Three terms that refer to the distribution of local grammage in the plane of a sheet of paper are introduced-the intensity, macroscale and microscale of mass distribution. The measurement of these quantities by an optical method and by beta radiography is discussed. Data are obtained by a specially designed microdensitometer and analysed by a frequency analyser. Results are presented as wavelength spectra of mass distribution.
Experimental spectra for handsheets and machine-made sheets are compared with theoretical spectra for sheets composed of fibres having randomly distributed orientations and positions. It is shown both theoretically and experimentally that it is important to use a small measuring area.
A poor mass distribution is demonstrated to have a negative influence on the scattering coefficient and opacity of paper. It is shown how the mass distribution and other paper properties affect the visual appearance of multi-ply board and the lookthrough and print-through of paper. The concept of mass distribution is extended to the distribution of ink on paper. The connections between the mass distribution of ink and the mottle of the prints are discussed.
Finally, the adverse effect of a poor mass distribution on the strength properties of handsheets and machine-made sheets is demonstrated using recent (partly unpublished) experimental data by Cavlin & Rudström.