteners are colorless dyes that work by emitting visible light when exposed to invisible ultra-violet light. They are used to make white or light-colored fabrics appear brighter. Most fully bleached commercial white fabrics have been treated with these brighteners. Fabrics and garments that are truly prepared for dyeing should not contain brighteners. Optical brighteners can interfere with some dyes by competing for the "dye sites" on the fibers.