Thesis (M.Sc.) - Cairo University - National Institute of Laser Enhanced Sciences - Department of Laser Application in Enivronmental Metrology Photochemistry , Photogiology & Agriculture

Document papers produced by digital printing methods, such as laser and inkjet printers, are used widely for different purposes. In forensic science, examination methods and scientific procedures are used to provide evidences about suspicious document papers. Nowadays, laser applications are widely used in industry, medicine, and communication. In this thesis, the role of laser application in counterfeiting and forgery for document papers is studied. Laser radiation was used to remove printed toner at different conditions and then different diagnostic techniques were employed to examine laser-treated document papers. Experiments were carried out using Q-switched Nd:YAG laser working at the second harmonic generation at wavelength of 532 nm. The full width at half maximum (FWHM) of the laser pulse was 7 ns and laser repetition rate was 10 Hz. Samples were performed at different laser fluencies of 1.02, 1.50, and 2.50 J/cm2 using plano-convex lens of f = 10 cm. The working distance was 11 cm from the focusing lens to avoid direct damaging of sample surface (i.e. out of focus). Laser beam was scanned over the printed toner on the sample surface using XYZ translation stage. Removing of toner and possible changes occurred on the white paper surface were investigated using optical microscope, scanning electron microscopy (SEM), energy dispersive electron spectroscopy (EDX), Fourier transform infrared (FTIR), and laser induced breakdown spectroscopy (LIBS)laser fluence of 2.50 J/cm2 at different laser exposure, as seen by the optical microscope. SEM images of samples treated with laser at 1.02 and 1.50 J/cm2 showed slight impact on the thin surface layer after removing the toner

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