Chapter 3.2

3D Scanning of Artworks

Lica Pezzati, Raffaella Fontana

CNR – INOA, Istituto Nazionale di Ottica Applicata, Firenze, Italia

Method Non-destructive Information obtained Structural integrity Type of object Any (layered) structure Sample size/amount needed Size: several m2 Sampling type Surface/subsurface layers Portable/transportable version available Yes

Contents 3.2.1 Introduction 3.2.2 Single-Line Laser Scanning 3.2.2.1 Case Study 1: The Minerva of Arezzo 3.2.2.2 Case Study 2: Poesia Cave Inscriptions 3.2.3 Micro-Profilometry 3.2.3.1 Case Study 3: David 3.2.3.2 Case Study 4: Perugino’s Paintings 3.2.3.3 Case Study 5: Greek and Roman Coins 3.2.4 Conclusions 3.2.5 Supporting Information 3.2.5.1 Literature 3.2.5.2 Useful Websites 3.2.6 Contact Information

3.2.1 Introduction Following the lines discussed in the previous Chapter 3.1, in this section we extend the review of 3D scanning techniques currently adopted in the diagnostics of artworks to 3D scanning of objects: statues, paintings, archaeological findings, and other small- and medium-scale pieces of historic and cultural interest.

Optical techniques play an important role in the field of Cultural Heritage diagnostics, because of their safety and effectiveness. Among them, 3D scanning techniques have recently found the way to a variety of relevant applications. They are presently used to measure buildings, statues, inscriptions, coins, and even paintings, whose apparent “flatness” attracted little attention from the pioneers of 3D measuring techniques. The growing technological progress of laser-based devices opened a new era in 3D survey, enabling the design of highly accurate instruments for quota measurements and allowing for extremely dense data sampling at high acquisition rate. The relative ease of acquisition of 3D arrays of several million points fostered the study of problems related to the representation and use of digital models. The uses for these models range now from historical and artistic studies to the assembling of easy-access and long-lasting digital archives, as well as from the analysis of the conservation condition to the monitoring of the restoration treatments. Furthermore, the digital model constructed with 3D data can be used for virtual reality applications. The work of art can be enjoyed outside its habitual surroundings and the image can be placed in different contexts. Any change resulting from a proposed restoration treatment can be visualised (so-called computer aided restoration) as well as the proposed reconstruction of losses can be visually evaluated. Moreover, by measuring the shape over time it is possible to obtain data on the object alterations. This allows mechanical stresses to be located, the effects of microclimatic variations (temperature and/or humidity) to be quantified, surface degradation or the monitoring of shape variations introduced by the restoration process to be measured.

Different 3D scanning techniques are generally based on different principles and capture different and often complementary information. As described in Chapter 3.1, the most common techniques used for 3D measuring are the optical triangulation, embedded in most of 3D measuring system, and the time-of-flight, but other means, such as interferometric methods (and among these, holography), were and are used. As a reference, see in Fig. 3.2.1 a classification of the more common 3D measuring technique, adopting the schematic division between active and passive methods, where active means that the measured object must be actively illuminated, while passive methods work with ambient light.

Another useful classification is the one based on the object scale: not all these techniques can indeed be used for all possible objects, if optimal results are to be attained. For instance, where time-of-flight (TOF) methods are well-suited for large and distant objects, like architectures and terrains, fringe projection is very good for medium-scale semi-flat objects with smooth continuous surfaces, such as paintings and inscriptions, single-line laser scanning is ideal for medium-scale objects like statues and vases, and finally micro-profilometry is well suited for highly-detailed nearly-flat surfaces, either of small objects or of big object details.

In this Chapter we present two prototype instruments designed and assembled at INOA for medium-scale and small-scale 3D acquisition, and a variety of their applications to the diagnostics of Cultural Heritage objects. For a general introduction on the use of 3D measuring methods and 3D data processing, see at the previous Chapter 3.1.

3.2.2 Single-Line Laser Scanning A laser-line scanner is a device for 3D measurements based on optical triangulation through single-line laser projection. It combines resolution with versatility, making it particularly suitable for in situ applications both under difficult lighting conditions and in all environments. A custom laser-line scanner has been designed at INOA for scanning medium-scale objects.

The prototype (Fig. 3.2.2) is composed of a diode-laser line-projector emitting red light (⎣ = 670 nm) shaped to a light sheet by an optical element (prism) and a HRES CCD camera (1300 x 1030 pixel) placed at a certain, known, angle with respect to the laser source. The camera is equipped with a 16 mm lens and a band-pass filter, chosen to match the wavelength of the laser in order to eliminate problems related to ambient light. A motorized stage allows for scanning the objects and closely-spaced parallel profiles are then acquired.

