Therefore, the amplitude of this signal for consecutive positions of the reference mirror carries information about the locations of scattering centres and/or reflective interfaces along the probing beam. 1b) defines the axial resolution of the technique. 1b, due to the integration over a broad spectral range, the interference signal will not average to zero only if the optical path difference between the two arms of the interferometer is small. As can be seen from the simplified graph in Fig. In this variation of the technique, the signal is integrated at a detector, usually a photodiode, and the photocurrent is registered over time simultaneously with translation of the reference mirror. Time-domain OCT (TdOCT) was developed first. There are two ways in which useful data can be extracted from this multispectral interference signal. Omitting the technical details for simplicity, information on the internal structure of a suitable object is generated by interference between light reflected and/or scattered by details of its internal structure and the reference beam (Fig. At present, various commercial instruments dedicated to medical diagnostics are available at prices ranging from 50,000 to 80,000 Euros. Among other medical applications, utilisation in dermatology, oncology, gastrology, and gynaecology are especially important. It is used mostly for imaging of human retinas in vivo through the iris of the eye, but also for examination of the geometry and diseases of the anterior chamber. , and since then has become a well established diagnostic method in medicine, especially in ophthalmology. The OCT technique was invented in the mid-1990s by Huang et al. Nevertheless, the variety of objects to which the technique is being applied is still gradually increasing. The major constraint arises from the limited transparency of the strata of the object to the light used for examination. The technique is especially well suited to inspection of the internal structure of stratified objects because the in-depth (axial) resolution falls into the range of 1 to 10 μm, even though the lateral resolution is significantly lower, usually from 10 to 30 μm. The resultant image has a convenient form, similar to the very popular photomicrographs of cross sections of samples collected from the objects. The examination is non-contact, fast, and does not require any preparation of the object examined. Since it utilises light of low intensity, it is harmless to all known types of artworks. Optical Coherence Tomography (OCT) is a white light interferometric technique providing high resolution cross-sectional views (tomograms) of objects which moderately absorb and scatter the probing light. Finally, the use of OCT combined with LIBS analysis and laser ablation of surface layers is presented. Applications to the structural imaging of semi-transparent subsurface layers such as varnishes and glazes, of underdrawings and of reverse painting on glass, are described first, and then applications in the examination of the structure and state of preservation of historic glass, jade, glazed porcelain and faience are discussed. The second part of the article comprises a review of the utilisation of OCT for structural examination of artworks, illustrated with some representative results. The parameters of the tomographs, such as axial and lateral resolution, wavelength and intensity of the probing light, imaging range, time of examination, and sensitivity are then defined, and a paradigm for interpreting the OCT tomograms provided. A brief introduction to Optical Coherence Tomography (OCT) is presented, stressing the origin of the tomographic signal and the detection methods defining various modalities of the technique.
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