Spectroscopic optical coherence tomography : application to the analysis of pigments from pictural layers
The knowledge of materials used in art works helps for their conservation. Optical coherence tomography (OCT) is a technique that was first developed for imaging in biological tissues. In this workit has been applied to the study of pigments in painting layers by the mean of the recent developmentof spatially resolved spectroscopic OCT. Our device is a time domain en face OCT, using a Mirau objective and a visible light source. We found that the optical system introduces aperture effects leading to spectral translation of the measured spectra towards higher wavelengths. We defined a general protocol in order to correct this effect that we used successfully for the measurement of the reflectivity of several gold samples. Pigments are made of scattering particles with randomly shape and micrometric size. Based on theorical and experimental considerations of spherical particles, we show that OCT spectroscopic measurements are morphology-dependent. The strong spread of the results recorded on a sample makes impossible the distinction between two particles of different materials. However, we showed that calculating the average over a large number of particles leads to distinguish two layers juxtaposed or overlaid, which has been verified experimentally with yellow and red pigments. Thus, coupling spectroscopic analysis with 3D imaging improves sample characterization