Visible-induced luminescence imaging: A user-friendly method based on a system of interchangeable and tunable LED light sources

Abstract

Imaging techniques represent an unavoidable analytical tool for the non-invasive investigation of cultural heritage objects providing useful information for the identification and distribution of materials on the investigated surface. In particular, photo-induced luminescence imaging can give important indications as to the distribution of organic (binders, resins, dyes and lakes) and inorganic (Egyptian blue, Han blue, Han purple, zinc oxide, cadmium based pigments) constituting materials. Recently the use of LEDs, generating a narrow emission band in the Visible range, has been introduced opening up a new perspective in the detection of inorganic luminescent pigments. In this work we propose a user-friendly tool for luminescence acquisition based on the use of a custom made interchangeable and tunable LED light source system, that allows us to select both the power and the wavelength of the excitation light. The study involved the analysis of panel painting replicas prepared applying several pigments and lakes commonly used by artists in different historical periods. The luminescence images were recorded by a high sensitivity CCD camera and a Vidicon camera in the Visible and NIR range, under three excitation narrow bands (red LEDs at 630nm; green LEDs at 517nm and blue LEDs at 465nm). The reliability of the developed experimental setup has been validated by spectrofluorimetric measurements. A very high correspondence between imaging and spectral response has been found concerning emission band position, emission intensity and excitation wavelength effect. Furthermore, the NIR luminescence of a modern yellow pigment, nickel titanate, has been for the first time documented. The defined methodology has been applied to the study of works of art of different historical periods where it has been proved to successfully detect luminescent inorganic and organic pigments, whose presence has been confirmed also by elemental analyses and spectrofluorimetry.