Method for the local determination of the thickness and density of papera

A new method has been developed for measuring a two-dimensional map of local thickness and local density of paper. A paper is placed on an coordinate-table with high precision, and moved in steps of 150 pm so that the total measurement area amounts to 77 mm X 77 mm. The thickness is recorded as the distance between two spherical measurement probes each with a diameter of 1 mm, which are pressed against the paper with a force of 35 mN. Using image processing technology, a two-dimensional picture of local thickness is then recorded. The local thickness is represented by 256 grey tone levels. Local grammage is defined as the mean value over an area of 150 pm X 150 pm, i.e. with the same resolution in the plane as the thickness measurement, and is measured in the image analysis equipment on a P-radiogram of the sheet over the same total measurement area, 77 X 77 mm. Local density is obtained by dividing local grammage by local thickness. The precision of the measurements is discussed and finally a few examples are presented to illustrate applications of the method within the field of paper technology.