WHITENESS NOT BRIGHTNESS - A NEW WAY OF MEASURING AND CONTROLLING PRODUCTION OF PAPER

White is primarily a sensation like blue, green or red and, as such, is not directly measurable. Only the physical property, spectral reflectance can be directly measured. If colour is to be integrated into a colour system, the physical properties have to be correlated with the known properties of light and of the human eye. Every colour, including white, can be defined exactly with a minimum of three numbers. What distinguishes white from other colours is its high lightness, its low saturation, and that it is felt to be more attractive with a bluish rather than a yellowish cast. Personal taste plays a big part in assessing whiteness. Anyone working with whiteness formulae has to be aware that every formula, however structured, has its specific 'whiteness preference'. Users are therefore urgently advised to characterize the hue in addition to the whiteness. High whiteness is obtainable only with the aid of fluorescent whitening agents (FWAs), and hence as a fluorescent colour demands specific qualities for the illumination. The present article outlines a method, successfully used for years in industry, with which the 'colour white' can be defined both precisely and graphically. In particular the results obtained are reproducible and largely comparable. Measuring instruments and illumination are of central importance.