White is primarily a sensation like blue, green, or red and, as such, is not measurable directly. Only a physical property, the spectral reflectance of a sample, can be measured directly. But this is not a standard fixed quantity; it depends on a number of individual properties of the measuring instrument used. The entire geometry of the illuminating chamber - generally speaking a sphere is used for samples with a structured surface - is incorporated in the measuring results. The size of the aperture and the exclusion or inclusion of gloss also influence reflectance. High whiteness is obtainable only, with the aid of fluorescent whitening agents (FWAs), and is hence a fluorescent color, which demands specific qualities of the illumination. The sample illumination must be identical with that for which the colorimetric values have been calculated. Nowadays, however, this is usually standard illuminant D65, which can be simulated only, approximately in measuring instruments. In addition, all lamps used are subject to changes in spectral energy, distribution. The problem is how to obtain constant, comparable results, namely whiteness, tint, and lightness for fluorescent materials using measuring instruments of different designs incorporating different means of stimulating standard illuminant D65 or other D illuminants. This article presents a method that has been in use in industry for about 20 years. The method in question comprises two parts: first, on the hardware side, sample illumination that has to meet specific requirements, match the UV excitation required, and remain stable; second, on the software side, the two critical dimensions of whiteness and tint are calculated indirectly from the measuring rests. Only in this way is it possible to achieve a large measure of comparability between different instruments. In principle, the method is also suitable for different illuminants and for any white preference. In all other methods of assessment the parameters are not matched to the instrument characteristics. If the results obtained with different measuring instruments are to be compared, difference values have to be used entailing the need for standards and involving all the drawbacks associated with them.