Optical Coherence Tomography (1960 nm)

Optical Coherence Tomography (OCT) provides fast, non-invasive, non-contact imaging of 3D surface and subsurface microstructure of materials providing stratigraphic information of objects. OCT offers a micrometre level resolution in the direction normal to the surface of an object and is well suited for the investigation of fine details of structures which moderately absorb infrared light, such as varnishes, glaze and paint layers and underdrawings of paintings, reverse paintings on glass, glazes on ceramics and faience, jade, historic glass and enamels, amber, rock art, parchment and paper etc. Images obtained by OCT are usually presented in the convenient manner of cross-sectional views, similar to microscopic images of sample cross-sections extracted from the object. If necessary, information of an entire volume (a cube) may be collected by combining a sequence of 100 to 1000 cross-sections across the surface. The major advantages of OCT are the ability to see the layers and microstructure below the object surface in a non-contact and non-invasive manner (intensity of light used for examination is in the order of a few milliwatts), rapid data collection with online display of images and with no need for object preparation. Given the non-invasive nature of the technique, the number of measurements across the entire surface can be unlimited, making it possible to have a representative view of the whole object. The long wavelength Fourier domain OCT operating at central wavelength of 1960nm provides a larger penetration depth in materials than OCTs at shorter wavelength. Systematic studies on the spectral transparency of historic artists' paint found that the most transparent spectral window to image pigmented layers is around 2200nm.