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Visual characteristics

In paper specifications, whiteness can be presented in three ways. Numeral data may range from 90 to 170. Why is it so? Whiteness measured in the light without the UV component has the lowest value. Such results come to approximately 90% and show what degree of whiteness the test has in comparison with the sample (bar sulphate). They also mirror the whiteness of fibers and the filler. The need for papers with better whiteness brought about the introduction of optical brighteners to paper production.
For testing the whiteness of such paper, different lightening method of the sample is required. Lamps with UV radiation in its own spectrum, were used. Such adiation is changed by optical brighteners into visible light and such results are higher in comparison with results of paper made without optical brighteners. According to the amount of used brightener, they can hesitate from 90 % to 110 %.
Another method of testing whiteness is CIE, offered for many years by International Lightening Organization. The results obtained by this method are much higher than those obtained by previously mentioned methods. The results hesitate between 130 and 170. How such results were obtained?
Special lamp with wide spectrum was used in the device. Special computer program counts the value by the integral calculus for whole scope of spectrum. The results are not given in percentage, like previous results. They are presented in units.

Those three methods obtain different results for the same paper. There is no direct numeral converter of results from such methods of signing whiteness. Paper with low degree of whiteness without UV may have very high degree of whiteness in using method with UV. It depends on the amount of used optical brightener. And vice versa - paper with little whiteness with UV may have higher degree of whiteness without UV. It depends on the whiteness of fibers and filler which was used in paper production. The change of filler from kaolin to carbon calcium with the same whiteness of fibers, may enhance the whiteness without UV even by several percent if the UV was not changed. Human eye perceives paper with optical brightener as whiter. Whiteness is one of the parameters, which have no impact on recomposing of paper, in spite of big visual effect. Another parameter, connected with the whiteness is color L, a, b (in other words, the shades of paper). Human eye is very specific in perceiving shades. In spite of the same results of whiteness, obtained by the devices, some papers seem to be whiter than other. If paper has yellow shade, it is perceived as less white than paper which has blue shade. Human eye perceives that whiteness through shades added to paper and called nuancing help. A bit of them changes the shade of paper into slightly pink (used earlier) or blue (used nowadays) – trying to escape from the yellow shade. The race to make whiter paper made that paper nowadays is so white, even flagrant. Such paper can be used with presentations, where we want to achieve the clarity of print and bright colors (desired effect). However, if we would have to read long essay, printed on such paper, our eyes would probably revolt. That is the reason why paper with lower degree of whiteness is used for such purposes. The same kind of paper is used as cheaper to make working prints.



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