Enrico Pampaloni

Scanning Multispectral IR Reflectography SMIRR: An Advanced Tool for Art Diagnostics

Spectral imaging technology, widely used in remote sensing applications, such as satellite or radar imaging, has recently gained importance in the field of artwork conservation. In particular, multispectral imaging in the near-infrared region (NIR) has proved useful in analyzing ancient paintings because of the transparency of most pigments and their varied reflectance changes over this spectral region. A variety of systems, with different detectors and filtering or dispersing technologies, have been implemented. Despite the recognized potential of multispectral NIR imaging, which provides information on both spectral and spatial domains (thus extending the capabilities of conventional imaging and spectroscopy), most of the systems currently used in art diagnostics have limitations. The technology is still in its early stages of development in this field. In this Account, we present the scanning multispectral IR reflectography (SMIRR) technique for artwork analysis, together with an integrated device for the acquisition of imaging data. The instrument prototype is a no-contact optical scanner with a single-point measurement of the reflectance, capable of simultaneously collecting a set of 14 spatially registered images at different wavelengths in the NIR range of 800-2300 nm. The data can be analyzed as a spectral cube, both as a stack of wavelength resolved images (multi-NIR reflectography) and as a series of point reflectance spectra, one for each sampled pixel on the surface (NIR spectrometry). We explore the potential of SMIRR in the analysis of ancient paintings and show its advantages over the wide-band conventional method. The multispectral option allows the choice of the most effective NIR bands and improves the ability to detect hidden features. The interband comparison aids in localizing areas of different pictorial materials with particular NIR reflectance. In addition to the analysis of single monochromatic images, enhancement procedures involving the joint processing of multispectral planes, such as subtraction and ratio methods, false color representation, and statistical tools such as principal component analysis, are applied to the registered image dataset for extracting additional information. Maintaining a visual approach in the data analysis allows this tool to be used by museum staff, the actual end-users. We also present some applications of the technique to the study of Italian masterpieces, discussing interesting preliminary results. The spectral sensitivity of the detection system, the quality of focusing and uniformity of the acquired images, and the possibility for selective imaging in NIR bands in a registered dataset make SMIRR an exceptional tool for nondestructive inspection of painting surfaces. The high quality and detail of SMIRR data underscore the potential for further development in this field.

Three-dimensional modelling of statues: the Minerva of Arezzo

Abstract The Minerva of Arezzo is an ancient bronze statue located at the Museo Archeologico in Florence and currently under repair at the Restoration Centre of the Soprintendenza Archeologica of the Tuscany Region. We assembled a complete three-dimensional (3D) digital model of the Minerva before the restoration started. More 3D models will be produced to keep track of the variations that occurred during the restoration process, up to the final acquisition of the form of the restored artwork. The modelling of the Minerva will be the focal point of an ambitious “Minerva Project” that involves the integration of data from other sources in a 3D digital model of the object. Besides this, the project is aimed at showing how 3D techniques can be used to design useful and easily manageable new tools for the diagnostics of archaeological objects. 3D measurements have been realized by means of a high-resolution laser scanner developed at National Institute for Applied Optics (INOA). The instrument is composed of commercial low-cost components in order to be competitive with the very expensive commercial devices. Besides this, our scanner is supported by an efficient and flexible software developed by Consiglio Nazionale delle Ricerche (CNR) that supports all the post-processing phases of a 3D scanning session (range data alignment, merge and simplification).

New high-resolution IR-color reflectography scanner for painting diagnosis

Infrared reflectography is a prominent optical technique for non-destructive diagnostics of paintings, which allows the visualisation of details hidden by the paint layers, because of their transparency characteristics to IR radiation. High-resolution reflectography was introduced around the end of the 80s by the Istituto Nazionale di Ottica Applicata, where a prototype of an innovative scanner device was developed. This technique was recently improved with the introduction of a new optical head, able to acquire simultaneously the reflectogram and the colour image, perfectly superimposing. The technical characteristics of the IR-colour scanner guarantee: a high spatial resolution (16 points/mm2), a high tonal dynamics (thousands of grey levels), a uniform lighting of the scanned area and the punctual superimposition of the colour and IR images. Moreover we can print distortion-free reflectograms, false-colour and colour images of paintings on a 1:1 scale. The quality of the acquired reflectogram is presently higher than that obtainable with any traditional detection system, like CCD or Vidicon cameras. The point-by-point comparison between the reflectogram and the colour image of the painting, along with digital processing of the recorded images, open new possibilities for the analysis of the reflectogram. Some examples of application to the study of ancient paintings are shown.

High-resolution 3D digital models of artworks

The measurement of the shape of an artwork usually requires a high-resolution instrumentation, in order to catch small details such as chisel marks, sculptural relieves, surface cracks, etc. 3D scanning techniques, together with new modeling software tools, allow a high fidelity reproduction of an artwork: these can be applied either to support and document its repair or for the realization of 3D archives and virtual museums. Starting from a high-resolution digital model of an object, a further step could be its reproduction by means of fast-prototyping techniques like stereo-lithography or electro-erosion. This work is aimed at showing the performance of a high-resolution laser scanner devoted to Cultural Heritage applications. The device is portable and very versatile, in order to allow in situ applications, accurate and reliable, so to capture intricate details. This laser profilometer has been used in a few surveys, the most significant of which are the monitoring the various phases of the restoration process of an ellenistic bronze (the Minerva of Arezzo, Florence), the cataloguing of some archaeological findings (from the Grotta della Poesia, Lecce) and the documenting of wooden panels surface conditions (the “Madonna del Cardellino” by Raffaello and “La Tebaide” by Beato Angelico).

