Photoluminescence excited at variable fluences: a novel approach for studying the emission from crystalline pigments in paints

Crystalline solids can exhibit photoluminescence when properly excited by sufficiently energetic light radiation. Following excitation, different radiative and non-radiative recombination pathways can occur that are informative of the energetic structure of the material as well as of the presence of crystal defects and impurities. Usually, the characterization of the optical emission of crystalline materials is achieved through the study of emission spectra as a function of the excitation wavelength. A different approach employs variable excitation fluence to populate the energetic levels until saturation, which promotes the emission from other radiative and non-radiative pathways. The method is particularly effective for understanding conduction phenomena and studying charge recombination channels in semiconductor materials. In this work, we propose its application for characterizing radiative recombination paths in crystalline pigments. The approach has been tested in spectroscopy mode for the identification of paints in a model painting and in micro-imaging modality for the study of paint stratigraphies. We demonstrate that the method is highly informative of the nature of different recombination paths in crystalline pigments and allows a deeper characterization of the emission from luminescent paints with respect to the conventional steady-state photoluminescence approach.