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Monday, April 6, 2015

Fluorescence: Greening

(top) Non-fl & (bottom) Fl under black light
Office lighting

At low concentrations of FWA's (Fluorescent Whitening Agents), each molecule is isolated such that the local environment around the FWA consists of the paper and paper products.  As concentrations increase, all the available attaching sites become full and the FWA molecules begin aggregating and reacting with other FWA molecules.  This results in altering the fluorescing effectiveness of each molecule.  The net result is that the quantum efficiency of the FWA decreases.  This means that the increase in fluorescing effect due to each added unit of FWA decreases.  Therefore, the addition of more FWA's will result in an increase in TOTAL fluorescence, but the wavelength of the emission peak will shift toward the green.

More work still needs to be done to better understand when the greening limit is being approached.  This may result in a measurement that will predict greening before it is too late.