The system was calibrated and corrections were applied to compensate for lens distortions. However, the accuracy of the image spot location, and hence the instrument depth accuracy, is degraded by laser speckle. With a stand-off distance of 50 cm and a scanned area of about 30 x 30 cm2, the quota resolution resulted to be 50 ⎧m, with an absolute error smaller than 0.3 mm.

3.2.2.1 Case Study 1: The Minerva of Arezzo The Minerva of Arezzo, shown in Fig. 3.2.3a, is a bronze statue presumably dated III century B.C. that was discovered in Arezzo in 1541. Normally at the Archaeological Museum in Florence, it is currently under repair at the Restoration Centre of the Soprintendenza Archeologica of the Tuscany Region. The lower part of the statue was entirely restored with wood and plaster, while the right arm, from shoulder, was integrated in 1785 and it is bronze made. The artwork is about 155 cm high and its weight is about 150 kg. The body is roughly a cylinder having a diameter of about 50 cm, with a protruding right arm, that extends outward for about 40 cm.

For the realization of the digital model of the statue, 119 separate measurements, or range maps, were acquired, each with a spatial density of about 16 points (vertex) per mm2, and each framing a different part of the statue surface. The range maps were then joined (registered) and merged in a single 3D model (mesh), that is shown Fig. 3.2.3b.

The 3D model of the Minerva stands as the only documentation of the statue’s form before the restoration process irreversibly changed it. For instance, during the statue’s repair the wooden inferior part as well as the right arm were removed, and the corroded surface has been smoothed by cleaning. The 3D model will be used either to reproduce and place exactly the missing parts, or to keep trace of possible further alterations of the shape.¸

The Minerva restoration of 1785 appears to be incorrect. The recovery circumstances are not well-known, as well as the original preservation conditions of the statue that most probably had suffered a previous restoration in the XV century. The pose of the right arm, that puts the Minerva on an oratorical move, probably imitates the one of the Arringatore, another major bronze statue of the Tuscan museum, together with the Chimera and the Idolino. A previous plaster repair, whose evidence is given by a printing dated the beginning of XVIII century, describes the right arm of the Minerva, goddess of war and wisdom, laid out along the trunk and the forearm up to hold a lance: this is a recurrent attribute for the warrior goddess, armed with the Corinthian helmet. The digital model allowed the realization of a software application capable to visualize the two hypothetical positions for the statue’s right arm (Fig. 3.2.5).

3.2.2.2 Case Study 2: Poesia Cave Inscriptions The Grotta della Poesia (literally: Cave of Lyrics) is an archaeological excavation near Roca Vecchia (Lecce, Italy) discovered in 1983. The shallow cave floor is nowadays totally flooded by sea water and the cave itself is accessible only through a hole in the collapsed vault, by means of a scaffolding. Its walls are covered by hundreds of pre-historical Messapic, Greek and Latin inscriptions of great archaeological interest, so it is very important to obtain a complete virtual copy of the cave and in particular a digital documentation of the carved surface.

The Cultural Heritage Department of the University of Lecce (Italy) is taking moulds of the wall inscriptions with silicone rubber. We acquired by laser scanning digital models of some of these for a preliminary study of the inscriptions, and of a possible documentation method. Fig. 3.2.6 shows two photos of the cave and a snapshot of the digital model of one mould.

Near the cave, in the extended archaeological site of Roca, many terracotta findings have also been recovered; they are currently kept in the Cultural Heritage Department of the University of Lecce. Fig. 3.2.7 shows a snapshot of the 3D digital model of one of these findings, an oil-lamp presumably dated III century B.C., found in a tomb in 1989. For the acquisition of the oil-lamp, approximately 7 x 4 x 3 cm3, 12 range maps were taken, each covering an area of about 7 x 30 cm2.

3.2.3 Micro-Profilometry A very accurate device, capable of measuring distances in the micrometric range, is necessary to reveal the finest details of an object surface. The prototype instrument realized at INOA for micro-profilometry makes use of a distance-meter device based on an interferometric technique (conoscopic holography). This method combines the capabilities of working on different materials, of measuring reflective and diffusive surfaces, and of being insensitive to colour changes. The probe, shown in Fig. 3.2.8 is a video camera coupled with a conoscopic module, consisting in a special optical crystal (uniaxial birefringent) sandwiched between two circular polarizers. A laser diode (λ = 680 nm) is focussed to a point on the object surface; the conoscopic module splits the light beam reflected from the investigated point, in two beams (the ordinary and the extraordinary beams), which due to an interference phenomenon produce a patterns of fringes, recorded by the video camera. The distance of the investigated point from the probe is then obtained by measuring the fringe spacing. The conoscopic micro-profilometer (Conoprobe, Optimet) used in the prototype is equipped with a 50 mm lens. This set up results in a dynamic range of 8 mm at a working distance of 40 mm. The out-of-plane resolution is about 1 μm, the accuracy is better than 6 μm, and the in-plane lateral resolution is about 20 ⎧m. The probe is mounted on two motorized linear stages, allowing a maximum scanning area of 280 x 280 mm2. The acquisition speed ranges from 100 to 400 points/sec, depending on the spatial sampling step and the acquisition area chosen.

