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Optimum spectral window for imaging of art with optical coherence tomography
Abstract
Optical coherence tomography (OCT) has been shown to have potential for important applications in the field of art conservation and archaeology due to its ability to image subsurface microstructures non-invasively. However, its depth of penetration in painted objects is limited due to the strong scattering properties of artists’ paints. VIS–NIR (400–2,400 nm) reflectance spectra of a wide variety of paints made with historic artists’ pigments have been measured. The best spectral window with which to use OCT for the imaging of subsurface structure of paintings was found to be around 2.2 μm. The same spectral window would also be most suitable for direct infrared imaging of preparatory sketches under the paint layers. The reflectance spectra from a large sample of chemically verified pigments provide information on the spectral transparency of historic artists’ pigments/paints as well as a reference set of spectra for pigment identification. The results of the paper suggest that broadband sources at ~2 μm are highly desirable for OCT applications in art and potentially material science in general.
... In fact, OCT proved to be a powerful tool in varnish drying studies [12], monitoring of laser ablation [13,14], in depth localization of varnish deterioration [15], varnish thinning [16,17]. On the other hand, only few studies deal with the transparency of pigments [18][19][20][21]. In fact, one major limitation of OCT imaging is the achievable probing depth through highly turbid paints. ...
... However, the survey is rather qualitative not specifying the paint thicknesses, whose structure can be visualized in OCT tomogram. Liang et al. [18] found a clear correspondence between the pigment composition and the particle size, pointing out a considerable advantage in using higher wavelengths, especially when a fine grain material composes the analysed object. Liang's research team has demonstrated that the optimal spectral window for a deeper OCT analysis is around 2200 nm, due to the maximum transparency of most of the pigments. ...
... The corresponding behaviour was observed in UL, PBC and IB paints. This result is in line with those published by Szkulmowska et al. [18], stating good transparency of ultramarine pigment. The RGB image of CB(H) paint and 1500 nm reflectogram are shown respectively in Figs. ...
Contemporary art is particularly delicate as the synthetic materials used for their realization are of poorer quality and durability than the traditional materials. It follows that the contemporary artworks often require imminent restorations as well as in-depth analytical studies of the constituting materials. The non-invasiveness of the scientific methods is a key issue in the diagnostics of contemporary art because it minimizes the need of sampling. In this respect, the potential of Optical Coherence Tomography (OCT) imaging to probe the internal structure of the commercial acrylic paint layers, as a function of their thickness, was investigated and compared with that of the near infrared (NIR) reflectography. In support of these measurements, micro-profilometry, an interferometric technique with micrometric depth resolution, was used to measure the paint layer thicknesses. We show that for some painted materials there is a limited possibility to visualize the underdrawings, when investigated by both OCT and NIR reflectography. This is owed to either the absorption of the illumination wavelength by the paint layer (Cobalt-based paints) or the multiple photon scattering effect (presence of TiO2).
... While mostly used in the context of biomedical application, OCT is also a highly relevant tool for Non-Destructive Investigation (NDI) in other fields, as shown by applications in fields ranging from industrial coating inspection [10] to art research and cultural heritage conservation [11]. In cultural heritage research, NIR light OCT is most common, as longer wavelengths lead to lower scattering, resulting in increased depth penetration, of importance for this particular application. ...
... In cultural heritage research, NIR light OCT is most common, as longer wavelengths lead to lower scattering, resulting in increased depth penetration, of importance for this particular application. Liang & al showed that for this conventional approach, the optimal wavelength range is around 2 µm [11]. Using long-wavelength OCT, several studies have demonstrated the existence of underdrawings or the presence of hidden characters or objects on old master paintings [12]. ...
... A collection of 53 common historic artist pigments were used to prepare reference paint samples in linseed oil, egg tempera or animal glue [17,18]. The pigment compositions were chemically analyzed with the main composition confirmed. ...
... The pigment compositions were chemically analyzed with the main composition confirmed. Details of the analysis along with the pigment composition are given in [17]. The level of fluorescence between the different binding media are not significantly different probably because they are not as aged as those found in historic paints. ...
Portable and mobile Raman spectroscopy systems are increasingly being adopted in in situ non-invasive examination of artworks given their high specificity in material identification. However, these systems typically operate within centimeter range working distances, making the examination of large architectural interiors such as wall paintings in churches challenging. We demonstrate the first standoff Raman spectroscopy system for in situ investigation of historic architectural interior at distances > 3 m. The 780 nm continuous wave laser-induced standoff Raman system was successfully deployed for the in situ examination of wall paintings, at distances of 3–15 m, under ambient light. It is able to identify most common pigments while maintaining a very low laser intensity to avoid light induced degradation. It is shown to complement our current method of standoff remote surveys of wall paintings using spectral imaging.
... Reference spectra for common historic artists' pigments and binders were collected with the same instruments as those used on the manuscript. The pigment samples were verified analytically as described in [53] and the paint samples were prepared in animal glue and painted on paper [52]. The binding media samples included egg white, egg yolk, gum Arabic, animal glue and linseed oil, each prepared in thick layers (> 30 μm) such that the FTIR-ATR spectra would not be influenced by the substrate. ...
... 20 In Ibn Hayyim's treatise [18], both lac and brazilwood were mentioned in the making of carmine. [52,53] (Fig. 5c). ...
The aim of this research was to use non-invasive scientific analysis to uncover evidence of the planning process and relationship between pigments used in text copying and artwork production in the Oppenheimer Siddur (Oxford Bodleian Library MS Opp. 776), an illuminated 15th-century Hebrew prayer book. In many medieval Hebrew illuminated manuscripts, the authorship of the artwork is unknown. This manuscript’s colophon states that it was copied by its scribe-owner for personal family use but does not confirm who was responsible for the artwork. Prior deductive analysis suggested that the scribe-owner may also have been the manuscript’s artist, based on common motifs and an apparent shared colour palette appearing in both texts and artwork. Visual examination using high resolution digital images also identified points of contact between pigments used in the manuscript’s texts and artwork, raising questions about the pigment application sequence, and concurrent versus sequential text copying and artwork production. An in-house developed remote spectral imaging system (PRISMS) with 10 filters spanning the spectral range from 400 to 880 nm was modified for close-range application to image two of the folios to examine the sequence of production, identify the pigments and compare the materials used for the illumination and the text. Optical microscopy and Fourier Transform Infrared spectroscopy in the attenuated total reflection mode (FTIR-ATR) were used directly on the folios to complement the spectral imaging data in binding media and pigment identification. The results revealed close matches in reflectance spectra for the colorants and inks used in both text copying and illuminations, suggesting that the same mixture of colorants and inks have been used. The spectral imaging in the near infrared bands revealed a hidden underdrawing, indicating a design change during production of the manuscript, and the outlining of letters prior to coloured pigment being applied. The pigment use, the variation in the binder for different pigments and some elements of its production were found to be consistent with those described in historical sources. The evidence from this study supports the hypothesis that the scribe applied pigments for the manuscript’s artwork at the same time he did some of the scribal work which has implications for understandings of Jewish medieval visual cultures.
... Several studies have summarized the results of experiments aimed at finding the 'optimal' spectral transmission window for pigments and paints used by artists [1,[4][5][6]. In general, such studies have found increasing transmission of light starting around ~700 nm and peaking at about ~2000 nm. ...
... The camera system had to operate over the complete spectral range identified as providing the most usable information: 1000 to ~2500 nm. The selected spectral range of 1000-2500 nm is based on the prior studies of the wavelength dependence of the transmission of the paint layer [1,6] and also a study that included the effect of the underdrawing material and the paint grounds [4]. This latter study examined only five spectral regions from 900 to 2500 nm. ...
Background
Infrared reflectography (IRR) remains an important method to visualize underdrawing and compositional changes in paintings. Older IRR camera systems are being replaced with near-infrared cameras consisting of room temperature infrared detector arrays made out of indium gallium arsenide (InGaAs) that operate over the spectral range of ~900 to 1700 nm. Two camera types are becoming prevalent. The first is staring array infrared cameras having 0.25–1 Megapixels where the camera or painting is moved to acquire tens of individual images that are later mosaicked together to create the infrared reflectogram. The second camera type is scanning back cameras in which a small InGaAs array (linear or area array) is mechanically scanned over a large image formed by the camera lens to create the reflectogram, typically 16 Megapixels. Both systems have advantages and disadvantages. The staring IR array cameras offer more flexible collection formats, provide live images, and allow for the use of spectral bandpass filters that can provide reflectograms with better contrast in some cases. They do require a mechanical system for moving the camera or the artwork and post-capture image mosaicking. Scanning back cameras eliminate or reduce the amount of mosaicking and movement of the camera, however the need to minimize light exposure to the artwork requires short integration times, and thus limits the use of spectral bandpass filters. In general, InGaAs cameras are not sensitive in the 1700 to ~2300 nm spectral region, which has been identified in prior studies as useful for examining paintings with copper green pigments or thick lead white paints. Prior studies using cameras with sensitivity from 1000 to 2500 nm have found in general the performance at wavelengths longer than 1700 nm degraded relative to the performance at shorter wavelengths. Thus, there is interest in a camera system having improved performance out to 2500 nm that can utilize spectral bandpass filters.
Methods
Design requirements for such an improved IRR camera system were determined by first re-examining the optimal spectral window for detecting underdrawing. Thus the wavelength dependence of the clarity of carbon black underdrawings on a chalk ground covered by various paint swatches was measured from 750 to 2500 nm. Second, analysis of the loss of light transmission (1000–2500 nm) and the impact of thermal radiation (3000–5000 nm) on the performance of IR arrays sensitive to the 1000–5000 nm region was analyzed. From the results of these studies, a high sensitivity, near-infrared (1000–2450 nm) indium antimonide (InSb) staring array camera with a cold filter that blocked light >2450 nm was constructed with a color-corrected macro lens capable of holding interference filters. The camera system was characterized in three spectral bands (1100–1400, 1500–1800, and 2100–2400 nm) using test targets and art objects.
