Stiffness and Stiffness Variation in Paper Measured by Laser-Generated and Laser-Recorded Bending Waves

Stiffness in paper is measured by holographic interferometry. The displacement field is numerically evaluated using a phase-stepping and phase-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 variations in thickness, stiffness or density, are identified using this technique.