This scanning micro-profilometer was applied on a variety of objects, ranging from panel paintings to details of statues, with different instrument configurations purposely designed. The methods can reveal details which are not visible at sight and often not detectable with other techniques.

Due to its very high resolution, the instrument stillness during measurements is crucial. Vibrations in the measurement environment can cause spurious details to appear in the acquired surface model.

High-density surface sampling with micrometric resolution, besides providing the users with an accurate reproduction of the surface micro-features, provides a data set that can be used for statistical calculations such as roughness computation. Roughness measurement of an artwork is important to document the surface condition, to assess either changes due to restoration or surface decay due to wearing agents and to monitor the time evolution of the shape.

3.2.3.1 Case Study 3: David Michelangelo’s David (Fig. 3.2.9a), exposed at the Museo dell’Accademia in Florence, was restored in 2002-2003. On this event, an extended diagnostic project was carried out to study and then to monitor the “health” of the sculpture, that had not been touched since 1873, when it was moved to the Accademia from Piazza della Signoria. The analysis of the surface roughness allows understanding how much the previous restorations or simply the ageing have compromised the marble surface of this masterpiece. Roughness measurements on the Michelangelo’s David were realized by means of the conoscopic micro-profilometer, and were an important item of the diagnostic project. Since the instrument is very sensitive to vibrations, induced both by the instrument motion and by the environment, a special solution was adopted for measuring on a 10 m high scaffolding (Fig. 3.2.9b). The micro-profilometer base was joined to the statue body (using felt pads for all the contact points) and held in place by a soft synthetic belt. The measurement started with a few seconds of delay to allow the operators to leave the scaffolding during the scanning time. The instrument was also modified on-site to reach all the areas to be measured: a special mechanical arm was realized to the purpose. The system at work during measurements is shown in Figs. 3.2.10 a-d. Six areas were sampled, representative of different surface conditions and expositions. In Fig. 3.2.11 images of these areas are shown, displaying different situations of roughness and corrosion.

Roughness measurements of statues is quite an unexplored field, due to the lack of measurement protocols or reference case studies. Roughness is defined in terms of deviations from a mean surface level and it is usually described by the RMS (Root Mean Square) value around it (Rq) and the wavelength (λq). Therefore Rq is a measure of the roughness amplitude and λq is a measure of its characteristic length. For roughness computation, we selected 1 x 1 cm2 locally flat areas. To correct for any minor shape variations (waviness) within the sample areas, for each acquired line a quadratic fit was performed and the result subtracted from the data. The RMS roughness value was computed for each acquired line. Its mean value and standard deviation were then finally calculated for each sampling area. The results are plotted in Fig. 3.2.12, together with the error bars representing the measurement variability range.

3.2.3.2 Case Study 4: Perugino’s Paintings The ‘Canaa weddings’ panel painting (Fig. 3.2.13a) is part of a ‘predella’ by Cosimo Vannucci, called il Perugino, from the Galleria Nazionale dell’Umbria in Perugia (Italy).

We performed a survey of the panel to analyse some peculiar carvings, apparently originated by toothed tools used in laying the preparation. Measurement results are shown in Figs. 3.2.13b and c. The sizes of two investigated areas are respectively 26 x 15 cm2 and 15 x 10 cm2 and the sampling density is 64 points/mm2. The three-dimensional model is displayed as a raking-light image. Parallel marks are visible as traces of toothed tools, having different depth and spacing. Fig. 3.2.14 shows the acquisition software interface. Numerical data can be obtained by the surface profiles: the maximum depth of the traces is 50 ⎧m and the spacing ranges from 0.5 to 1.5 mm.

The integration of the 3D relief with a colour image is shown in Figs. 3.2.13d and e for the two investigated areas. The integration of the colour information with the 3D model increases the readability of the data, thus making the analysis easier for the art historian or the restorer that is studying the object.