Results
The experimental results of the contrast difference between the model underdrawing on the chalk ground showed the optimal spectral window for a given pigment varied over the range from 700 to 2300 nm. The contrast in all cases was found to be low from 2300 to 2600 nm and even lower from 2600 to 5000 nm. This is attributable in part to broad absorption by the drying oil paint binder. Performance testing of the IRR camera found high signal-to-noise was observed in all three spectral bands due to the optimized macro lens having high transmission from 1000 to 2600 nm, a 100% efficiency cold stop, and a cold filter that blocks light >2450 nm. The camera was found to have high light sensitivity, requiring only ~30 to 50 lx from incandescent lamps having a color temperature of ~2060 K. The images produced in each spectral band were sharp, and the modulation at Nyquist (1/2 the sampling frequency) was 20%. IRR images of Old Master paintings, works on paper, and a warped panel painting were collected to test the practical utility of the camera system.
Conclusions
The IR camera system presented here was found to produce a variety of high-resolution image products. The ability to extend image collection over the 1700–2450 nm spectral range was found in some examples to provide improved visibility of underdrawing. The ability to register the resulting multi-band IRR images with the color image offers opportunities to produce new image products such as false-color images and principal component images. These image products were found to give new insights into the construction of works of art. A collection scheme based on a ‘Z-stack’ method was found to solve the problem of producing high-resolution IRR images of highly warped panel paintings.
... Table 1 shows the list of 28 pigments thus selected that were mixed with linseed oil or egg tempera. These were painted in 2006 by the National Gallery in London on both glass microscope slides and boards sized with chalk in rabbit skin glue [32]. Samples of pure binding media including linseed oil, egg tempera, animal glue and gum Arabic were also applied on glass. ...
... Figure 4 shows that the full depth of the lapis lazuli layer to be ~100-150 microns as measured with the 1960nm OCT, while the multi-photon excitation fluorescence image only shows the top 30 micron. Most pigments become more transparent in the near infrared at longer wavelength [32], therefore the longer the central wavelength the better depth of penetration for OCT imaging, which was why the full thickness of the lapis lazuli sample was revealed in the 1960nm OCT but not in the 810nm OCT image (Fig. 4(c)). In the case of multi-photon excitation fluorescence imaging, even if the excitation wavelength is in the near infrared at a long wavelength and therefore likely to penetrate deeper into the sample to excite fluorescence, the multi-photon fluorescence emission would still be at a much shorter wavelength in the visible range and therefore likely to be absorbed or scattered by the material above where it originated and not be detected. ...
This paper examines for the first time the potential complementary imaging capabilities of Optical coherence tomography (OCT) and non-linear microscopy (NLM) for multi-modal 3D examination of paintings following the successful application of OCT to the in situ, non-invasive examination of varnish and paint stratigraphy of historic paintings and the promising initial studies of NLM of varnish samples. OCT provides image contrast through the optical scattering and absorption properties of materials, while NLM provides molecular information through multi-photon fluorescence and higher harmonics generation (second and third harmonic generation). OCT is well-established in the in situ non-invasive imaging of the stratigraphy of varnish and paint layers. While NLM examination of transparent samples such as fresh varnish and some transparent paints showed promising results, the ultimate use of NLM on paintings is limited owing to the laser degradation effects caused by the high peak intensity of the laser source necessary for the generation of non-linear phenomena. The high intensity normally employed in NLM is found to be damaging to all non-transparent painting materials from slightly scattering degraded varnish to slightly absorbing paint at the wavelength of the laser excitation source. The results of this paper are potentially applicable to a wide range of materials given the diversity of the materials encountered in paintings (e.g. minerals, plants, insects, oil, egg, synthetic and natural varnish).
... Our study shows the successful coordination and timewise efficient application of four non-invasive optical techniques to the monitoring of the cleaning operations performed on a painting currently under restoration at the OPD Laboratories. 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. ...
... OCT, an interferometric technique widely applied in medicine and the biomedical field for probing biological tissues, has been introduced in the conservation field in the early 2000s [31][32][33][34][35][36][37]. The analysis output is a high-resolution stratigraphic image of the examined surface resulting in a visual and numerical quantification of the paint layers thickness, allowing the progressive assessment of foreign material's thinning during the cleaning process, thus eliminating any risk of damage to the original [38][39][40][41][42][43][44][45]. ...
The cleaning of painted artworks, i.e. the critical operation whereby materials are selectively removed from a painted surface by partial thinning or complete elimination of varnish, is one of the most debated conservation operations, being an irreversible process, which may result in chromatic and morphological variations in the painted surface. Due to ageing, the upper layer is subject to darkening and yellowing because of blanching and fading from ultraviolet exposure, dust deposition, and overpainted layers due, for instance, to restoration interventions. This degradation can either alter the original appearance of painting polychromy or cause mechanical failure of the finishes. To address these adverse conditions, a process of examination and analysis is critical to the definition and interpretation of the varnish layer. When investigating the ageing process of old paintings, it is of great importance to obtain insight into the painting technique as practiced in the past, and the first step in gaining this knowledge is, to a large extent, based on the study of the varnish film. An effective control of the process and objective evaluation of its outcome requires therefore instrumental/analytical support. The present study illustrates the successful application of non-invasive optical techniques—such as colorimetry, multispectral reflectography, laser scanning micro-profilometry, and optical coherence tomography—to the monitoring of an Italian fourteenth-century painting cleaning process. Results presented here confirm that optical techniques play a pivotal role in artwork diagnostics, especially with regard to conservation operations, while also indicating their validitywhen applied to the monitoring of the cleaning process.
... It can be used for dynamic monitoring of the wetting and drying of different varnishes, varnish removal using solvents, real time laser ablation of varnish layers and tracking of canvas deformation due to environmental changes or in the examination of ancient glass, enamel, ceramics, jade, faience and parchment. [3][4][5][6][7][8][9][10][11][12] The OCT capability to probe the material substructure is limited by a combination of scattering and absorbing effects (i.e. material opacity). ...
... material opacity). Since the OCT depth (axial) resolution (Δz) is related to the source wavelength and its spectral bandwidth (Δz~λ 2 /Δλ) and the scattering coefficient of a material generally decreases with increasing wavelength, the best OCT probing depth is obtained with longer wavelength sources at the cost of lower axial resolution [8,9,11]. ...
Calcium oxalates are insoluble colourless or whitish salts constituting noble patina, on both natural
and artificial stone artworks’ surfaces, the presence of which is extremely valued. The oxalates are
not considered detrimental to the substrate, however, being often accompanied by other substances
such as gypsum, silicates, pigmented particles. They may form very adherent, relatively thick and
coloured layers creating disfiguring effects and hindering legibility of the pictorial surface. For this
reason it may be appropriate to diminish their thickness, but patina’s partial preservation is
particularly required calling for extremely gradual and controllable cleaning approach. Thinning of
calcium oxalate patina from a detached 16th century fresco (from Sansepolcro) was performed
through the use of laser (Nd:YAG and Er:YAG) systems and chemical means (Carbogel loaded 5
wt% of tetrasodium EDTA). Optical Coherence Tomography (OCT), providing a non-invasive
stratigraphic cross-section of the examined surface, allowed to distinguish the oxalate from the
underlying original layers and therefore to have an overview about its distribution, to numerically
evaluate patina’s thickness range and to provide the information on the amount of the material both
removed and left on the artwork’s surface. Laser scanning conoscopic microprofilometry allowed
for a high-density sampling of the artwork’s surface providing a three-dimensional model of the
surface pattern. Theobtained 3D models were used to estimate the amount of material removed and
to compare them with those provided by OCT. The successful exploitation of the proposed
exceptional cleaning monitoring methodology may be seen as an innovative and valid support for
the restorers in the conservation of mural painting or other surfaces covered by oxalate layers and
may pilot more targeted, cautious and respectful cleaning intervention.
... The 850 nm OCT system was capable of imaging the smalt model systems down to the ground layer but could not penetrate sufficiently below the surface for the iron oxide model systems. Iron oxide oil paint has low transparency in this wavelength region 43 . In Fig. S12 in S2.4 a comparison is shown of an 850 ...
Long-term condition monitoring of works of art can provide new insights into object-specific deterioration mechanisms. Detecting change over time allows us to determine whether deterioration is active, to investigate its cause and to establish the efficacy of conservation interventions. However, long-term condition monitoring poses both logistical and technical challenges. To address the latter, a 6-month pilot study using model systems has been performed to investigate the approach to long-term monitoring of chemical dynamic processes in oil paintings. The focus was placed on metal soap protrusions: a condition phenomenon encountered in oil paintings that results from dynamic chemical pigment-binder interactions. Eight portable non- or minimally invasive examination technologies available via the MOLAB facility of IPERION HS were used to detect change in model systems. These model systems were designed to form lead soap protrusions in situ in a short time frame by including reactive components in their stratigraphy, providing changes on a scale more typical of years or decades in real paint systems. Raking light imaging or commercial colorimetry did not provide sufficient resolution for detecting small-scale changes associated with lead soap protrusions. X-radiography with consistent acquisition parameters in combination with a form of automated recognition of protrusions was found to provide a relatively accessible method for monitoring changes in the spatial distribution of protrusions. 3D techniques such as optical coherence tomography and micro-profilometry were found to be suitable for detecting change in lead soap protrusions, provided that they reach sufficient spatial resolution in the plane of a paint layer (≤20 μm) and depth (≤2–3 μm). Acoustic microscopy was found to provide insufficient spatial resolution for this purpose. More specificity for lead soaps was provided by techniques that couple high resolution 2D or 3D imaging to spectral information, such as micro-profilometry coupled to VIS-NIR spectroscopy.
