Type: 2D/3D Analysis
When investigating the aging process of old paintings, it is of great importance to obtain insight into the painting technique as practiced in the past, and the first step in gaining this knowledge is, to a large extent, based on the study of the varnish film. Besides that, measuring the varnish thickness is often the starting point of the cleaning operation, the process whereby materials are selectively removed from a painted surface by partial thinning or complete elimination of varnish, being, thus, an irreversible process which may result in chromatic and morphological variations of the painted surface.
OCT is an interferometric technique whose output is a high-resolution stratigraphic image of the examined surface resulting in a visual and numerical quantification of the varnish layer thickness.
The prototype combines confocal microscope optics with the OCT-technique set-up: confocal operation allows to set the beam focus position inside the sample. The excessive reflectance of some of the varnish layers, that may cause a signal intense enough to cover the one produced by interfaces at deeper levels, can be bypassed by displacing the beam focus inside the sample rather than on the outer surface.
The confocal OCT microscope is based on an optical-fiber interferometer that uses a low-coherence source at 1550 nm, with 100 nm FWHM spectral band.
The instrument has a working distance of about 3 mm. The maximum scanning length (y direction) is 25 mm (about 20 min for a single tomographic section). The lateral resolution is about 2.5 micron. The dynamics, that is the axial length (z direction) along with the instrument yields tomographic results, is 1 mm and the axial resolution is less than 10 micron in air.
Many profiles can be acquired along the x direction to acquire a tomocube, whose maximum dimension is 1´1 mm with a sampling step of 10 micron, due to the huge dimension of the output data (nearly 2 hours).
The instrument (see pictures below) needs a stable positioning to work properly.
- Latour, J.P. Echard, B. Soulier, I. Emond, S. Vaiedelich, M. Elias, Appl. Opt. 48, 6485 (2009)
- Targowski, M. Iwanicka, L. Tyminska-Widmer, M. Sylwestrzak, E.A. Kwiatkowska, Acc. Chem. Res. 46, 826 (2010)
- H. Liang, B. Peric, M. Hughes, A.G. Podoleanu, M. Spring, S. Roehrs, Optical coherence tomography in archaeological and conservation science—a new emerging field. Proc. SPIE 7139, 713915 (2008).
MOLAB Italy: CNR-INO