Method for detecting free fiber ends in tissue paper

Softness is one of the major properties of tissue paper. Tissue softness can be divided into bulk and surface softness. The bulk softness can be measured quite reliably by measuring the thickness and elasticity of a sheet. However, the measurement of surface softness is not straightforward. One significant factor influencing surface softness is the fibers extending from the surface of the tissue paper. This paper presents a novel imaging method to detect the amount of such fibers on the tissue paper. The method is based on the detection of shadows caused by the free fiber ends. The shadows cannot be detected as such from the reflectance image of the sheet because of the wavy surface of tissue paper. Thus, the 3D information of the surface was estimated based on the photometric stereo, and the intensity variations caused by the wavy surface were filtered out. The method has the advantages over previous methods of improved accuracy and the possibility of implementation in a running paper machine.