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 cultural heritage, providing morphological, structural and compositional information on a large variety of artistic materials and objects. Being non-invasiveness the main requirement in artwork diagnostics, it becomes increasingly necessary to verify the absence of physicochemical alterations possibly induced by the use of high intensity radiation sources, and thus to find the exposure limits for safe applications. In this work, we propose a new methodology to set the criteria of damage evaluation for near infrared femtosecond pulsed lasers used for examining painted samples by means multi-photon excited fluorescence (MPEF), modality of NLOM. The method is based on the detection of changes in the MPEF signal upon repetitive irradiation on a series of modern acrylic painted 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 of thickness in paint layers.