Fig. 6
![Fig. 6](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs40494-020-00440-w/MediaObjects/40494_2020_440_Fig6_HTML.png)
Hybrid PA and optical monitoring during the laser-assisted removal of encrustation from stonework. a Cross-correlation between PA waveforms recorded after the incidence of Nth and 1st pulse, which is used as reference (first 6 pulses are presented). The 1st pulse curve (black color) corresponds to an autocorrelation function. An optimum laser fluence is applied at 0.8Â J/cm2. b Maximum amplitude of cross-correlation function graph estimated for 15 pulses. c Percentage change of cross-correlation maximum amplitude; the peak indicates the pulse for which the marble substrate has been reached. d Optical images of the laser spot recorded for 15 sequential pulses. Red border indicates the point (5th pulse) where the maximum change has been observed according to (c). e Percentage change of cross-correlation maximum amplitude estimated for 15 pulses at 3 different fluences. Figure adapted from [61]