Optimization of graffiti removal on natural stone by means of high repetition rate UV laser

The use of laser for graffiti removal is a promising alternative to conventional cleaning methods, though irradiation parameters must be carefully selected in order to achieve the effective cleaning without damaging the substrate, especially when referring to natural stone. From a practical point of view, once a safe working window is selected, it is necessary to determine the irradiation conditions to remove large paint areas, with minimal time consumption. The aim of this paper is to present a systematic procedure to select the optimum parameters for graffiti removal by means of the 3rd harmonic of a high repetition rate nanosecond Nd:YVO4 laser. Ablation thresholds of four spray paint colors were determined and the effect of pulse repetition frequency, beam diameter and line scan separation was analyzed, obtaining a set of values which optimize the ablation process.