Ultra-high resolution Fourier domain optical coherence tomography for old master paintings

Abstract

Abstract:In the last 10 years, Optical Coherence Tomography (OCT) has been successfully applied to art conservation, history and archaeology. OCT has the potential to become a routine non-invasive tool in museums allowing cross-section imaging anywhere on an intact object where there are no other methods of obtaining subsurface information. While current commercial OCTs have shown potential in this field, they are still limited in depth resolution (> 4 μm in paint and varnish) compared to conventional microscopic examination of sampled paint cross-sections (~1 μm). An ultrahigh resolution fiber-based Fourier domain optical coherence tomography system with a constant axial resolution of 1.2 μm in varnish or paint throughout a depth range of 1.5 mm has been developed. While Fourier domain OCT of similar resolution has been demonstrated recently, the sensitivity roll-off of some of these systems are still significant. In contrast, this current system achieved a sensitivity roll-off that is less than 2 dB over a 1.2 mm depth range with an incident power of ~1 mW on the sample. The high resolution and sensitivity of the system makes it convenient to image thin varnish and glaze layers with unprecedented contrast. The non-invasive 'virtual' cross-section images obtained with the system show the thin varnish layers with similar resolution in the depth direction but superior clarity in the layer interfaces when compared with conventional optical microscope images of actual paint sample cross-sections obtained microdestructively.

The origins of the Selden map of China: scientific analysis of the painting materials and techniques using a holistic approach

“…The 810 nm ultra-high resolution OCT (UHR-OCT) developed at NTU has an axial resolution of 1.2 μm (in the direction perpendicular to the painting surface) in varnish and paint, and a transverse resolution of 7 μm (in the plane of the painting surface) [19]. OCT was used on the fragments to verify if there was any potential coating layers on the map, as it looked very glossy for an East Asian painting [13].…”

Application of non-invasive optical monitoring methodologies to follow and record painting cleaning processes

“…Techniques involved-multispectral reflectography [1][2][3][4][5][6][7][8][9][10][11][12][13][14], colorimetry [15][16][17][18][19][20], laser scanning microprofilometry [21][22][23][24][25][26][27][28][29][30], and optical coherence tomography (OCT) [31][32][33][34][35][36][37][38][39][40][41][42][43][44]-allowed for a non-invasive quantitative analysis of significant alterations resulting from the cleaning process, while enabling the restorer/conservator to objectively assess the overall outcome.…”

Using non-invasive non-destructive techniques to monitor cultural heritage objects

“…A recent AHRC/EPSRC research project developed OCT instrumentation optimised towards cultural heritage applications [10,11] . This included an ultra-high-resolution OCT (UHR OCT) with an axial resolution of 1.2 microns (in glass) for the highest resolution to distinguish thin layers [10] and a longwavelength OCT with a central wavelength of 1960 nm for deeper penetration into highly scattering material [11] . In this work, UHR OCT was used to examine the gel layers and internal structures of the enamel.…”

“…In this work, UHR OCT was used to examine the gel layers and internal structures of the enamel. The UHR OCT operating at a central wavelength of 810 nm has a depth resolution of 1.2 microns in glass, a transverse resolution of 7 microns and is capable of collecting a 3D image cube of a 5 mm by 5 mm area at a transverse sampling resolution of 10 microns in 10 s [10] . A virtual cross-section image and a virtual cut parallel to the surface of the bottom right blue drapery in the enamel shown in Figure 4 are shown in Figures 7(a) and 7(b), respectively.…”