Fluorescence spectroscopy: a powerful technique for the noninvasive characterization of artwork.

After electronic excitation by ultraviolet or visible radiation, atoms and molecules can undergo thermal or radiative deactivation processes before relaxing to the ground state. They can emit photons with longer wavelengths than the incoming exciting radiation, that is, they can fluoresce in the UV-vis-near-infrared (NIR) range. The study of fluorescence relaxation processes is one of the experimental bases on which modern theories of atomic and molecular structure are founded. Over the past few decades, technological improvements in both optics and electronics have greatly expanded fluorimetric applications, particularly in analytical fields, because of the high sensitivity and specificity afforded by the methods. Using fluorimetry in the study and conservation of cultural heritage is a recent innovation. In this Account, we briefly summarize the use of fluorescence-based techniques in examining the constituent materials of a work of art in a noninvasive manner. Many chemical components in artwork, especially those of an organic nature, are fluorescent materials, which can be reliably used for both diagnostic and conservative purposes. We begin by examining fluorimetry in the laboratory setting, considering the organic dyes and inorganic pigments that are commonly studied. For a number of reasons, works of art often cannot be moved into laboratories, so we continue with a discussion of portable instruments and a variety of successful "field applications" of fluorimetry to works of cultural heritage. These examples include studies of mural paintings, canvas paintings, tapestries, and parchments. We conclude by examining recent advances in treating the data that are generated in fluorescence studies. These new perspectives are focused on the spectral shape and lifetime of the emitted radiation. Recent developments have provided the opportunity to use various spectroscopic techniques on an increasing number of objects, as well as the ability to fully characterize very small amounts of sample, either in a laboratory setting or on site. Thus, a new technological highway is open to scientists; it is still difficult to navigate but offers an enormous potential for investigating objects without touching them. Fluorescence spectroscopy is one of the most important of these techniques.