Fluorescent whitening agents improve the appearance of cloth or paper through two separate visual effects, a bluing effect and a lightening effect; the former is much more important than the latter. For any one shade of whitener, both the bluing and lightening effects are directly proportional to the effective fluorescence (defined here as the sum of the tristimulus values of the fluorescent light from the dyeing). However, when whiteners of different shades are compared at the same effective fluorescence, the greener-shade whitener has more of a lightening effect, but the redder-shade whitener has more of a bluing effect. These two advantages, one for the greener shade and the other for the redder shade, almost exactly cancel each other, so that at the same effective fluorescence both whiteners improve the cloth to the same degree. Effective fluorescence, as here defined, is therefore a reliable indication of whitening power irrespective of shade. This quantity may be measured on a fluorimeter, but an appropriate correction factor must be applied when two different whiteners are being compared.
© 1957 Optical Society of America
Equations on this page are rendered with MathJax. Learn more.