... (3) Contrast visual illusion: Contrast visual illusion usually refers to two objects of the same size and shape under the influence of two different opposing environments, such as length, size, depth, etc., which can cause the human brain perceptual system to produce incorrect visual perceptions of unequal graphic areas [13]. A circle surrounded by a large circle environment and one surrounded by a small circle environment are shown in figure 1(c), where the two circles in the middle are equal in size, but the middle circle on right side of the figure is much smaller due to being in different opposing environments. ...
This research adopted literature analysis methods to explain the principles and types of line illusion and, secondly, using Grasshopper software generated some fashion design samples with visual illusion element application. Then semantic analysis was applied to investigate audiences’ cognition, with the aim to improve the design. Lastly, Grasshopper software was used to generate new design patterns by changing the set of generated points through algorithm operation and dotted line interference to change the presented linear illusion forms and patterns. The results of this research show that with the support of parametric technology, the opportunities and space for creating line elements in clothing will be further expanded, enabling that real-time pattern or apparel design changes, and adjustments can be realized by parameter variable controlling. The research comprised a new attempt at combining linear visual illusion, fashion design and parametric design, showing the content and design case studies for relevant research in this area.
... Figure 3 shows the OCT can reveal the surface and under-layer information of the paint sample. However, different binders and pigments have different infrared light absorption coefficients, therefore, OCT's ability to reveal under-layer structural information is material dependent (Liang et al. 2013). ...
Optical coherence tomography (OCT) is a non-invasive imaging method that can be used to study the surface features and subsurface structures of delicate cultural heritage objects. However, the field of view of OCT severely limits the system’s scanning area. Previously, we have presented a hybrid scanning platform combined with an imaging stitching algorithm to achieve macroscopic OCT (macro-OCT) imaging. This paper further demonstrates the potential applications of the OCT data by rendering 3D volumetric data into standard virtual reality (VR), augmented reality (AR), and 3D printing formats. The 3D model can be 3D printed or interactively displayed through various platforms such as VR and AR headsets, smartphones, and web pages. The high-resolution 3D models obtained from the macro-OCT system can potentially improve the experience of accessing artworks online and assist individuals with visual impairments to study art through tactile feedback.
... The opacity of these pigments in the infrared severely hindered access to possible underdrawings and/or colour notations, especially in dark blue and green areas. It has been shown that the spectral transparency of both these pigments is highly attenuated by both pigment mass concentration and particle size [50]. A further limitation of the mostly non-invasive approach was the lack of information at the molecular level that may have enabled the identification of both the origin of the binder and the organic colourants used. ...
Scientific analyses of the traditional materials and methods in thangka production are uncommon, as thangkas are sacred objects, the sampling of which is discouraged, in order to preserve their integrity. This study builds on this important ethical challenge and presents a three-stage methodology that systematically delves deeper into each layer of the composition, successfully enabling the investigation of different phases of production. In the first stage, visual examination of the painting, including observations under magnification, was used to assess its condition. In the next step, the infrared reflected (IRR) and short-wave infrared (SWIR) images revealed the underdrawing and instances of modifications as well as colour notations. Additionally, ultraviolet-induced visible luminescence (UVL), infrared-reflected false colour (IRRFC) and X-ray images provided important preliminary information on the colourants present, the nature of the underdrawing, and the painting technique. In the final stages, fibre optic reflectance (FORS), Fourier-transform infrared (FTIR) and Raman spectroscopies confirmed the identity of many of the pigments (cinnabar/vermillion, minium, iron oxide, malachite, azurite, indigo, Indian lac), the ground and the binder used for the blue and green paint layers. In addition, key details of practices and materials were revealed, that may indicate provenance or other information of scholarly importance. These will constitute a helpful comparison to existing and future studies of other thangkas.
... Five spectra of each sample were collected with 15 s of real time acquisition. Although many of the pigment reference samples used are commercially available pigments that differ in composition from historical pigments or are samples that may not have been processed using traditional method, the elemental compositions of most of the pigment reference samples used have been previously characterised in other studies (see Additional file 1) [39][40][41]. It should be noted that the compositions of these pigment reference samples were not used to inform the elemental compositions noted in Table 2, although as discussed later in the paper, these spectra were used to refine the CNN model. ...
X-ray fluorescence (XRF) spectroscopy is an analytical technique used to identify chemical elements that has found widespread use in the cultural heritage sector to characterise artists' materials including the pigments in paintings. It generates a spectrum with characteristic emission lines relating to the elements present, which is interpreted by an expert to understand the materials therein. Convolutional neural networks (CNNs) are an effective method for automating such classification tasks—an increasingly important feature as XRF datasets continue to grow in size—but they require large libraries that capture the natural variation of each class for training. As an alternative to having to acquire such a large library of XRF spectra of artists' materials a physical model, the Fundamental Parameters (FP) method, was used to generate a synthetic dataset of XRF spectra representative of pigments typically encountered in Renaissance paintings that could then be used to train a neural network. The synthetic spectra generated—modelled as single layers of individual pigments—had characteristic element lines closely matching those found in real XRF spectra. However, as the method did not incorporate effects from the X-ray source, the synthetic spectra lacked the continuum and Rayleigh and Compton scatter peaks. Nevertheless, the network trained on the synthetic dataset achieved 100% accuracy when tested on synthetic XRF data. Whilst this initial network only attained 55% accuracy when tested on real XRF spectra obtained from reference samples, applying transfer learning using a small quantity of such real XRF spectra increased the accuracy to 96%. Due to these promising results, the network was also tested on select data acquired during macro XRF (MA-XRF) scanning of a painting to challenge the model with noisier spectra Although only tested on spectra from relatively simple paint passages, the results obtained suggest that the FP method can be used to create accurate synthetic XRF spectra of individual artists' pigments, free from X-ray tube effects, on which a classification model could be trained for application to real XRF data and that the method has potential to be extended to deal with more complex paint mixtures and stratigraphies.
... When deciding on which wavelength to use, it is important to consider the purpose of the investigation. A prior survey conducted for common historical artists' pigments in oil and egg tempera binders found the optimum wavelength to maximise penetration depth to be around 2.2 μm over the spectral range 400-4500 nm (Liang et al. 2013). The light scattering coefficient of materials tends to decrease with increasing wavelength over this range, making materials more transparent at longer wavelength. ...
Egyptian faience is a non-clay ceramic semi-transparent material formed of a quartz core and alkali-lime glaze. Previous investigations have identified production techniques by using microstructure images obtained from invasive methods. Optical coherence tomography (OCT) is a non-invasive 3D imaging technique that produces virtual cross-sections of transparent and semi-transparent materials. A previous study by one of the authors demonstrated the feasibility of non-invasive investigation of microstructures of Egyptian faience using 930 nm OCT, but the limited probing depth prevented viewing down to the quartz core of the objects. This paper shows that an in-house developed OCT system using a longer wavelength (2 µm) was able to image the full microstructure from the top glaze layer down to the core, allowing rapid and non-invasive studies of intact objects and demonstrating the potential for surveying large museum collections. OCT virtual cross-section images at 5 wavelengths, 550, 810, 930, 1300 and 1960nm, were compared and the optimum wavelength for OCT investigation of Egyptian faience microstructure was found to be 2 μm.
... In 2013, Liang et al. [173] investigated the best spectral window for the en-face imaging of subsurface materials in painting with IRR and OCT. Based on the spectral properties of a set of historic artists' pigments/paints, the 2.2 μm wavelength turned out to be the most suitable for visualizing preparatory sketches, both for direct infrared imaging and OCT, suggesting that broadband sources at around 2 μm are highly desirable for OCT applications in art and in material science in general. ...
The present paper focuses on the reflectance spectral imaging of painted
surfaces in the visible-near infrared spectral region (400–2500 nm). Other spectral
ranges and methods are mentioned, to contextualize the spectral investigation of works
of art.
... This is because the ability to distinguish and visualize thin layers is a decisive factor for using in this area of applications. Therefore, shortinfrared radiation was chosen even though the transparency of typical pigments is limited in this area and the optimum wavelength range lays around 2 μm [41]. By using portable commercial sources composed of coupled superluminescent diodes (Broadlighters: Q-870-HP or M-T-850-HP-I both from Superlum, Ireland) emitting in the range 770-970 nm (the former) and 750-960 (the latter), it was possible to achieve 3.3 μm of (measured) axial resolution in air, and 2.2 μm in medium of n R = 1.5. ...
... The spectral window of around 2 µm is a well-known range optimally suitable for OCT imaging of art objects [48]. In order to introduce the developed 2 µm OCT sub-system as a reasonable and affordable alternative for non-invasive imaging in art conservation (the state-of-the-art systems [33,49] are based on the expensive MCT or InSb cameras), the multilayer mock-up that is made up of different commercial paints was prepared. ...
Recent developments and commercial availability of low-noise and bright infrared (IR) supercontinuum sources initiated intensive applied research in the last few years. Covering a significant part of near- and mid-infrared spectral ranges, supercontinuum radiation opened up unique possibilities and alternatives for the well-established imaging technique of optical coherence tomography (OCT). In this contribution, we demonstrate the development, performance, and maturity of a cost-efficient dual-band Fourier-domain IR OCT system (2 µm and 4 µm central wavelengths). The proposed OCT setup is elegantly employing a single supercontinuum source and a pyroelectric linear array. We discuss adapted application-oriented approaches to signal acquisition and post-processing when thermal detectors are applied in interferometers. In the experimental part, the efficiency of the dual-band detection is evaluated. Practical results and direct comparisons of the OCT system operating within the employed sub-bands are exhibited and discussed. Furthermore, we introduce the 2 µm OCT sub-system as an affordable alternative for art diagnosis; therefore, high resolution and sensitive measurements of the painting mock-ups are presented. Finally, potentials of the dual-band detection are demonstrated for lithography-based manufactured industrial ceramics.
... It may be possible to obtain layer structure to a greater depth and higher resolution by varying the parameters of the instrument, such as wavelength and bandwidth of the light source. An optimum spectral window for OCT was found to be~2.2 µm, through the study of the transparency of pigments as a function of wavelength on various historic pigments in linseed oil and egg tempera [51]. ...
