Safe limits for the application of nonlinear optical microscopies to cultural heritage: A new method for in-situ assessment

Nonlinear optical microscopies (NLOM) are emerging techniques for the monitoring and preservation of culturalheritage, providing morphological, structural and compositional information on a large variety of artistic materialsand objects. Being non-invasiveness the main requirement in artwork diagnostics, it becomes increasinglynecessary to verify the absence of physicochemical alterations possibly induced by the use of high intensityradiation sources, and thus to find the exposure limits for safe applications. In this work, we propose a newmethodology to set the criteria of damage evaluation for near infrared femtosecond pulsed lasers used for examiningpainted samples by means multi-photon excited fluorescence (MPEF), modality of NLOM. The method isbased on the detection of changes in the MPEF signal upon repetitive irradiation on a series of modern acrylicpainted samples, complemented by Raman spectroscopy measurements performed before and after irradiation.The obtained results allowed determining a safe average laser power range for the non-invasive determination ofthickness in paint layers.