Controlled laser cleaning of artworks via low resolution LIBS and linear correlation analysis

Based on a fibre optic miniature spectrometer (spectral range 200-1100 nm, λ-resolution 2 nm) we have built up a LIBS-system (Laser Induced Breakdown Spectroscopy) for online monitoring during laser cleaning of artworks. This system is unsuitable for high resolved elemental identification but is applicable for the detection of “spectral fingerprints”. A powerful method of comparison online spectra during laser cleaning process with reference spectra is the analysis of correlation. Based on “Pearson’s correlation” the linear relationship between the entire data sets of both spectra are classified by the correlation coefficient r. In practise of laser cleaning a permanent comparison of the spectrum from the ablated layer with a reference spectrum of the layer to be preserved is necessary. Thus, online controlling of the cleaning process is enabled by permanent estimation of r. Laser removal will continue until r exceeds a predefined value subsequently the ablation process is paused. In a next step, the sample is translated and the cleaning process continues. Successfully we have automatically cleaned glass covered with encrustation and polluted parchment.