Multipass laser ablation of three coloured ink from a paper substrate

This paper reports the effects of multipass laser ablation over large areas and single lines of multiple layered coloured inks which have been printed upon ordinary white wood pulp paper. The aim of the work is to investigate the characteristics and feasibility of selectively removing subsequent layers to reveal different coloured inks and ultimately to generate a full coloured image with a tactile quality. An image generated in such a way would be considered to have features for anti-counterfeiting by colour copying. Ablation was performed using a Q-switched Nd:YAG laser. The results show that it is possible to selectively expose cyan, magenta and yellow layers, respectively, with repeated passes of the laser beam. SEM and colour analysis were carried out to examine the effect of laser ablation on the colour properties of the pigments. It was found that under the fundamental mode of the laser (1.06 μm wavelength) and a pulse width of 8 μs, the colours exposed by laser ablation were paler than those of the original prints and that the laser-treated surface is highly porous. An evaporation mechanism is proposed, whereby the laser energy is absorbed physically into the ink layers which then sublime, causing the pock marked surface.