A range of sophisticated imaging techniques have been developed in recent years that can reveal the surface structure of cultural heritage objects with varying precision. In combination with various spectroscopic methods, they allow the study of the chemical composition of the object; thus, conclusions can be drawn about the origin of the object or its initial components, method, or time of creation, authenticity, mechanisms of degradation, and ways of further conservation. At present, different techniques can be applied to a wide range of cultural heritage objects, such as varnishes, paintings, archaeological objects, binding media, paper-based documents, parchments, marbles, frescoes, as well as various objects made of leather, fabric, stone, ceramics and glass, wood, or metal. One of the main needs in the study of cultural heritage (CH) is the transportability/portability of the research equipment, since many pieces under investigation cannot be moved to the laboratory, either because of their size, inseparability (for example, frescoes on walls, mural paintings in caves), or the threat of damage. In this work, we briefly overview the main optical- and laser-based methods used for the study of cultural heritage objects indicating the scope of their application, and we focus on the applications of non-linear microscopic methods for the investigation of a series of artifacts. We also discuss all the requirements for the construction of a prototype transportable non-linear optical system that will be used as a novel diagnostic tool for in situ studies of CH assets. The availability of such a transportable workstation will significantly improve the study and characterization of various types of CH objects and will constitute an extremely useful diagnostic tool for heritage scientists dealing with a variety of investigations.
... Though used less often, the 1.7 μm optical window has even lower scattering, and attains the lowest attenuation coefficient for deep microscopy in tissues such as the brain [2,[4][5][6]. The NIR optical window around 2.2 μm is even more rarely used for tissue imaging because of light source limitations [7] and high water absorption [8]. However, the benefits of 2.2 μm for in vivo imaging through bone, a biological tissue with moderate water mass content (12% [9]) compared to other tissues (>70% [10]), have not been investigated, to our knowledge. ...
In biological tissue, longer near-infrared wavelengths generally experience less scattering and more water absorption. Here we demonstrate an optical coherence tomography (OCT) system centered at 2.1 microns, whose bandwidth falls in the 2.2 micron water absorption optical window, for in vivo imaging of the rodent brain. We show in vivo that at 2.1 microns, the OCT signal is actually attenuated less in cranial bone than at 1.3 microns, and is also less susceptible to multiple scattering tails. We also show that the 2.2 micron window enables direct spectroscopic OCT assessment of tissue water content. We conclude that with further optimization, 2.2 micron OCT will have advantages in low-water-content tissue such as bone, as well as applications where extensive averaging is possible to compensate absorption losses.
... 1,2 While OCT systems with the central wavelengths from 900 nm to 1.3 µm are commercially available for biomedical uses, a central wavelength at 2.2 µm is better suited to detect pigment layers, due to the larger penetration depth. 3 Liang et. al. have demonstrated that a custom-built 2.2 µm Fourier domain OCT (FD-OCT) system significantly improved the imaging depth through oil-paint pigment layers such as titanium white, cobalt blue, and yellow ochre. ...
... Unfortunately, the diversity of objects being within a circle of interest is in case of OCT limited by the properties of the materials constituting the artwork. The major limitation of what may be examined with OCT lies in the transparency of subsurface structures of the object [7,8] to infrared radiation. This drawback practically excludes some areas of objects or even whole groups of objects. ...
Optical coherence tomography is a fast, non-invasive technique of structural analysis utilising near-infrared radiation. Examples of using OCT, for obtaining cross-sectional images of objects of craftsmanship and an easel painting have been shown. Issues regarding the technique of execution and destruction phenomena were resolved non-invasively. In some cases, the secondary alterations can be identified and localised within the object’s structure which helps in authentication of the artwork.
... In most cases,the use of long-wavelength infrared radiation results in its higher penetration into the paint layer. [5] Nevertheless, in OCT there is at rade-off between the penetration and the axial resolution, the latter of which deteriorates with the square of the central wavelength. Herein, an earinfrared system [6] (770-970 nm) was used to provide an axial resolution of 2.5 mmi nv arnish and painting media. ...
Optical Coherence Tomography (OCT) was used for non-invasive examination of a well-known, yet complex, painting from the studio of Leonardo da Vinci in combination with routine imaging in various bands of electromagnetic radiation. In contrast with these techniques, OCT provides depth-resolved information. Three post-processing modalities were explored: cross-sectional views, maps of scattering from given depths, and their 3D models. Some hidden alterations of the painting due to past restorations were traced: retouching and overpainting with their positioning within varnish layers as well as indications of a former transfer to canvas.
... In most cases,the use of long-wavelength infrared radiation results in its higher penetration into the paint layer. [5] Nevertheless, in OCT there is at rade-off between the penetration and the axial resolution, the latter of which deteriorates with the square of the central wavelength. Herein, an earinfrared system [6] (770-970 nm) was used to provide an axial resolution of 2.5 mmi nv arnish and painting media. ...
Optical Coherence Tomography (OCT) was used for non-invasive examination of a well-known, yet complex, painting from the studio of Leonardo da Vinci in combination with routine imaging in various bands of electromagnetic radiation. In contrast with these techniques, OCT provides depth-resolved information. Three post-processing modalities were explored: cross-sectional views, maps of scattering from given depths, and their 3D models. Some hidden alterations of the painting due to past restorations were traced: retouching and overpainting with their positioning within varnish layers as well as indications of a former transfer to canvas.
... OCT is widely used in biomedical research and in clinical studies, especially in the fields of ophthalmology [6][7][8][9][10][11][12] and cardiology [13][14][15][16][17]. Besides biomedical fields [18][19][20], OCT has been applied in characterization of polymer materials [21,22] and in studies of art conservation [23][24][25]. ...
Supplementary Figures, Supplementary Notes and Supplementary References
... The output of micro-SORS is compared with the results of the optical coherence tomography (OCT) technique that has emerged in substitution of invasive methods capable of recovering the cross-sectional structural information. OCT is a well established optical interferometric technique, particularly suited for probing semi-transparent materials in the medical field, mainly in ophthalmology, and recently also in cultural heritage to study stratified systems of paintings [5][6][7][8][9]. ...
We present a method for estimating the thickness of thin turbid layers using defocusing micro-spatially offset Raman spectroscopy (micro-SORS). The approach, applicable to highly turbid systems, enables one to predict depths in excess of those accessible with conventional Raman microscopy. The technique can be used, for example, to establish the paint layer thickness on cultural heritage objects, such as panel canvases, mural paintings, painted statues and decorated objects. Other applications include analysis in polymer, biological and biomedical disciplines, catalytic and forensics sciences where highly turbid overlayers are often present and where invasive probing may not be possible or is undesirable. The method comprises two stages: (i) a calibration step for training the method on a well characterized sample set with a known thickness, and (ii) a prediction step where the prediction of layer thickness is carried out non-invasively on samples of unknown thickness of the same chemical and physical make up as the calibration set. An illustrative example of a practical deployment of this method is the analysis of larger areas of paintings. In this case, first, a calibration would be performed on a fragment of painting of a known thickness (e.g. derived from cross-sectional analysis) and subsequently the analysis of thickness across larger areas of painting could then be carried out non-invasively. The performance of the method is compared with that of the more established optical coherence tomography (OCT) technique on identical sample set.
This article is part of the themed issue ‘Raman spectroscopy in art and archaeology’.
... The cross-sectional image of the white base coat sample shown in Fig. 5(c) demonstrates that it has a transparent clear coat layer as well. However, different from the black and onyx samples, the strongly multiple scattering property of the titanium dioxide in the white base coat limits the penetration depth of the incident light [19]. Light entering this layer is scattered many times within a short distance, meaning no structure ...
We have demonstrated for the first time, to our knowledge, the use of optical coherence tomography (OCT) as an analytical tool for nondestructively characterizing the individual paint layer thickness of multiple layered automotive paints. A graph-based segmentation method was used for automatic analysis of the thickness distribution for the top layers of solid color paints. The thicknesses measured with OCT were in good agreement with the optical microscope and ultrasonic techniques that are the current standard in the automobile industry. Because of its high axial resolution (5.5 μm), the OCT technique was shown to be able to resolve the thickness of individual paint layers down to 11 μm. With its high lateral resolution (12.4 μm), the OCT system was also able to measure the cross-sectional area of the aluminum flakes in a metallic automotive paint. The range of values measured was . In summary, the proposed OCT is a noncontact, high-resolution technique that has the potential for inclusion as part of the quality assurance process in automobile coating.
... Previous SECM schemes were mainly operated at 1.0 µm wavelength window because of the widely available optical components at this window and the good compromise between water absorption and tissue scattering for biological applications. However, its potential use in a more favorable wavelength window (∼2µm) for imaging of highly scattering materials with low water content is yet to be explored so far [2]. In this paper, we present a SECM system at the 1.9-µm wavelength window based on the supercontinuum (SC) source generated by a 13-cm homemade Tm 3+ -doped germanate glass fiber. ...
We demonstrate a spectrally encoded confocal microscopy (SECM) system at 1.9 µm using broadband supercontinuum (SC) source, which is capable of imaging at resolution of ~2 µm and field of view (FOV) of ~500 µm.
... Previous SECM schemes were mainly operated at 1.0 µm wavelength window because of the widely available optical components at this window and the good compromise between water absorption and tissue scattering for biological applications. However, its potential use in a more favorable wavelength window (∼2µm) for imaging of highly scattering materials with low water content is yet to be explored so far [2]. In this paper, we present a SECM system at the 1.9-µm wavelength window based on the supercontinuum (SC) source generated by a 13-cm homemade Tm 3+ -doped germanate glass fiber. ...
