Joseph A. Izatt
Duke University, United States
OCT, Varnish, Paintings, Er:YAG Laser, Conservation.
Computational Optical Sensing and Imaging
Photonics, Optics and Laser Technology
Optical Coherence Tomography (OCT) was used to visualize and quantify the varnish layer of paintings.
And to verify the success of efforts by conservators to remove the varnish using laser ablation. An Er:YAG
laser (λ = 2.94μm) with a repetition rate of 15Hz and an optical power of 1mW was used to remove the
varnish. A spectral domain OCT system with a Michelson topology was constructed using a broadband
super-luminescent diode (λ = 840±25nm). The OCT system provided an 8μm resolution, and a field of view
of 5x5mm. Samples, including pigment, varnish and substrate, approximately 1mm2 in size, were removed
from the oil painting, San Giorgio Maggiore by Martin Rico (1833-1908). Varnish thickness obtained from
OCT was validated by similar measurements obtained from SEM analysis. Other paintings, including a late
18th century landscape, signed Thomas Gainsborough, were imaged with OCT to compare neighboring
regions before and after laser treatment and to examine the layering of the artist
’s signature in an effort to
determine its authenticity. In conclusion, the non-invasive OCT technique is an efficient tool for
measurement of varnish layer thickness, for imaging over-paint under varnish layers, and for assessing the
effectiveness of laser assisted varnish removal.