One reason might be how much the colour saturation of the bleached pulp contains after bleaching with different chemicals. The objective of bleaching is to get white pulp without colour saturation.
One of the suitable programs of "excitation purity" - one of the tristimus Helmholtz parameter - estimation can be found in: Dávid A., Hamala M., Paulínyová E. and Katuščák S.: Stanovenie Helmholtzových farebných súradníc pre oblasť prírodnej farby drevín. Drevársky výskum/Wood Research 107, 43-65 (1985). Estimation of Helmholtz parameters: dominant wavelength, excitation purity and lightness. Application in the area of defining the color space coordinates for natural color of lignocellulosics, wood species.
If you want white paper, you need an excitation purity of 0.00. See also CIE whiteness and yellowness indices. They are used in much the same way in different industries.
Actually the purpose of bleaching is to increase brightness. If you want to get White Paper, after bleaching you need to use optical whitener. Many Paper makers (especially White Paper producers) use optical whitener, Blue color, also some fillers such as calcium carbonate, titanium dizide, etc.
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