Published online by Cambridge University Press: 25 November 2020
Optical coherence tomography (OCT) is an imaging technique offering a non-invasive alternative to the traditional analysis of artworks by means of sampling. A cross-section photomicrograph of a sample collected from a painting has its own limitations, mostly due to the restricted number of samples it is possible to collect. There is a need, therefore, for non-invasive verification of locally acquired data, even at the price of not attaining the fuller information given by sample examination. OCT offers such an opportunity, as a non-invasive, fast and contactless technique that provides cross-sectional images of sub-surface structures over relatively large areas. OCT has been used for the examination of artworks since 2004, but recent progress in imaging techniques has made it more applicable to resolving practical issues raised by art conservators and curators. While most applications are connected with the examination of transparent and semi-transparent layers on easel paintings, other objects, such as wall paintings, historic and archaeological glass, ceramics, semi-precious stones like jade, and even parchment, have also been successfully examined using OCT.
A major limitation of OCT for the examination of artworks is the limited transparency of their structure to the probing light utilized by the technique. OCT is an interferometric technique that uses broadband infrared radiation to determine the distance to a structure that scatters or reflects light. If the spectrum of the source is broad enough (c. 200 nm in near infrared) the precision of layer thickness measurements (the axial resolution of technique) is about 3 μm in air and 2 μm in media such as varnish. This permits detection of thin transparent layers, for example varnishes on the surface of a painting. The unique instrument used to examine the Amsterdam Sunflowers was constructed at Nicolaus Copernicus University in Toruń especially for the examination of artworks within the EU CHARISMA project. It utilizes a superluminescent light source with a spectral range of 770–970 nm. The total power measured at the object is less than 0.8 mW and the beam is never focused at the same spot for longer than 50 μs. The narrow beam of infrared light penetrates the object as far as is possible for a given absorbance of the structures at this spot – usually a fraction of a millimetre – and is collected by the instrument's optics.