Energy Conversion
Physical phenomena describe the difference in wavelength between the energy an atom absorbs and the energy it subsequently emits as light. The stokes shift occurs when molecules absorb high-energy ultraviolet radiation and release it at a lower energy, which corresponds to a longer wavelength in the visible spectrum. This process is the driving force behind the performance of optical brightening agents in paper manufacturing.
Brightness Enhancement
Paper mills use chemicals that exhibit a large stokes shift to make products appear whiter and brighter. These molecules absorb invisible UV light and re-emit it in the blue part of the visible spectrum, which compensates for the natural yellow tint of wood fibers. The magnitude of the shift determines how effectively the paper can transform ambient light into additional brightness.
If the shift is too small, the emitted light might stay in the UV range and remain invisible to the eye. This effect is most noticeable under daylight conditions which are rich in the ultraviolet energy required to trigger the reaction.
Fluorescence Measurement
Instruments measuring color on brightened board must account for this shift by using calibrated UV filters. Failure to manage the excitation energy leads to inconsistent brightness readings across different spectrophotometers.