We demonstrate a spectrally encoded confocal microscopy system using a wideband supercontinuum source at a central wavelength of 1870 nm over a bandwidth of 180 nm. It is illustrated to achieve a better depth of field (DOF) and penetration depth at , compared with the 1.5- spectral window when using the same experimental setup. Our scheme shows the potential of optical source operating - wavelength range to be applied in other established imaging modalities, such as time-stretch imaging for the imaging of highly scattering materials with a large depth range, where a large DOF and penetration depth are required.
... An obvious disadvantage, though, lies in the limited transparency of many media to the penetrating infrared beam. For maximum penetration through the pigments, the optimum spectral window lies around 2 μm (Liang et al., 2013), but then the available axial resolution is limited. Nevertheless, OCT has been successfully employed in the examination of components of objects of cultural heritage, such as the varnish and glaze layers of easel paintings (Targowski et al., 2010a;Liang et al., 2005), stained glass (Targowski et al., 2010b), jade (Yang et al., 2012) and similar stones, as well as many others (oct4art, 2014). ...
Macro-X-ray fluorescence (XRF) is a newly commercially available research tool very useful in the examination of artwork. Its novelty lies in its ability to create maps of the distribution of chemical elements on scales of a few milimetres. In this contribution, its use together with optical coherence tomography (OCT) in the inspection of an illuminated manuscript is reported for the first time. The former technique is used both for mapping the elemental distribution over large parts of the folios - including illuminated initials - and for quantitative analysis of the composition of the smalt pigment, as well as of changes in the composition of iron-gall ink at different pages. The latter, by providing cross-sectional images of painted details, helps in interpreting the XRF results. All of the results shown relate to the examination of a late sixteenth-century-illuminated parchment manuscript (a gradual) originating from the Convent of the Benedictine Sisters in Lviv in the Ukraine. © The International Institute for Conservation of Historic and Artistic Works 2015.
... Initially full field time domain OCTs using white light sources in the visible part of the spectrum were constructed to reach high depth resolutions of ~1 µm [10,11], however, they have low sensitivity compared with systems using laser sources and have very limited depth penetration as most paints are opaque in the visible range [12]. ...
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 ultra-high 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 micro-destructively.
... Rapid 'real-time' spectroscopic characterization of materials and swept-source optical coherence tomography (SS-OCT) [5] are two important examples of such applications. The use of OCT for non-invasive investigation of paintings to provide the information necessary for effective restoration and to aid conservation, art history and archaeology is one emerging application where operation in the longer wavelength band around ~2 μm brings the advantage of increased penetration depth due to reduced scattering for typical artists' pigments [6]. Tm-doped silica fiber laser provide an efficient way to access the relevant wavelength region due to the wide emission band for Tm-doped silica glass. ...
A wavelength-swept thulium-doped fiber laser system employing two parallel cavities with two different fiber gain stages is reported. The fiber gain stages were tailored to provide emission in complementary bands with external wavelength-dependent feedback cavities sharing a common rotating polygon mirror for wavelength scanning. The wavelength-swept laser outputs from the fiber gain elements were spectrally combined by means of a dichroic mirror and yielded over 500 mW of output with a scanning range from ~1740 nm to ~2070 nm for a scanning frequency of ~340 Hz.
... Wavelength-swept lasers (WS-lasers) have important applications in a number of areas, including spectroscopic characterization of materials and swept-source optical coherence tomography (SS-OCT) [1]. The use of OCT for non-invasive investigation of paintings to provide the information necessary for effective restoration and to aid conservation is one example of an emerging application where operation in the longer wavelength band around ~2 μm brings the advantage of increased penetration depth due to reduced scattering for typical artists' pigments [2]. SS-OCT does not require a detector array which is an advantage in long wavelength-OCT, where a standard Si CCD camera cannot be used and short-wave infrared-sensitive cameras are expensive. ...
A wavelength-swept thulium-doped silica fiber laser using an intracavity rotating slotted-disk wavelength scanning filter in combination with an intracavity solid etalon for passive control of temporal and spectral profiles is reported. The laser yielded a wavelength swept output in a step-wise fashion with each laser pulse separated from the previous pulse by a frequency interval equal to the free-spectral-range of the etalon and with an instantaneous linewidth of <0.05 nm. Scanning ranges from 1905 nm to 2049 nm for a cladding-pumping laser configuration, and from 1768 nm to 1956 nm for a core-pumping laser configuration were achieved at average output powers up to ~1 W.
... Other applications of spectral imaging in art include imaging of underdrawings beneath the paint layers. Most paints are more transparent in the near infrared and hence images in the infrared are useful for revealing the preparatory drawings beneath the paint (van Asperen de Boer, 1968;Liang et al., 2013). A comparison between images in the visible spectral range with those in the near infrared can also reveal past interventions and damages to the paintings, since conservators colour match the paint for retouching to the original without necessarily using the same paint material. ...
PRISMS (Portable Remote Imaging System for Multispectral Scanning) is designed for in situ, simultaneous high resolution spectral and 3D topographic imaging of wall paintings and other large surfaces. In particular, it can image at transverse resolutions of tens of microns remotely from distances of tens of metres, making high resolution imaging possible from a fixed position on the ground for areas at heights that is difficult to access. The spectral imaging system is fully automated giving 3D topographic mapping at millimetre accuracy as a by-product of the image focusing process. PRISMS is the first imaging device capable of both 3D mapping and spectral imaging simultaneously without additional distance measuring devices. Examples from applications of PRISMS to wall paintings at a UNESCO site in the Gobi desert are presented to demonstrate the potential of the instrument for large scale 3D spectral imaging, revealing faded writing and material identification.
... Rapid 'real-time' spectroscopic characterization of materials and swept-source optical coherence tomography (SS-OCT) [5] are two important examples of such applications. The use of OCT for non-invasive investigation of paintings to provide the information necessary for effective restoration and to aid conservation, art history and archaeology is one emerging application where operation in the longer wavelength band around ~2 μm brings the advantage of increased penetration depth due to reduced scattering for typical artists' pigments [6]. Tm-doped silica fiber laser provide an efficient way to access the relevant wavelength region due to the wide emission band for Tm-doped silica glass. ...
Wavelength-swept operation of cladding-pumped and core-pumped thulium-doped fiber lasers using a novel intracavity rotating disk wavelength-scanning arrangement is reported. Scanning ranges from 1905-2049nm (for cladding pumping) and 1768-1956nm (for core pumping) were obtained.
A mobile remote standoff Raman spectroscopy system operational at typical distances of 10 m was developed specifically for research of historical sites and wall paintings recently. Here we present an upgrade to that system informed by a thorough experimental investigation of the relevant laser-induced degradation issues. Reflectance spectroscopy as a more sensitive technique than Raman spectroscopy was used for monitoring and a new phenomenon of reversible alterations was detected in many paint samples at very low laser intensities of less than 1 W/cm ² when Raman measurements detected no changes. Contrary to conventional wisdom, the intensity threshold for safe operation was found to decrease significantly for larger incident irradiation area in the case of a vermilion oil paint sample. Damage threshold in intensity for each material needs to be determined for different spot sizes, which can be orders of magnitude lower for 1 mm spot size compared with micro-Raman. Results from this study is also relevant to portable Raman systems which use similarly large spot sizes. However, the larger spot size still generates more Raman photons overall under safe operation than micro-Raman systems. Continuous-wave (CW) lasers are found to be best suited to efficient, that is more Raman signal detected over a given measurement time, and safe Raman operation than ns-pulse lasers at the same wavelength. While the damage threshold in intensity for ns-pulse lasers is much higher than that of CW lasers, the pulse energy allowed in one pulse for safe operation is still too low to allow detection of Raman signal, and the need for multiple pulses makes pulse laser inefficient owing to the low repetition rate necessary to ensure adequate heat dissipation between pulses. The safety of the upgraded system was evaluated and found that no permanent laser-induced degradation was detected within 60 s of laser irradiation for any of the paint samples.
Ensuring self-driven mode-locking and broadband wavelength tuneability in all-fibre-integrated femtosecond laser sources enables a new level of their versatility and extends areas of their applications. Principle limitations for this are traditionally available ultrafast modulators and tuneability techniques. Here, we exploit Thulium-doped fibre to perform three roles in the cavity: laser gain, saturable absorber, and tuneability element via controlling its excitation level. We confirmed that Tm-doped fibre saturable absorption is defined by a reinforced quenching of Tm3+ pairs. As a result, we present both numerically and experimentally a highly stable sub-picosecond pulse generation with a ~90 nm tuneability range spanning from 1873 to 1962 nm via adjusting the cavity feedback. The maximum laser efficiency corresponds to 25% cavity feedback, enabling the highest output energy of 1 nJ in 600-fs solitons at 1877 nm. Overall, the presented laser system establishes a compact and straightforward approach for ultrafast generation, which can be translated to other fibre laser operation wavelengths. Active tuning of laser output properties promises to broaden the versatility and applications of these light sources. Here, a variable fibre-optical coupler is introduced in a self-mode-locked Tm-doped fibre laser to achieve active wavelength tuning over 90 nm.
In this contribution, we present the first analytical study of the painting "Christ in Gethsemane" (from a parish church of the Assumption of the Blessed Virgin Mary in _ Zaga n, Silesia, Poland) which attribution to Michael Willmann (1630-1706) has been questioned recently by art historians. After some preliminary non-invasive investigations, the painting was subjected to Macro XRF scanning. The further detailed study of micro-samples became essential although of limited scope since, given the painting's historical value, their number had to be strictly limited. The application of optical coherence tomography (OCT) allowed selecting areas of the original paint layer and collecting samples representative of the artist's technique. Further study analysis of the cross-sections stratigraphy using UV-VIS optical microscopy and SEM-EDS analysis supported the conclusions derived from MA-XRF images. All the results were compared with a reference database containing information about various painting materials used by M. Willmann during all his artistic periods which finally allowed for negative attribution of the painting to the master.
We demonstrate a compact all-fiber mode-locked ultrashort pulse laser at 1.8 pm. This cost-effective pulsed laser is expected to be promising for deep-brain multiphoton imaging at the third near infrared optical tissue window.
