Hyperspectral Imaging as a Non Destructive Technique to Control the Laser Cleaning of Graffiti on Granite

The progress of the laser cleaning of two graffiti paints on coarse-grained granite was in situ followed up by the hyperspectral imaging technique, through the calculation of a quantitative index based on a linear spectral mixture analysis model, which provided an estimation of the cleaning level attained along the process. The laser used was a nanosecond pulsed Nd:YVO\(_{4}\) source, at 355 nm wavelength and, in order to validate the results of laser cleaning, conventional analytical techniques as optical microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry, Fourier transform infrared spectroscopy and spectrophotometer colour measurements were also implemented. The results indicated that hyperspectral imaging technique is a reliable tool to in situ control the process of laser cleaning of graffiti in coarse-grained granites in order to determine the laser scans enough to achieve the best possible cleaning without modifying the granitic substrate.