Optical Coherence Tomography has been successfully applied to the non-invasive imaging of subsurface microstructure
of a variety of materials from biological tissues to painted objects of art. One of the limitations of the technique is the
low depth of penetration due to the strong scattering and absorption in the material. Previous studies found that for paint
materials, the optimum window for large depth of penetration is around 2.2 microns. This is also true for many other
materials with low water content. We have previously demonstrated OCT systems in this wavelength regime for imaging
with improved depth of penetration. In this paper, we present an improved 2 micron high resolution Fourier domain OCT
system using a broadband supercontinuum source. The system achieved a depth resolution of 9 microns in air (or 6
microns in paint or any polymer).
Optical Coherence Tomography has been successfully applied to the imaging of painted objects in recent years. However, a significant limitation is the low penetration depth of OCT in paint because of the high opacity of paint due to either scattering or absorption. It has been shown that the optimum spectral window for OCT imaging of paint layers is around 2.2μm in wavelength. In this paper, we demonstrate a 1950nm OCT for imaging painted objects using a superfluorescent fiber source at low power.
While OCT has been applied to the non-invasive examination of the stratigraphy of paint layers in recent years, it has been recognized that the resolutions of commercially available OCT cannot compete in depth resolution with conventional microscopic examination of cross-sections of paint samples. It is necessary to achieve resolutions better than 3 microns to resolve the thinnest layers of paint and varnish. In this paper, we demonstrate a Fourier domain ultrahigh resolution OCT at 810nm with depth resolution of 1.8 μm in air (or 1.2μm in varnish or paint).
Multispectral and hyperspectral imaging are efficient methods of measuring spectral reflectance at high spatial
resolution. This non-invasive technique has been applied to the imaging of paintings over the last 20 years. PRISMS
(Portable Remote Imaging System for Multispectral Scanning) was designed specifically for imaging wall paintings.
Optical Coherence Tomography (OCT) is a low coherence interferometric technique capable of fast non-invasive
imaging of subsurface microstructure. This paper shows the first application of in situ OCT imaging of a wall painting.
The combination of PRISMS and OCT gives information on the varnish and paint layer structure, pigment identification,
the state of degradation of the paint and varnish layers and informing curators on the painting schemes and techniques.
PRISMS (Portable Remote Imaging System for Multispectral Scanning) is a multispectral/hyperspectral imaging system
designed for flexible in situ imaging of wall paintings at high resolution (tens of microns) over a large range of distances
(less than a meter to over ten meters). This paper demonstrates a trial run of the VIS/NIR (400-880nm) component of the
instrument for non-invasive imaging of wall paintings in situ. Wall painting panels from excavated Tang dynasty (618-
907AD) tombs near Xi'an were examined by PRISMS. Pigment identifications were carried out using the spectral
reflectance obtained from multispectral imaging coupled with non-invasive elemental analysis using a portable XRF.
This paper will introduce a new application of Optical Coherence Tomography (OCT) to the monitoring of vulnerability
of rock art monuments in-situ. The porosity of the host rock is an important factor affecting the susceptibility of rock art
monuments to decay. Pore characteristics of rocks are one of the main factors that control the intensity of physical
deterioration. OCT has successfully been applied to paintings and archaeological objects, including geological materials,
to produce cross sectional images non-invasively. The stack of cross sectional images can be rendered as a volume to
visualise the structure in depth over an extended area. Preliminary studies show that it can directly image the pores and
subsurface structure to within 500microns of the surface depending on lithology. This study aims to analyse this stack of
cross sectional images computationally to enable the description of the pore space distribution which will be compared
with spatially resolved NMR porosity measurement for the samples.
A Fourier domain (FD) optical coherence tomography (OCT) system is shown to be capable of profilometry with two
orders of magnitude better accuracy than the axial imaging resolution of the system. High precision OCT profilometry
not only achieves similar accuracy as commercial white light interferometry based profilometers but is also capable of
profilometry on complex subsurface structures with multiple interfaces of low reflectance. An accuracy of 55nm was
achieved with a ThorLabs SROCT on a lab bench without special
anti-vibration devices. This technique has the potential
for a range of applications, such as high precision refractive index measurements and simultaneous dynamic monitoring
of the interface structure of a drying varnish and the substrate.
There has been a long tradition of applying biomedical imaging techniques to the examination of historical artefacts,
owing to similar demands for non-invasive methods in both fields. Optical Coherence Tomography (OCT) is no
exception. We review the achievements on OCT applications to art conservation and archaeology since the publication
of the first papers in 2004. Historical artefacts include a much broader range of materials than biological tissues, hence
presenting a greater and somewhat different challenge to the field of OCT. New results will be presented to illustrate the
various applications of OCT including both qualitative and quantitative analysis.
Results are presented in using low coherence interferometry in quantifying the reflectivity and imaging of different
objects, such as tissue, paintings and fruits. All images have been obtained using en-face flying spot technology. This
allows simultaneous generation of optical coherence tomography and confocal scanning images.