3.2.3.3 Case Study 5: Greek and Roman Coins

Application of 3D techniques to archaeology faces two main problems, often shared by other fields but particularly critical in this case: the first is the huge variety (dimensional, chromatic, of materials and shapes) of the objects, ranging from buildings to skeletons, from wreckages to statues, etc. The second is a complete lack of generally accepted measurement standards for innovative methods such as 3D surveys. Archaeological findings are usually documented by means of photographs and drawings, thus relying on the photographer and on the illustrator skill. These two methods are scarcely descriptive of the real shapes and sizes of objects, because they are based on plane representations. 3D modelling being an objective means for measuring and recording an object shape represents a big step up. The information contained in a 3D digital model largely surpass that stored in a photograph, so 3D techniques are very promising for archaeological documentation, and new standards for this must be studied.

We present an application of the optical micro-profilometry to two bronze coins, kept at the Cultural Heritage Department of the University of Lecce. Besides a high-resolution three-dimensional documentation, not achievable with any photographic method, this technique offers a means of data processing that increases the readability of eaten away surfaces.

The first coin (Figs. 3.2.15a and b), from Capua mint, is Greek and probably dates back to the beginning of III century B.C. The second one is a Roman coin (Figs. 3.2.15c and d) probably from the end of III sec. B.C. Range maps of 40 x 50 mm2 with 400 points/mm were acquired and snapshots from the 3D models were taken and processed, to emphasize the inscriptions on the coin surfaces.

In Fig. 3.2.16 a few examples of image processing are shown, where an embossing filter was used for obtaining raking light simulations. On the obverse side of the Greek coin (Fig. 3.2.16a) a human head is clearly visible: Apollo, according to the archaeologist’s opinion: On its reverse side a lyre and the inscription kapu are marked (Fig. 3.2.16b). Due to the highly worn out surface of the Roman coin, interpretation of the results was difficult. On the obverse side, the two-faced Janus head can be hardly seen (Fig. 3.2.16d), whereas on the other side a prow of a ship is probably present (Fig. 3.2.16e). The two images of the drawing of the coin type are also shown (Figs. 3.2.16c and f).

For the numismatist the primary source of knowledge of a coin history is contained in the inscriptions. The quantitative morphological characterization of coin surface and the subsequent possibility of image processing is, thus, a new approach that could completely change the way to document these findings.

3.2.4 Conclusions Non-invasive optical techniques are particularly suitable for Cultural Heritage diagnostic applications. In particular, 3D scanning of objects and of surfaces has recently found the way to a variety of relevant applications, because it allows fast measurements of the shape of artworks with both high accuracy and high resolution. 3D scanning techniques, together with new modelling software tools, allow a high fidelity reproduction of artworks that can be applied either to support and document its repair, or for the realization of 3D archives and virtual museums. Moreover, 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.

In this Chapter we presented two laser-scanner prototypes designed and assembled at INOA for medium-scale and small-scale 3D acquisition: a single-line laser scanner and a conoscopic micro-profilometer. The former has a scanning surface 30 x 30 cm2 wide with a stand-off distance of about 50 cm and a resulting depth resolution of 50 μm. Its hardware simplicity and daylight working capabilities, and, above all, the capability to acquire range maps of very complex 3D objects, make it useful for a variety of Cultural Heritage applications. The latter allows measurements on a maximun area of about 28 x 28 cm2 with a stand-off distance of about 4 cm, a maximum transversal resolution of 20 ⎧m and a 1 μm quota resolution. The high-density surface sampling with micrometric quota resolution, besides a high fidelity reproduction of the surface characteristics, provides a data set that can be used for statistical calculations.

A few applications of both devices to the diagnostics of Cultural Heritage objects were shown. 3D data sets were used either for creating digital models of the artworks, or for studying the finest surface characteristics by means of both image analysis and roughness computation.

3.2.5 Supporting Information

3.2.5.1 Literature J.F. Asmus: More light for art conservation. IEEE Circuit and Devices Magazine, March 1986 p.6. A. Cabrucci: La scansione laser nel rilevamento architettonico, Quaderni di restauro – Tecniche

di rilevamento nuove frontiere della tecnologia, ed. E.S.I, Napoli, 157/2001. C. Rocchini, P. Cignoni, C. Montani, P. Pingi, R. Scopigno: A Suite of Tools for the Management

of 3D scanned Data. Proceedings of the 2001 Workshop of Italy-Canada on 3D Digital Imaging and Modelling Applications of Heritage, Industry, Medicine & Land, Padua, Italy, 3-4 April 2001.

