Is the optical brightener a white dye?

This is strange, since my product itself is white, why do I need to use optical  brightener? In fact, a large number of organic materials, natural or artificial textiles or other materials, such as paper, plastic, etc., do not look white, usually they absorb more blue-violet light (400 to 480nm) than yellow light, so they appear more or less yellowish.From the name of optical brightener, we can know that its effect is whitening, rather than dyeing. In other words, the prerequisite for optical  brightener to play the whitening effect is that the product itself is white.

Optical brightener is dyed on substrates with certain whiteness (such as textiles, plastics, paper, detergents, etc.) in a way that absorbs the energy of ultraviolet light invisible to the naked eye (about 340~380nm) under sunlight (or light source containing enough ultraviolet light). It is then emitted as a very bright, showy blue-purple fluorescence (about 410~460nm). This blue-purple fluorescence is mixed with the slightly yellow light on the matrix (yellow is a complementary blue-purple color) to obtain white light, and has a whitening effect. Due to the enhanced ability of the matrix whitened by  optical brightener to reflect visible light, the whiteness and brightness of the matrix are increased, making the matrix whitened look white, pleasing to the eye and bright. Therefore, it can be seen that the whitening of optical  brightener is optical complement brightening, rather than dyeing.
The fluorescence of  brightener is produced by ultraviolet light, so the ultraviolet content of the irradiation source has an effect on the fluorescence intensity and whitening effect of optical whitening agent. If the ultraviolet content of the irradiation light is high, the whitening effect is particularly significant; However, in the light source with low content of UV light or no UV light, the whitening effect is little or no. Therefore, Ji 'nan Ogilvy chemical suggested that when comparing whiteness, we should try to be under natural light.