Fluorescent whitening agent 191 absorbs high-energy near-ultraviolet light, causing its molecules to enter an excited state. The excited molecules then transition to a lower-energy ground state and emit fluorescence. Due to the energy loss, the emitted fluorescence wavelength becomes longer, approximately 450 nm blue light. The yellow color of yellowed items can be compensated by the blue light reflected by the fluorescent whitening agent, thereby increasing the apparent whiteness of the items.