We present a proto-type portable remote multispectral imaging system, PRISMS (Portable Remote Imaging System for
Multispectral Scanning), that is light-weight, flexible and without any cumbersome mechanical structure for in situ high
resolution colour and spectral imaging of large and inaccessible paintings such as wall paintings. This is the first
instrument to be able to image paintings at inaccessible heights in situ from ground level to produce not only high
resolution colour images but also multispectral images.
The design and experimental method for the use of a novel instrument for lightfastness measurements on artwork is
presented. The new microfadometer design offers increased durability and portability over the previous, published
design, broadening the scope of locations at which data can be acquired. This reduces the need for art handling or
transportation in order to gain evidence-based risk assessments for the display of light-sensitive artworks. The
instrument focuses a stabilized high powered xenon lamp to a spot 0.25 millimeters (FWHM) while simultaneously
monitoring color change. This makes it possible to identify pigments and determine the lightfastness of materials
effectively and non-destructively. With 2.59mW or 0.82 lumens (1.7 x107 lux for a 0.25mm focused spot) the instrument
is capable of fading Blue Wool 1 to a measured 11 ΔEab value (using CIE standard illuminant D65) in 15 minutes. The temperature increase created by focused radiation was measured to be 3 to 4°C above room temperature. The system was
stable within 0.12 ΔEab over 1 hour and 0.31 ΔEab over 7 hours. A safety evaluation of the technique is discussed which
concludes that some caution should be employed when fading smooth, uniform areas of artworks. The instrument can
also incorporate a linear variable filter. This enables the researcher to identify the active wavebands that cause certain
degradation reactions and determine the degree of wavelength dependence of fading. Some preliminary results of fading
experiments on Prussian blue samples from the paint box of J. M. W Turner (1755-1851) are presented.
Optical coherence tomography (OCT) is a fast scanning Michelson interferometer originally designed for in vivo
imaging of the eye. In 2004, our group along with two other groups first reported the application of OCT to art
conservation and archaeology. Since that time we have been conducting a project to investigate systematically the
potential of OCT as a new tool for non-invasive examinations of a wide range of museum objects and to design an OCT
optimised for in situ use in museums. Here we present the latest results from this ongoing project, which include the
determination of the optimum spectral windows for OCT imaging of paintings and painted objects executed using
traditional techniques, and non-invasive imaging of the subsurface stratigraphy of painted layers at multiple
wavelengths. OCT imaging in assisting spectral pigment identification and in measuring refractive indices of paint will
also be presented to illustrate the potential of the technique.
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 non-destructively 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.
The new SIRIS (Scanning InfraRed Imaging System) camera developed at the National Gallery in London allows high-resolution images of paintings to be made in the near infrared region (900-1700 nm). Images of 5000 × 5000 pixels are made by moving a 320 × 256 pixel InGaAs array across the focal plane of the camera using two orthogonal translation stages. The great advantages of this camera over scanning infrared devices are its relative portability and that image acquisition is comparatively rapid - a full 5000 × 5000 pixel image can be made in around 20 minutes. The paper describes the development of the mechanical, optical and electronic components of the camera, including the design of a new lens. The software routines used to control image capture and to assemble the individual 320 × 256 pixel frames into a seamless mosaic image are also mentioned. The optics of the SIRIS camera have been designed so that the camera can operate at a range of resolutions; from around 2.5 pixels per millimetre on large paintings of up to 2000 × 2000 mm to 10 pixels per millimetre on smaller paintings or details of paintings measuring 500 × 500 mm. The camera is primarily designed to examine underdrawings in paintings; preliminary results from test targets and paintings are presented and the quality of the images compared with those from other cameras currently used in this field.
It is current practice to take tiny samples from a painting to mount and examine in cross-section under a microscope. However, since conservation practice and ethics limit sampling to a minimum and to areas along cracks and edges of paintings, which are often unrepresentative of the whole painting, results from such analyses cannot be taken as representative of a painting as a whole. Recently in a preliminary study, we have demonstrated that near-infrared Optical Coherence Tomography (OCT) can be used directly on paintings to examine the cross-section of paint and varnish layers without contact and the need to take samples. OCT is an optical interferometric technique developed for in vivo imaging of the eye and biological tissues; it is essentially a scanning Michelson's interferometer with a "broad-band" source that has the spatial coherence of a laser. The low temporal coherence and high spatial concentration of the source are the keys to high depth resolution and high sensitivity 3D imaging. The technique is non-invasive and non-contact with a typical working distance of 2 cm. This non-invasive technique enables cross-sections to be examined anywhere on a painting. In this paper, we will report new results on applying near-infrared en-face OCT to paintings conservation and extend the application to the examination of underdrawings, drying processes, and quantitative measurements of optical properties of paint and varnish layers.
We present results of applying low coherence interferometry to gallery paintings. Infrared low coherence interferometry 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.