It has been theoretically shown that opacity decreases with increasing variation coefficient of the grammage. However, the quantitative relationship between opacity and variation coefficient has been uncertain for two reasons. First, the value of the variation coefficient depends on the aperture of the formation measurement. Secondly, the theoretical studies are based on the assumption that the local scattering coefficient does not correlate with the local grammage, even though, for example, it has been found for a newsprint sample that, due to greater packing, the local scattering coefficient decreases with increasing local grammage. In order to incorporate the sheet packing into the analysis, the recently presented simulation model was used in this study. The model was applied in analyzing the reflectances of four pulp sheets having different values of variation coefficient of grammage. The results suggest that, with the illuminating conditions used in the instrument measuring reflectance, the sideways diffusion of light in paper effectively obscures sheet non-uniformities. In practice, therefore, the effect of formation on opacity is small.
