Characterization of Glass Surface Morphology by Optical Coherence Tomography

Abstract

Optical coherence tomography (OCT) has been applied as a tool to assist glass conservators and researchers in monitoring and predicting the evolution of glass surfaces – the crucial issue in the correct maintenance of historic glass. This technique of examination of the internal structure of objects with infrared light originates from diagnostic medicine but application to any object that weakly absorbs and/or scatters light is straightforward. Since the intensity of the examining light is very low, this remote method is also completely non-invasive. The object is usually penetrated by a narrow beam of light of broad spectrum. The distribution of the backscattering and reflecting centres within the object is extracted by means of interferometry. The resolution achieved is in the range 2–15 μm axially and usually between 15 and 40 μm transversely. In the case of glass, the technique allows the identification and characterization of leached and hydrated glass surface layers and thus helps to describe the condition of so-called unstable glass. Five historic artefacts were examined to show the advantages and limitations of the method.