In-fiber optical coherence tomograph for examination of paintings

Abstract

Scientific examinations of paintings are routinely carried out in major galleries and museums to assist in conservation treatment and as part of technical or art historical examinations. Care is taken to examine the paintings nondestructively as far as possible. However, in order to study the paint and varnish layers, it is still currently necessary to take tiny samples of a painting to examine the cross section of a small area of the painting under a microscope. In an attempt to solve this problem we evaluate the potential of optical coherence tomography (OCT) in providing high resolution information about paint layers. Two OCT systems have been assembled, operating at 850 nm and 1300 nm, each using two single mode in-fiber couplers. Both systems can produce A (reflectivity profile in depth), T (lateral reflectivity profile), B (cross section image) and C-scans (constant depth image). Using superluminiscent diodes, a depth resolution better than 9 microns is achieved. We present results of applying OCT to sample panels and paintings. We show that infrared OCT is capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer but also the paint layers. The OCT images present better microscopic tomography of the surface of the varnish and paint layers than any system currently employed in the examination of paintings. OCT could also be used for accurate measurement of the thickness of the varnish layer on a painting.

Multi-scale optical coherence tomography imaging and visualization of Vermeer’s Girl with a Pearl Earring

“…The OCT scattering strength i scat is determined by summing the OCT signal of the first 50 pixels in depth (approximately 100 µm) starting at the pixel position of the air/varnish interface, i scat = i=50 i=0 |i OCT (z i )| 2 , with i OCT (z i ) the complex-valued OCT signal at z i with the start of the summation at z 0 = z air/varnish . This is similar to the depth averaging [7] or summed voxel projection [22], but in this case is aimed at creating scattering contrast in the near surface region.…”

“…OCT has, since 2004, been applied to the analysis of works of art [3][4][5]. Focusing on the application of OCT to paintings, it has been used for a variety of purposes such as: quantification of varnish drying [6], imaging of underdrawings [7], investigation of canvas deformation [8], control of varnish ablation [9], and examination of stratigraphy [10].…”