Determination of thresholds for safe analyses of acrylic paintings by non-linear optical microscopy

Abstract

Non-linear optical microscopy (NLOM) techniques have been recently introduced for the study of cultural heritage objects due to its non-invasive nature and imaging capabilities with 3D micrometric resolution. For paintings, it has been recently reported that valuable information about composition, layer thickness and state of conservation can be obtained by NLOM [1-3]. Ensuring correct analytical protocols for the application of this novel technique requires the determination of the laser power thresholds that allow measurements under safe conditions, an aspect especially important when studying sensitive materials such as paintings. In this work, we present a novel methodology to determine the laser power thresholds for safe analyses of painting layers by the NLOM modality of multiphoton excitation fluorescence (MPEF). We also present the results obtained in a set of acrylic paints, extensively used by artists over the past century thanks to their properties and low cost of manufacture. To that purpose, samples were prepared as thin layers over a glass substrate and MPEF signals were induced with two different femtosecond laser sources: a Ti:Sapphire laser with wavelength of 800 nm, repetition rate of 80 MHz and pulses of 70 femtoseconds; an optical parametric oscillator pumped by a Yb-based laser with repetition rate of 80 MHz and dual output: at 800 nm with pulses of 100 fs and at 1040 nm with pulses of 140 fs. The excitation wavelength affects the determined thresholds and the results obtained show a strong dependence on the light absorption properties and chemical composition of the painting material.