Fundamental Parameters Affecting the Opacity and Brightness of Uncoated Paper

Abstract

A recently introduced theory of light reflectance based on a paper sheet behaving in its reaction to light as a stack of spaced parallel layers is described briefly. Utilising the theory, the contributions of cell wall thickness, lumen size, degree of lumen collapse, absorption coefficient of cell wall material and extent of interfibre bonding to the light-scattering ability of a sheet are discussed in a quantitative way. The conclusions are illustrated by examples of paper sheets in which there is clear evidence that their particular optical properties may be traced to one or more of the above parameters. Thus, for example, it is shown that to make sheets of high opacity while sacrificing neither brightness nor strength, fully bleached fibres with thin cell walls and narrow lumens are the most desirable.