The examination of documents is based on the analysis and comparison of the markings left by the writing tool. Depending on the method the text is produced, we observe various characteristic features of writing or printing: relief (ball pen), the amount of allowance (laser printer toner), a dye and pigment (printer ink, markers, and stamps) and change of the structure of the paper. The quantitative aspects of these features demand that the equipment covers both broad range and high resolution, for example depending on the primer and the contact force, the relief depth of ball pen stroke may vary in the range of 10μm to 120μm, whereas the colour difference of less than 1 (ΔE CIELAB), cannot be distinguished by human eye. The forensic analysis of documents is not trivial and the results are often inconclusive. Further complications are caused by the obligation to preserve the integrity of the tested document, which attracts attention to non-destructive methods, including optical technology.
Based on the above requirements, the four main domains of applications of optical technology for forensic documents examination have been proposed: 2D imaging, surface topography, spectroscopy and light scattering. Scanning systems, image processing, visualization and data analysis, were developed for all of the above methods.
As a results of our R&D project, a prototype of forensic scanner, including a hardware and software platform, was developed. The solution supports an examination of documents to detect the alteration and forgery as well as establish the writing chronology.
