Abstract: The invention relates to a procedure for determining a quantity dependent on the rate of diffusion in the fibre wall for diffusion occurring through it, in which procedure a diffusible tracer is introduced into the fibres under measurement, the fibres are suspended in an aqueous phase and the tracer concentration is determined after a certain time has elapsed since the time of suspension of the fibres, and the quantity dependent on the rate of diffusion is calculated on the basis of the concentration and time.

Abstract: Distribution of mass density of paper or other materials is determined by irradiating a sample with a burst of electron beams from an electron gun. The intensity levels of electron beams emerging from said sample are detected by an areal detector located immediately behind the sample. These intensity levels are processed to derive distribution of mass density information for the sample. The electron beam detector may be a photographic film or a realtime electron beam detector assembly coupled to computer controlled digital data processing and displaying equipment.