IPERION HSIntegrating Platforms for the European Research Infrastructure ON Heritage Science
Technique: Optical Coherence Tomography (850 nm)

OCT (850 nm)

The 850 nm Fourier domain OCT (NCU) comprises a broad-band light source made up of optically coupled super luminescent LEDs emitting in a band of 750-960 nm. The intensity of radiation at the object never exceeds 1.5 mW, and due to fast scanning is focused at any given spot on the object for 45 μs only. The axial imaging resolution is 3.3 μm in air (2.2 μm in varnish and similar media), with an axial imaging range of 1.4 mm. The lateral resolution, in the standard configuration, is about 13 μm with a field of view of 17 mm x 17 mm and the distance to the object from the most advanced element of the device equal 43 mm. If necessary, the alternate configuration may be used, providing better lateral resolution (ca 6 μm) but for the price of smaller field of view (5 mm x 5 mm) and distance to the object (7.5 mm). The acquisition time of volume information comprised of 100 cross-sections is ca 13 s.

Potential Results

Results of examination by OCT are, in their basic form, provided as cross-sectional images or B-scans, up to about 15 mm width and showing structures up to about 1.5 mm in depth from the surface, on condition that they are at least partially transparent to the probing light (near IR). The contrast of imaging in OCT is provided by differences in refractive indices of structures and/or by embedded light scattering particles. Generally OCT alone is not capable of resolving chemical composition of the structures. The limit of the in-depth imaging is in transparency of layers: the last visible is the upper boundary of the opaque one. The penetration ability of the probing light increases with the increase of the wavelength but the axial resolution simultaneously decreases due to the physical limitations. OCT is well suited to detect number and thickness of transparent layers (e.g. varnishes) especially if there is a dirt embedded at the boundary, the refractive indices are significantly different or one of the layers comprises some embedded scattering particles, eg. pigments. OCT scans can be collected also from a surface (usually up to 15 mm x 15 mm) aa a series of parallel adjacent B-scans and saved as a 3D data cube, from which surface topography as well as thickness and scattering maps can be generated in post processing.