STIFFNESS AND STIFFNESS VARIATION IN PAPER MEASURED BY LASER-GENERATED AND LASER-RECORDED BENDING WAVES

Stiffness in paper is measured by holographic interferometry. The disp lacement field is numerically evaluated using a phase-stepping and pha se-unwrapping reconstruction technique. Transient bending waves in the paper sample are generated and recorded by the same pulsed laser. The proposed method can be developed into a commercial on-line measuring device to measure paper stiffness. A number of defects, like variation s in thickness, stiffness or density, are identified using this techni que.