Determination of mechanical characteristics of sheets of paper using the optical coherence tomography technique and its applications in forensic sciences

In this work, we present a methodology to assess wrist pressure during the handwriting process. Using optical coherence tomography, it was possible to determine the deformation suffered by the paper during the handwriting process. A calibration of the deformation suffered by the paper as a function of the pressure exerted by the pen was performed, making it possible to determine several parameters of interest, such as the radius of the pen tip sphere R = 488 $\pm$ 2 $\mu$m, the Poisson coefficient $\nu$ = 0.30 $\pm$ 0.02 and the effective Young’s modulus of bond paper 75 g/m².

To this end, we developed a method for determining the Hooke modulus for a spring, with which we built a device for applying a known force to a ballpoint pen during the handwriting process.

We also performed the study depending on the type of substrate, with five volunteers trying to copy the same signature on four different types of paper. Using the optical coherence tomography technique, we demonstrate that it is possible to differentiate the authorship of a signature also by the depth of deformation on the paper. The experimental procedure also made it possible to determine the Poisson’s coefficient and determine the effective Young’s Modulus of the paper.

Keywords:
Optical coherence tomography; Young’s modulus; Poisson’s coefficient; mechanical strain; forensic measurements