The Study of Force in Digitally Captured Signatures

Digitally Captured Signatures (DCS) provide biometric data—typically X and Y pen positions, applied force (F), and time (T)—that allow forensic handwriting examiners to assess authenticity with greater precision than traditional pen-andpaper signatures (PPS). Among these parameters, force data is particularly significant: while PPS reveals pen pressure only indirectly through ink distribution and line quality, DCS records it quantitatively. However, a major obstacle arises from the inconsistency of how different devices register force. Many systems label outputs as “pressure levels,” but these do not correspond to absolute values. The same “pressure level” can represent different actual forces across devices, rendering raw data incomparable.

To address this limitation, experimental calibration methods have been developed to link device-specific “pressure levels” to physical force values. The Zeta Function (and its inverse, the Zeta⁻¹ Function) provides a mathematical model for this mapping. By applying known forces through a mechanical array and recording the corresponding sensor outputs, a logarithmic relationship is derived that characterizes the response curve of each digitizing device. This Zeta Function enables conversion from arbitrary “pressure levels” to standardized Newton values, while the inverse function allows re-normalization of raw data to the scale of another device.

The implications for forensic practice are substantial. In real-world casework, disputed signatures and reference samples may originate from different hardware platforms or software versions. Without normalization, cross-comparison of force data is unreliable, potentially undermining evidentiary conclusions. By applying the Zeta Function, examiners can standardize datasets, ensuring comparability and preserving the evidentiary value of dynamic biometric features. This approach parallels established forensic practices in other disciplines, where calibration and normalization safeguard the integrity of measurements.

Moreover, use of the Zeta Function enhances error detection. Calibration curves can reveal defective sensors, software misinterpretations, or anomalous scaling, preventing misattribution of irregularities to the writer. This provides both scientific reliability and legal robustness, particularly under frameworks such as the EU eIDAS Regulation and national laws (e.g., Greece’s Law 4961/2022), which grant evidentiary force to advanced and qualified electronic signatures.

In summary, while DCS technology offers unparalleled access to dynamic writing features, its forensic utility depends on the reliable interpretation of force data. The Zeta Function serves as a normalization tool that bridges hardware inconsistencies, supports valid cross-platform comparisons, and strengthens the role of DCS in forensic handwriting examination. By standardizing force analysis, it advances both the scientific credibility and legal admissibility of DCS evidence.