Digitally captured signatures: A method for the normalization of force through calibration and the use of the zeta function

Digitally captured signatures (DCS) are entering the scope of the Forensic Handwriting Examiner (FHE), and the meaningful comprehension and comparison of the captured information is essential in order to proceed to the examination of authenticity of DCS. In DCS solutions, force is captured and presented as pressure levels, but the relation between exercised force and the assigned pressure levels is not provided by the manufacturers of these solutions. For this purpose, we constructed an experimental array that allows the correlation of exercised force and assigned pressure levels, in a combination of three different digitizers, six different styli and four different capturing software. This process let us calculate the correlation function that assigns pressure levels to force for each solution (called the Zeta Function). Through this process, it was observed that different solutions follow different Zeta functions. To address this problem, a methodology for normalization of captured data between different solutions was created and demonstrated, using the calculated Zeta function and its inverse.