This book details the application of advanced characterisation techniques and diagnostic tools to heritage science, including the evaluation of heritage assets’ condition, their preservation and restoration.
It examines the use of electrochemical techniques in conservation science, with a particular focus on how to solve problems in taking on-site measurements. Specifically, it introduces readers to a new gel polymer (GPE) electrochemical cell developed by the authors for the characterisation of metallic heritage objects. Other techniques used to characterise and monitor reinforced concrete objects in more modern buildings are also covered, including non-destructive electrochemical techniques that allow steel corrosion to be assessed in these structures, and in those that are used to protect and repair such buildings.
The usefulness of the NMR-Mouse nuclear magnetic resonance sensor in the assessment and preservation of softer heritage materials, such as wood, parchment, bone, and painted walls, is covered, as well as Infrared reflectography for examining paintings and laser cleaning for restoring them. The book introduces ultra-High Performance Liquid Chromatography (u-HPLC) with a diode-array (DAD) and mass–mass (MS-MS) quadruple time-of-flight spectroscopy (QTOF). This new technique can be applied to the analysis and identification of natural and synthetic organic pigments and its use is demonstrated in several case studies.
This book provides a rigorous scientific grounding in the application of state-of-the-art techniques in heritage science and conservation, and offers a practical handbook for practitioners.
A Novel OCT Design for Cultural Heritage Applications - Volume 24 Supplement - Danielle Duggins, Fengqiang Li, Maurice Aalders, Oliver Cossairt, Aggelos Katsaggelos, Marc Walton
The performance of ultrafast time-stretch imaging at long wavelengths (beyond 1.5 μm) has suffered from low detection sensitivity due to the increasing loss of optical dispersive fibers. Here, we report an ultrafast optical imaging system with a line scan rate of at the 2.0-μm wavelength window by combining second-harmonic generation (SHG) with the highly sensitive time-stretch detection at 1.0 μm. In this imaging system, the sample is illuminated by the pulsed laser source at 2.0 μm in the spectrally encoding manner. After SHG, the encoded spectral signal at 2.0 μm is converted to 1.0 μm and then mapped to the time domain through a highly dispersive fiber at 1.0 μm, which provides a superior dispersion-to-loss ratio of , larger than that of the standard fibers at 2.0 μm (typically ). These efforts make it possible for time-stretch technology not only being translated to longer wavelengths, where unique optical absorption contrast exists, but also benefitting from the high detection sensitivity at shorter wavelengths.
In this paper, by means of a well-established parallelism between MWP and LCI, a novel MWP-LCI technique is proposed and experimentally evaluated. This technique is based on the slicing of a broadband source which is transmitted through a dispersive element. The optical path differences (OPD) under test modifies the electrical transfer function of the MWP-LCI subsystem through a interferometric structure. The OPD range is increased due to the right allocation of the interferometric structure and extended close to zero for removing the basedband component due to the avoidance of the Carrier Suppression Effect (CSE) and the incorporation of a balanced detection, respectively. In this sense, an OPD range of more than 1.4 cm has been experimentally achieved. In the whole range, a considerable improvement of sensitivity has been achieved in comparison to previous contributions. For low OPD values, the improvement is around 6 dB, but more importantly, for high OPD values, a sensitivity improvement larger than 20 dB is achieved. On the other hand, it is also demonstrated that the proposed scheme maintains the resolution through the complete measurement range. These key parameters only depends on the operation bandwidth of the RF and electrooptical components.
We experimentally observe dissipative soliton resonance in a thulium-doped fiber laser operating in the anomalous dispersion regime. The laser output exhibits broad quasi-Gaussian spectra (~38 nm) centered at 1970 nm. The system is subsequently applied to spectrally encoded confocal microscopy.
We present results from an all-fibre thulium laser system that can be tuned to any wavelength between 1710 – 2110 nm, without using any moving mechanical parts. An Acousto-Optic Tunable Filter (AOTF) is used as the tuning element, which allows for the wavelength to be tuned in ~ 20 μs. Core-pumped and cladding pumped thulium fibres are used to enable lasing action across the wavelength range. We use in-house fabricated fused fibre couplers and combiners that have a flattened coupling response with wavelength to allow for the system to be built in an all fibre design. These couplers have a coupling response that only varies by +/- 10% over the 400 nm operating range. The laser can output powers between 1-5 mW over 1710 – 2110 nm and has a linewidth of <0.2 nm. An Acousto-optic modulator is used as a switch on the output of the laser to switch the signal between core-pumped and cladding-pumped amplifier stages. This allows for the output signals to be amplified to ~1W levels.
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).
This work presents the results of an application of optical coherence tomography (OCT) to examine a 13th century Byzantine reliquary of unparalleled artistic and historical value. The aim of this work, performed at the initial stage, before the restoration, was focused on the resolution of cleaning procedures regarding both the thick, old varnish and the gold leaf details finely applied on the painted parts of the artwork by means of an integrated approach of non-invasive and invasive analyses and diagnostics.
A holistic approach using non-invasive multimodal imaging and spectroscopic
techniques to study the materials (pigments, drawing materials and paper) and
painting techniques of watercolour paintings is presented. The non-invasive
imaging and spectroscopic techniques include VIS-NIR reflectance spectroscopy
and multispectral imaging, micro-Raman spectroscopy, X-ray fluorescence
spectroscopy (XRF) and optical coherence tomography (OCT). The three
spectroscopic techniques complement each other in pigment identification.
Multispectral imaging (near infrared bands), OCT and micro-Raman complement
each other in the visualisation and identification of the drawing material. OCT
probes the microstructure and light scattering properties of the substrate
while XRF detects the elemental composition that indicates the sizing methods
and the filler content. The multiple techniques were applied in a study of
forty six 19th century Chinese export watercolours from the Victoria & Albert
Museum (V&A) and the Royal Horticultural Society (RHS) to examine to what
extent the non-invasive analysis techniques employed complement each other and
how much useful information about the paintings can be extracted to address art
conservation and history questions. A micro-destructive technique of microfade
spectrometry was used to assess the vulnerability of the paintings to light
exposure.
Full-field optical coherence microscopy (FF-OCM) is an established optical technology based on low-coherence interference microscopy for high-resolution non-invasive three-dimensional imaging of semi-transparent samples. We present an extension of the technique setting up an achromatic imaging system over a spectral range extending from 530 nm to 1700 nm, to provide tomographic images in three distinct bands centered at 635 nm, 870 nm and 1170 nm. Image contrast enhancement as well as sample characterization is performed using the conventional RGB color channel representation. Light is emitted by a halogen lamp and then separates into two arms of a Linnik-type interferometer with microscope objectives placed in each arm. The images are projected onto a visible to short-wavelength infrared detector based on an InGaAs photodiode array. En-face oriented tomographic images are obtained by arithmetic combination of four phase-shifted interferometric images. Great care was taken to reach similar performances in the three bands. An axial resolution of similar to 1.9 mu m and a transverse resolution of similar to 2.4 mu m are achieved in the three bands. A dynamic dispersion compensation system is set up to preserve axial resolution and signal intensity level when the imaging depth is varied. Images of biological samples revealing their spectral properties are shown as illustration of improved detection capability with enhanced contrast.
A 220 nm bandwidth supercontinuum source in the two-micron wavelength range has been developed for use in a Fourier domain optical coherence tomography (FDOCT) system. This long wavelength source serves to enhance probing depth in highly scattering material with low water content. We present results confirming improved penetration depth in high opacity paint samples while achieving the high axial resolution needed to resolve individual paint layers. This is the first FDOCT developed in the 2 μm wavelength regime that allows fast, efficient capturing of 3D image cubes at a high axial resolution of 13 μm in air (or 9 μm in paint).
An optical coherence tomography system using a compact fiber source emitting amplified spontaneous emission at central wavelength of 1960 nm with bandwidth of 40 nm is developed to enhance the probing depth in a highly scattering material with low water content. Examples of application to paint are used to demonstrate significantly improved penetration depth in high opacity materials in the 2-μm wavelength regime.
Multispectral imaging has been applied to the field of art conservation and art history since the early 1990s. It is attractive as a non-invasive imaging technique because it is fast and hence capable of imaging large areas of an object giving both spatial and spectral information. This paper gives an overview of the different instrumental designs, image processing techniques and various applications of multispectral and hyperspectral imaging to art conservation, art history and archaeology. Recent advances in the development of remote and versatile multispectral and hyperspectral imaging as well as techniques in pigment identification will be presented. Future prospects including combination of spectral imaging with other non-invasive imaging and analytical techniques will be discussed.
A brief introduction to Optical Coherence Tomography (OCT) is presented, stressing the origin of the tomographic signal and the detection methods defining various modalities of the technique. The parameters of the tomographs, such as axial and lateral resolution, wavelength and intensity of the probing light, imaging range, time of examination, and sensitivity are then defined, and a paradigm for interpreting the OCT tomograms provided. The second part of the article comprises a review of the utilisation of OCT for structural examination of artworks, illustrated with some representative results. Applications to the structural imaging of semi-transparent subsurface layers such as varnishes and glazes, of underdrawings and of reverse painting on glass, are described first, and then applications in the examination of the structure and state of preservation of historic glass, jade, glazed porcelain and faience are discussed. Finally, the use of OCT combined with LIBS analysis and laser ablation of surface layers is presented.
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.
One of the present challenges in optical coherence tomography (OCT) is the visualization of deeper structural morphology in biological tissues. Owing to a reduced scattering, a larger imaging depth can be achieved by using longer wavelengths. In this work, we analyze the OCT imaging depth at wavelengths around 1300 nm and 1600 nm by comparing the scattering coefficient and OCT imaging depth for a range of Intralipid concentrations at constant water content. We observe an enhanced OCT imaging depth for 1600 nm compared to 1300 nm for Intralipid concentrations larger than 4 vol.%. For higher Intralipid concentrations, the imaging depth enhancement reaches 30%. The ratio of scattering coefficients at the two wavelengths is constant over a large range of scattering coefficients and corresponds to a scattering power of 2.8 ± 0.1. Based on our results we expect for biological tissues an increase of the OCT imaging depth at 1600 nm compared to 1300 nm for samples with high scattering power and low water content.
