How Does Optical Brightener Work?

Fluorescent Whitening Agent (OBA) or fluorescent Whitening Agent (FWA) is a colorless to weakly colored organic compound that absorbs ultraviolet light and absorbs most of the absorbed energy in a solution or matrix, such as between 400 and 500 nm. Blue fluorescence.
From the human eye, the substance that uniformly reflects most of the light at all wavelengths of its surface is white. For example, natural fibers typically absorb more light in the blue region of the visible spectrum ("blue defects") because they contain impurities (natural pigments). Therefore, natural fibers exhibit unwanted yellow castings. Synthetic fibers also have this pale yellow casting, although not very obvious.
The whiteness of the substrate can be improved by (1) increasing reflection (reflectance) or (2) compensating for blue defects. Bleaching has these effects to some extent, but always leaves a portion of pale yellow gypsum. Even the most thorough bleach does not remove all traces of yellow castings. Prior to the appearance of optical brighteners (OBA), it is common practice to apply a small amount of a blue or violet dye (referred to as "blue") to enhance the visual impression of whiteness. . These dyes absorb light in the green-yellow region of the spectrum, thereby reducing brightness. However, since they change the hue of the pale yellow material to blue, the human eye feels an increase in whiteness.
Unlike dyes, fluorescent whitening agents can counteract yellow light polarization and increase brightness because their blue effect is not based on subtracting yellow and green light, but on increasing blue light. Fluorescent whitening agents (OBA) are actually a colorless compound. When it is present on the substrate, it mainly absorbs invisible ultraviolet light in the range of 300-400 nanometers (nm) and re-emits light with visible violet-blue fluorescence. This fluorescent whitening agent absorbs invisible short-wave radiation and re-emits in visible blue light, making the light reflected by the substrate brighter, which is the key to the effectiveness of the fluorescent whitening agent.