Optical Mechanism
Chemical compounds incorporated into paper pulp or surface coatings absorb invisible ultraviolet radiation and re-emit it as visible light in the blue spectrum. This phenomenon, referred to as stilbene fluorescence, is the primary mechanism used to offset the natural yellow tint of wood fibers. Most optical brightening agents used in papermaking are derivatives of di- or tetra-sulfonated stilbene compounds.
By shifting energy from the ultraviolet region to the visible region, these chemicals increase both the brightness and whiteness of the sheet. This addition creates a more vibrant surface that enhances the contrast of printed text and images.
Ultraviolet Utilization
Efficiency of the brightening effect relies heavily on the presence of ultraviolet wavelengths in the viewing environment. When paper that exhibits stilbene fluorescence is viewed under natural daylight or under spectrophotometers using M1 conditions, the blue emission is highly pronounced. Under incandescent or M2 measurement conditions which lack ultraviolet light, the emission is absent, and the paper appears in its natural unbrightened state.
This variation makes it necessary to control the illumination source when evaluating papers treated with these compounds. Understanding this dependency helps print houses avoid mismatches when packaging is moved from a brightly lit production floor to a retail display case.
Quenching Constraint
Excessive loading of whitening agents can lead to a self-absorption process known as optical quenching where the compounds absorb their own emitted light. When paper sheets are formulated with too much stilbene fluorescence, the perceived brightness stops increasing and the sheet takes on a grey or violet tint. This limit dictates the maximum amount of brightener that can be economically added to the pulp during the papermaking process.