IR camera systems can be used for reflectographic analyses to detect and visualize underdrawings in paintings. Five technologies have been compared: CCD Si, FPA InGaAs, FPA HgCdTe and InSb detectors, and a vidicon tube. The comparison has been performed studying the relative contrast measured on homogeneous pictorial layers in infrared regions between 800 and 5000 nm. This comparison was completed through measurements of spectral transmittance (400–2500 nm) made with a spectrometer. The results have been related to the kind of pigment and of medium and to the thickness of the layer. All the systems have been tested also on the field, on paintings belonging to different museums. Some results are discussed.
Optical coherence tomography (OCT) has mostly been used for high-speed volume imaging but its profilometry potentials have not been fully exploited. This paper demonstrates high precision (as good as ∼ 50 nm ) multi-interface profilometry using a Fourier domain OCT system without special antivibration devices. The precision is up to 2 orders of magnitude better than the depth resolution of the OCT. Detailed analysis of the precision achieved for different surfaces is presented. The multi-interface profiles are obtained as a by-product of the tomography data. OCT has the advantage in speed and sensitivity at detecting rough and internal interfaces versus conventional optical profilometry. An application of the technique to the dynamic monitoring of varnish drying on paintlike substrates is demonstrated, which provides a better understanding of the formation of surface roughness. The technique has potential benefits in the fields of art conservation, coatings technology, and soft matter physics.
In the past decade we have observed a rapid development of ultrahigh-speed optical coherence tomography (OCT) instruments, which currently enable performing cross-sectional in vivo imaging of biological samples with speeds of more than 100,000 A-scans/s. This progress in OCT technology has been achieved by the development of Fourier-domain detection techniques. Introduction of high-speed imaging capabilities lifts the primary limitation of early OCT technology by giving access to in vivo three-dimensional volumetric reconstructions on large scales within reasonable time constraints. As result, novel tools can be created that add new perspective for existing OCT applications and open new fields of research in biomedical imaging. Especially promising is the capability of performing functional imaging, which shows a potential to enable the differentiation of tissue pathologies via metabolic properties or functional responses. In this contribution the fundamental limitations and advantages of time-domain and Fourier-domain interferometric detection methods are discussed. Additionally the progress of high-speed OCT instruments and their impact on imaging applications is reviewed. Finally new perspectives on functional imaging with the use of state-of-the-art high-speed OCT technology are demonstrated.
The capability of optical coherence tomography (OCT) to perform "optical biopsy" of tissues within a depth range of 1 to 2 mm with micron-scale resolution in real time makes it a promising biomedical imaging modality for dermatologic applications. Three high-speed, spectrometer-based frequency-domain OCT systems operating at 800 nm (20,000 A-scans/s), 1060 nm, and 1300 nm (both 47,000 A-scans/s) at comparable signal-to-noise ratio (SNR), SNR roll-off with scanning depth, and transverse resolution (<15 microm) were used to acquire 3-D tomograms of glabrous and hairy human skin in vivo. Images obtained using these three systems were compared in terms of penetration depth, resolution, and contrast. Normal as well as abnormal sites like moles and scar tissue were examined. In this preliminary study, skin pigmentation had little effect on penetration accomplished at three different wavelengths. The epidermis and dermal-epidermal junction could be properly delineated using OCT at 800 nm, and this wavelength offered better contrast over the other two wavelength regions. OCT at 1300 nm permits imaging of deeper dermal layers, critical for detecting deeper tumor boundaries and other deeper skin pathologies. The performance at 1060 nm compromises between the other wavelengths in terms of penetration depth and image contrast.
A method has been developed to detect underdrawings in paintings more completely than is possible with ir photography. A modified Barnes ir camera has been used equipped with a lead sulfide detector. The results can be explained in terms of an increase in hiding thickness of paint layers with the wavelength. Practical aspects of reflectography of paintings are briefly described and an example is given.
Gold punchwork and underdrawing in Renaissance panel paintings are analyzed using both three-dimensional swept source/Fourier domain optical coherence tomography (3D-OCT) and high resolution digital photography. 3D-OCT can generate en face images with micrometer-scale resolutions at arbitrary sectioning depths, rejecting out-of-plane light by coherence gating. Therefore 3D-OCT is well suited for analyzing artwork where a surface layer obscures details of interest. 3D-OCT also enables cross-sectional imaging and quantitative measurement of 3D features such as punch depth, which is beneficial for analyzing the tools and techniques used to create works of art. High volumetric imaging speeds are enabled by the use of a Fourier domain mode locked (FDML) laser as the 3D-OCT light source. High resolution infrared (IR) digital photography is shown to be particularly useful for the analysis of underdrawing, where the materials used for the underdrawing and paint layers have significantly different IR absorption properties. In general, 3D-OCT provides a more flexible and comprehensive analysis of artwork than high resolution photography, but also requires more complex instrumentation and data analysis.
Optical Coherence Tomography (OCT) is an optical interferometric technique developed mainly for in vivo imaging of the eye and biological tissues. In this paper, we demonstrate the potential of OCT for non-invasive examination of museum paintings. Two en-face scanning OCT systems operating at 850 nm and 1300 nm were used to produce B-scan and C-scan images at typical working distances of 2 cm. The 3D images produced by the OCT systems show not only the structure of the varnish layer but also the paint layers and underdrawings (preparatory drawings under the paint layers). The highest ever resolution and dynamic range images of underdrawings are presented and for the first time it is possible to find out non-invasively on which layer the underdrawings were drawn.
The influence of depth dependent dispersion by the main component of biological tissues, water, on the resolution of OCT was studied. Investigations showed that it was possible to eliminate the influence of depth dependent dispersion by water in tissue by choosing a light source with a center wavelength near 1.0 µm. Ultrahigh resolution ophthalmic imaging was performed at this wavelength range with a microstructure fiber light source.
The description of optical properties of light-scattering materials has made extensive use of radiative transfer models. One of the most successful and simplest models is that of Kubelka and Munk (KM). With this model, optical properties of particulate films under diffuse illumination can be predicted from effective absorption and scattering coefficients of the material. We consider the applicability conditions of this kind of model. An extended KM model for the case of perpendicular collimated illumination is compared with results from a more general four-flux approach, and the differences between them are characterized in terms of a correction factor that depends on particle scattering and absorption, concentration of the scatterers, and film thickness. It is proved formally that the extended KM model under perpendicular illumination is a good approximation for the cases of optically thick films that contain weakly or nonabsorbing particles.
Optical coherence tomography (OCT) is the optical analog of ultrasound imaging and is emerging as a powerful imaging technique that enables non-invasive, in vivo, high resolution, cross-sectional imaging in biological tissue. A new generation OCT technology has now been developed, representing a quantum leap in resolution and speed, achieving in vivo optical biopsy, i.e. the visualization of tissue architectural morphology in situ and in real time. Functional extensions of OCT technology enable non-invasive, depth resolved functional assessment and imaging of tissue. These new techniques should not only improve image contrast, but should also enable the differentiation of pathologies via metabolic properties or functional state. The book introduces OCT technology and applications not only from an optical and technological viewpoint, but also from biomedical and clinical perspectives. The chapters are written by leading international research groups, in a style comprehensible to a broad audience. It will be of interest not only to physicists, scientists and engineers, but also to biomedical and clinical researchers from different medical specialties.
Infra-red reflectography, a method of improving the detect ability of underdrawings in medieval paintings, is discussed. It is shown that the results can be explained with the Kubelka-Munk analysis of the optical properties of paint films. Measurements are described providing a plot of the paint layer thickness required to hide an underdrawing, against the wavelength. Such curves show a maximum around λ = 2.0 microns. Infra-red to visible image translation systems are surveyed to illustrate the choice of an infra-red vidicon television system to obtain reflectograms. This system is responsive to radiation up to 1.9 microns. The interpretation of infra-red reflectograms is briefly discussed. Limitations of the method are indicated.
A method is described for the evaluation of the performance of near-infrared imaging devices to detect underdrawing and reworking of paintings. System analyses have been performed on two different near-infrared imaging devices: a Hamamatsu lead sulphide vidicon camera and a Photometries silicon charge coupled device (CCD) camera. The analyses included modelling of the system spectral response curve by analysis of the system components, as well as considering the limiting resolution and modulation transfer function (MTF). The modelling was confirmed by examination of a test panel at various wavelengths. Calculation of visibility is introduced as a quantitative measure of system performance. This calculation is also discussed as a means to define more clearly the actual performance requirements of these systems. This analysis has revealed the strengths and weaknesses of current systems, and provides guidance in the construction of improved devices.
A new camera - SIRIS (scanning infrared imaging system) - developed at the National Gallery in London, UK allows highresolution images to be made in the near infrared region (900-1700 nm). The camera is based on a commercially available 320 × 256 pixel indium gallium arsenide area array sensor. This relatively small sensor is moved across the focal plane of the camera using two orthogonal translation stages to give images of c. 5000 × 5000 pixels. The main advantages of the SIRIS camera over scanning infrared devices or sequential image capture and mosaic assembly are its comparative portability and rapid image acquisition - making a 5000 × 5000 pixel image takes less than 20 minutes. The SIRIS camera can operate at a range of resolutions, from around 2.5 pixels per millimetre over an area of up to 2 × 2 m to 10 pixels per millimetre when examining an area measuring 0.5 × 0.5 m. The development of the mechanical, optical and electronic components of the camera, including the design of a new lens, is described. The software used to control image capture and to assemble the individual frames into a seamless mosaic image is mentioned. The camera was designed primarily to examine underdrawings in paintings; preliminary results from test targets and paintings imaged in situ are presented and the quality of the images compared with those from other cameras currently used for this application.
