Laser cleaning of paint layers on white marble surface based on cooperative use of laser-induced breakdown spectroscopy and image binarization

Abstract

The key to precisely removing paint layers sprayed onto stone surfaces using laser cleaning without damaging the stone substrate is to select the optimized laser parameters based on the online monitoring methods. In this paper, a pulsed laser beam with a high-pulse repetition frequency of 100 kHz, a pulse width of 200 ns, and a wavelength of 1064 nm was used to clean 39 µm- and 31 µm- thick of gold and silver paint layers off of white marble surfaces. The optimized average laser power for removing the paint layers was determined to be 40 W using laser-induced breakdown spectroscopy (LIBS) method and an image binarization method. Under the optimized average laser power of 40 W, the optimized spot overlap ratios for cleaning the layers of gold and silver paint were determined to be 10% and 30% respectively, using the image binarization method. The corresponding optimized number of cleaning shots was 5 and 4, respectively. The laser cleaning efficiency for both the gold and silver paint layers exceeded 95%. It was thereby demonstrated that this research protocol based on the cooperative use of two online monitoring methods to determine the optimized laser parameters in sequence (average laser power → spot overlap ratio → number of cleaning shots) not only optimizes the laser cleaning efficiency for different paint layers but does so without causing any damage to the white marble substrate.