An apparatus and method for making quantitative measurements of the amounts of optical brighteners in water samples using fluorescence measurements at multiple wavelengths. First and second emission wavelength raw measurements are corrected for sample absorption to provide absorption-corrected first emission wavelength and second emission wavelength fluorescence emission values. The absorption-corrected first emission wavelength value is compared to the absorption-corrected second emission wavelength value and to similarly-determined and similarly absorption-corrected emission wavelength values or a ratio thereof obtained from a comparison water sample in which optical brighteners are not present or are only minimally present, to provide a quantitative optical brightener measurement. The apparatus and method may be field-based or laboratory-based, and may operate on a flow-through basis or on discrete samples.