High-precision surface analysis of the roughness of Michelangelo's David

The knowledge of the shape of an artwork is an important element for its study and conservation. When dealing with a statue, roughness measurement is a very useful contribution to document its surface conditions, to assess either changes due to restoration intervention or surface decays due to wearing agents, and to monitor its time-evolution in terms of shape variations. In this work we present the preliminary results of the statistical analysis carried out on acquired data relative to six areas of the Michelangelo’s David marble statue, representative of differently degraded surfaces. Determination of the roughness and its relative characteristic wavelength is shown.

Optical micro-profilometry for archaeology

A quantitative morphological analysis of archaeological objects represents an important element for historical evaluations, artistic studies and conservation projects. At present, a variety of contact instruments for high-resolution surface survey is available on the market, but because of their invasivity they are not well received in the field of artwork conservation. On the contrary, optical testing techniques have seen a successful growth in last few years due to their effectiveness and safety. In this work we present a few examples of application of high-resolution 3D techniques for the survey of archaeological objects. Measurements were carried out by means of an optical micro-profilometer composed of a commercial conoprobe mounted on a scanning device that allows a maximum sampled area of 280×280 mm2. Measurements as well as roughness calculations were carried out on selected areas, representative of the differently degraded surface, of an ellenestic bronze statue to document the surface corrosion before restoration intervention started. Two highly-corroded ancient coins and a limestone column were surveyed to enhance the relief of inscriptions and drawings for dating purposes. High-resolution 3D survey, beyond the faithful representation of objects, makes it possible to display the surface in an image format that can be processed by means of image processing software. The application of digital filters as well as rendering techniques easies the readability of the smallest details.

Multi-spectral IR reflectography

A variety of scientific investigation methods applied to paintings are, by now, an integral part of the repair process, both to plan the restoration intervention and to monitor its various phases. Optical techniques are widely diffused and extremely well received in the field of painting diagnostics because of their effectiveness and safety. Among them infrared reflectography is traditionally employed in non-destructive diagnostics of ancient paintings to reveal features underlying the pictorial layer thanks to transparency characteristics to NIR radiation of the materials composing the paints. High-resolution reflectography was introduced in the 90s at the Istituto Nazionale di Ottica Applicata, where a prototype of an innovative scanner was developed, working in the 900-1700 nm spectral range. This technique was recently improved with the introduction of an optical head, able to acquire simultaneously the reflectogram and the color image, perfectly superimposing. In this work we present a scanning device for multi-spectral IR reflectography, based on contact-less and single-point measurement of the reflectance of painted surfaces. The back-scattered radiation is focused on square-shaped fiber bundle that carries the light to an array of 14 photodiodes equipped with pass-band filters so to cover the NIR spectral range from 800 to 2500 nm

Integrating 2D and 3D data for diagnostics of panel paintings

Imaging techniques are widely used for the diagnostics of paintings. The results, collected by applying various imaging techniques are generally compared by the art historian who studies the object. These 2D data can be effectively integrated to form a multi-dimensional dataset, and added to a 3D digital model of the painting, thus creating a complete package of information about the opera. In this work we present some examples of applications of data integration with measurements performed on some important panel paintings. The imaging analysis was carried out by applying techniques such as infrared reflectography, colour and false colour imaging and ultraviolet fluorescence. The IR reflectogram, the false colour and the colour image are simultaneously acquired with a scanning device characterized by a high resolution (16 dots/mm2), a high tonal dynamic and point-to-point correspondence between these three images. A multi-spectral device, based on a high-resolution CCD camera, is used for UV fluorescence. The 3D relief was obtained by means of scanning micro-profilometry with a quota resolution of about 1 micron. The integration of the 3D model with the results given by several imaging techniques greatly increases the information and it eases the analysis of the painting under investigation.

Optical and spectroscopic tools for evaluating Er:YAG laser removal of shellac varnish

Abstract We report on tests to remove naturally and artificially aged shellac varnish by laser and traditional chemical cleaning from the substrates of mural paintings. Optical tools were used for the evaluation of cleaning processes, in particular laser microprofilometry to assess the changes in the surface morphology and time-domain confocal optical coherence tomography (OCT) to evaluate varnish thickness. The cleaning assessment was integrated with molecular characterization provided by portable Fourier transform infrared spectroscopy operating in reflectance mode, and colorimetric measurements. This complete analytical approach led to optimized laser-based cleaning tests at 1.9 and 2.6 J/cm2 in the presence of isopropanol, which ensured controllable and safe partial removal of shellac varnish without alteration to the substrate morphology.

A 3D scanning device for architectural survey based on time-of-flight technology

This work is intended to show the results of a few architectural and archaeological surveys realized by means of a 3D scanning device, based on TOF (Time-Of-Flight) technology. The instrument was set up by the Art Diagnostic Group of the National Institute for Applied Optics (INOA) and it is composed by a high precision scanning system equipped with a commercial low-cost distance-meter. This device was projected in order to provide the following characteristics: reliability, good accuracy and compatibility with other systems and it is devoted to applications in Cultural Heritage field.