The application of Fourier domain optical coherence tomography (FDOCT), a new implementation of incoherent light interferometry, to the examination of museum objects is described. The technique has been applied to the examination of porcelain and faience ceramics, and to the investigation of paintings. Time-resolved optical coherence tomography was used to study the surface profile of a painting during changes in the environment surrounding the object. The method provides precise section and surface profiles with micrometer resolution, and is complementary to other optical methods.
Optical coherence tomography (OCT), a method for depth-resolved imaging
within turbid media based on the concept of low-coherence
interferometry, rapidly evolved in the recent years with the development
of a multitude of new functionalities and modalities. Biomedical
research and diagnostics have been up to now the main driving forces for
the reported applications and progress in OCT. The characteristics of
OCT, precisely the ability to provide high-resolution images in a
contact-free way, render this technique also attractive for a broad
spectrum of research topics and applications outside the biomedical
field. Consequently, a variety of novel applications for OCT and
developments for the method itself have started to emerge. In this
review we will give a detailed overview of the so far presented
OCT-based methods and applications, ranging from dimensional metrology,
material research and non-destructive testing, over art diagnostics,
botany, microfluidics to data storage and security applications, and
include new data from a study on penetration depths in various polymer
materials as well as on birefringence imaging of different crystalline
polymer structures. Finally, advanced and related OCT techniques are
presented with high potential for future applications outside the
biomedical field.
The eye is essentially transparent, transmitting light with only minimal optical attenuation and scattering, providing easy
optical access to the anterior segment as well as the retina. For this reason, ophthalmic and especially retinal imaging has
been not only the first, but also most successful clinical application for optical coherence tomography (OCT). This chapter
focuses on the development of OCT technology for retinal imaging. OCT has significantly improved the potential for early diagnosis,
understanding of retinal disease pathogenesis as well as monitoring disease progression and response to therapy. Development
of ultrabroad bandwidth light sources and high speed detection techniques have enabled significant improvements in ophthalmic
OCT imaging performance, demonstrating the potential of three-dimensional, ultrahigh resolution OCT (3D UHR OCT) to perform
noninvasive optical biopsy of the living human retina, i.e., the in vivo visualization of microstructural, intraretinal morphology in situ approaching the resolution of conventional histopathology.
Significant improvements in axial resolution and speed not only enable three-dimensional rendering of retinal volumes, but also high definition, 2D tomograms, topographic
thickness maps of all major intraretinal layers as well as volumetric quantification of pathologic intraretinal changes. These
advances in OCT technology have also been successfully applied in several animal models of retinal pathologies. The development
of light sources emitting at alternative wavelengths, e.g., around ∼l,050 nm, not only enabled three-dimensional OCT imaging
with enhanced choroidal visualization, but also improved OCT performance in cataract patients because of reduced scattering
losses in this wavelength region. Adaptive optics using deformable mirror technology, with unique high stroke to correct higher
order ocular aberrations, with specially designed optics to compensate chromatic aberration of the human eye, in combination
with 3D UHR OCT, recently enabled in vivo cellular resolution retinal imaging.
We demonstrate the potential of optical coherence-domain tomography (OCT) for noninvasive imaging of living skin simultaneously at two wavelengths in the near infrared range (830 and 1285 nm). The technical details of a prototype monomode fiber-optic coherence tomographic scanner providing rapid 2D and D imaging of biological tissues are described. The effects of both instrumentation parameters and the dynamic characteristics of living tissue on image contrast and resolution and on speckle reduction are discussed. The impact of imaging speed on OCT image quality is studied by a comparison between a single scan and the corresponding frame-averaged OCT images, with the latter resulting in decreased speckle noise as well as loss of some subtle structures. Both theoretical predictions and experimental results in human skin imaging show that longer wavelength can minimize the influence of multiple scattering on image contrast and resolution and thus increase the effective penetration depth of OCT imaging to about 2 mm. Some high- resolution 2D and 3D images of microscopic anatomic structures of living human skin are presented and analyzed, illustrating the unique capability of OCT for in depth, noninvasive visualization of living skin microscopic morphology in vivo.
The main objective of our research is the development of totally non-destructive methodologies for the investigation of art works. We propose the use of fiber optic reflectance spectroscopy (FORS) in the visible and near-infrared region as a tool for the identification of the pigments used in painting. We have considered several representative inorganic artists' pigments (mainly till the Renaissance period); powder X-ray diffractograms and diffuse reflectance spectra in the visible, near-, middle- and far-infrared range were recorded for pure pigments. Then we prepared suitable samples with the same pigments using fresco, tempera and oil techniques. The visible and near-infrared spectra of these samples were recorded using an optical fiber spectrum analyzer; color analysis (chromaticity, dominant wavelength and purity) was also performed. The obtained results were used in the interpretation of the spectra obtained from some paintings collected in the Uffizi Gallery, Florence (Giotto and Luca Signorelli). The pointwise information obtained by FORS analysis was also used to calibrate and tune an image spectroscopy system, based on sequences of band-pass filtered images in the near infrared range.
Nondestructive techniques have seen successful growth in the last few years, and, among them, optical ones are widespread
and extremely well received in the field of painting diagnostics because of their effectiveness and safety. At present, many
techniques for nondestructive investigations of paintings are available; nevertheless, none of them is suitable for a quantitative
characterization of varnish. However, varnish removal, either partial or complete, is a fundamental part of the cleaning process,
which is an essential step in painting conservation. This critical process has been carried out, up to now, without the possibility
of any non-destructive measurement for assessing the actual varnish thickness, but with microscopic observation of a detached
microfragment. Optical coherence tomography (OCT) is a noninvasive technique that is well established for biomedical applications.
In this work, we present a novel application of OCT to measure the varnish film thickness for painting diagnostics.
The system of differential equations of Kubelka-Munk, -di= -(S+K)idx+Sjdx, dj= -(S+K)jdx+Sidx (i, J... intensities of the light traveling inside a planeparallel light-scattering specimen towards its unilluminated and its illuminated surface; x... distance from the unilluminated surface S, K... constants), has been derived from a simplified model of traveling of light in the material. Now, without.simplifying assumptions the following exact system is derived: -di= -½(S+k)uidx+½Svjdx, dj= -½(S+k)vjdx+½Suidx, (u ≡ ∫0π/2(∂i/i∂φ)(dφ/cosφ), υ ≡ ∫0π/2(∂j/j∂φ)(dφ/cosφ) φ≡angle from normal of the light). Both systems become identical when u = υ = 2, that is, for instance, when the material is perfectly dull and when the light, is perfectly diffused or if it is parallel and hits the specimen under an angle of 60° from normal. Consequently, the different formulas Kubelka-Munk got by integration of their differential equations are exact when these conditions are fulfilled. The Gurevic and Judd formulas, although deriv d in another way by their authors, may be got from the Kubelka-Munk differential equations too. Consequently, they are exact under the same conditions. The integrated equations may be adapted for practical use by introducing hyperbolic functions and the secondary constants a=½(1/R∞+R∞) and b=½(1/R∞-R∞), (R∞≡reflectivity). Reflectance R, for instance, is then represented by the formula [equation] (Rg≡ reflectance of the backing, X = thickness of the specimen) and transmittance T by the formula [equation] In many practical cases the exact formulas may be replaced by appropriated approximations.
The effectiveness of an optical coherence tomography (OCT) system depends largely on the light source chosen. Published data on the optical properties of tissues are used to quantify the exponential attenuation of broadband light on transport through tissue. The effective attenuation coefficient is taken to be the sum of the absorption and scattering coefficients. This is used to demonstrate the effect on the spectra of a wide range of published OCT sources and the change in system resolution induced, and hence to comment on the suitability of different sources for OCT. The tissues studied include skin dermis, liver, and gallbladder. Sources at higher wavelengths are shown to be capable of high-resolution OCT imaging at greater depths. Titanium:sapphire lasers would be most suited for high-resolution OCT over comparatively shallow depths into tissue. For lower-resolution applications of OCT, a semiconductor optical amplifier and ytterbium fiber sources have better powers and bandwidths than superluminescent diodes. The resolution of OCT systems is not reduced significantly with imaging depth.
This paper addresses fundamental issues that underlie the interpretation of images acquired from turbid tissues by optical-coherence tomography (OCT). The attenuation and backscattering properties of freshly excised rat arteries and their dependence on the focusing and collection optics of the OCT system were measured at two wavelengths in the near infrared (830 nm and 1300 nm). Determined from the ratio of the magnitudes of the reflections from glass plates placed on both sides of the arteries, the mean attenuation coefficient of the arterial wall was found to be in the range 14 < microt < 22 mm(-1) at 830 nm and 11 < microt < 20 mm(-1) at 1300 nm. The measured values of microt were lowest for the longer source wavelength and for probe beams with the smallest average diameters. The observed dependence of microt on beam size indicates that relatively large-scale variations in the index of refraction of the tissue contributed to degradation of the tranverse spatial coherence of the beam. We introduce a framework for understanding and quantifying beam-size effects by way of the mutual-coherence function. The fact that spatial variations in backscattering and attenuation (which includes spatial-coherence losses) have similar effects on OCT signals makes the origin of the signals difficult to determine. Evidence is given that suggests that, in spite of this difficulty, certain features of microstructures embedded several hundred micrometres deep in a turbid tissue can still be detected and characterized.
Optical coherence tomography (OCT) is a powerful, noninvasive biomedical technique that uses low-coherence light sources to obtain in-depth scans of biological tissues. We report results obtained with three different sources emitting at 1570, 1330, and 810 nm, respectively. Attenuation and backscattering measurements are obtained with these sources for several in vitro biological tissues. From these measurements, we use a graphical method to make comparisons of the penetration depth and backscattering intensity of each wavelength for the studied samples. The influence of the coherence length of each source is also taken into account in order to make a more relevant comparison.
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