Fig. 5
PA monitoring of laser-assisted removal of black graffiti from white marble. a Experimental configuration for the on-line monitoring of laser cleaning. b SEM images depicting the irradiated regions at the onset of graffiti ablation, effective cleaning and substrate’s damage using 4 different fluences. Top right values represent the measured roughness of the substrate. White arrows highlight signs of initial graffiti ablation, whereas the white circles indicate regions with clear laser induced damage. c Linear regression models correlating the average normalized PA signal measured at the onset of each stage with the applied fluence. Error bars correspond to the standard deviation out of 5 measurements. d–g Color-coded graphs of the normalized PA signal evolution as a function of the applied pulse number for 0.6, 1.0, 1.5 and 2.0 J/cm2. Dashed lines indicate the predictions of the linear regression models, as presented in (c), at the onset of graffiti ablation (black), effective cleaning (green) and substrate’s damage (red). The determination of each stage’s extent is displayed using similar colours (grey, green and red), with its limits defined by the intersection of the prediction lines with the PA curve. Figure adapted from [60]