Photoluminescence imaging of modern paintings: there is plenty of information at the microsecond timescale

Luminescent emission from artworks is usually employed by conservators and restorers as a visual and quick method for assessing the presence of protective varnishes and restoration materials, as well to evaluate the painting conservation status. Besides these mere qualitative observations, the analysis of both spectral and temporal features of the optical emission from a painting can provide useful information about the pictorial materials employed by artists. In this work we propose Multi-spectral Time-gated Photoluminescence Imaging to differentiate between luminescent pigments, on the basis of their different emission spectra and lifetimes. We present the application of the approach on two modern paintings from Fondazione Maimeri Private Collection, on which we differently probe the fast-living emission from surface varnish and lake pigment and the long-living trap state emissions from different semiconductor pigments. With the aid of the time gated detection we record the trap state emission of semiconductor pigments, otherwise covered by the intense fluorescence of the organic compound. The method is complemented with Macro-XRF analysis, to confirm the nature of the identified inorganic pigments, as zinc white and cadmium yellow. The protocol represents a rapid, non-invasive and reliable way for assessing the identification of luminescent pigments and their spatial distribution.