Determination of the layered structure of baryta based heritage photographs by infrared ellipsometry

Variable angle-infrared spectroscopic ellipsometry is proposed as a reliable tool for the characterization of heritage photographic prints. It is shown that the proposed technique has access to both the chemical composition and the physical structure of the photograph. In particular, the physical structure can be determined by interference related spectral oscillations and the behavior, for different angles of incidence, of the peaks corresponding to different chemical components. Emphasis is made on gelatin/baryta samples, and particularly, in the role of the baryta layer. A relatively simple model is used to simulate the ellipsometric spectra. It shows that the thickness and location of different layers in photographic prints can be assessed by optical means.