S.F. El-Hakim: A practical approach to creating precise and detailed 3D models from single and multiple views. Proceedings of the XIX ISPRS Congress, Amsterdam, Holland, July 16-23, 2000.

P. Cignoni, C. Montani, C. Rocchini, R. Scopigno: External Memory Management and Simplification of Huge Meshes, IEI-CNR Tech. Rep., Jan. 2001, pp.10.

B. Curless, M. Levoy: A volumetric method for building complex models from images. SIGGRAPH ’96 Proceedings, pp. 303-312, ACM SIGGRAPH, 1996.

M. Garland, P.S. Heckbert: Surface simplification using quadric error metrics. In: SIGGRAPH 97 Conference Proceedings, ACM SIGGRAPH, 1997, pages 209 –216.

M. Levoy, K. Pulli, B. Curless, S. Rusinkiewicz: The Digital Michelangelo Project: 3D scanning of large statues. Comp. Graph. Proc., Annual Conf. Series (Siggraph’ 00), pp. 131-144, Addison Wesley, July 24-28 2000.

H. Rushmeier, F. Bernardini, J. Mittleman, G. Taubin: Acquiring input for rendering at appropriate levels of detail: digitizing a Pieta. Eurographics Rendering Workshop 1998, G. Drettakis and N. Max (ed.), Springer Wien, 1998.

M. Gaiani, M. Balzani, F. Uccelli: Reshaping the Coliseum in Rome. Computer Graphics Forum, Vol. 19(3), pp 369-378, 2000.

R. Baribeau, M. Rioux: Influence of speckle on laser range finders. Appl. Opt. Vol. 30 No. 20, pp 2873-2878, July 1991.

R. Baribeau, M. Rioux, G. Godin: Recent Advances in the Use of a Laser Scanner in the Examination of Paintings. Restoration '92, V. Tood et al. (Editors), United Kingdom Institute for Conservation, United Kingdom, pp. 69-73, NRC 35040 (1992).

K. Pulli: Multiview registration for large datasets. Proc. Second Int. Conf. on 3D Digital Imaging and Modeling, Ottawa, Canada, October 1999, pp.160-168.

C. Rocchini, P. Cignoni, C. Montani, R. Scopigno: The Marching Intersections Algorithm for merging range images. IEI-CNR Tech. Rep., submitted paper, July 2000.

M. Garland, P. S. Heckbert: Surface simplification using quadric error metrics. SIGGRAPH 97 Conference Proceedings, ACM SIGGRAPH, 1997, pp. 209-216.

R. Fontana, M.C. Gambino, M. Greco, E. Pampaloni, L. Pezzati, R. Scopigno: High-resolution 3D digital models of artworks. Proc. SPIE. 2003. V. 5146. p. 34-43.

R. Fontana, M.C. Gambino, M. Greco, L. Marras, M. Materazzi, E. Pampaloni, L. Pezzati: The diagnostics of statues: a high-precision surface analysis of roughness of Michelangelo’s David. Proc. SPIE. 2003. V. 5146. p. 236-243.

R. Fontana, M.C. Gambino, M. Greco, L. Marras, M. Materazzi, E. Pampaloni, L. Pezzati, P. Poggi: Integrating 2D and 3D data for diagnostics of panel paintings. Proc. SPIE V. 5146, 88 (2003).

D.J. Whitehouse: Handbook of Surface Metrology. Institute of Physics Publishing. Bristol and Philadelphia.

UNI-ISO 468, Rugosità delle superfici – Parametri, loro valori e regole per la descrizione delle caratteristiche (1991).

G.Y. Sirat, D. Psaltis: Conoscopic holograms. Opt. Comm. 9(65), 243-245, 1988. G.Y. Sirat, D. Psaltis: Conoscopic holography. Optics Letters 10(1), 4-6, 1985. J. F. Asmus: Holograph interferometry of painted surfaces. Proc. Int. Cong. on ‘Conservation of

Wood, in Painting and the Decorative Arts’, Oxford, United Kingdom, 1978, Vol. IIC, 141-144.

D. Charlot et al.: Holographie conoscopique – reconstructions numeriques. Annales des Telecommunications 43 (1988) 23-26.

D. Bertani, M. Cetica, M. Melozzi, L. Pezzati: High-resolution optical topography applied to ancient paintings diagnostics. Optical Engineering 34/4 (1995) 1219-1225.

Y.Y. Hung: A new optical method for strain measurement and nondestructive testing. Opt. Eng. 21 (1982) 391–5.

C. Lahanier, G. Aitken, J. Taylor, F. Blais, L. Borgeat, J.-A. Beraldin, M. Rioux, B. Breuckmann, P. Colantoni, L. Cournoyer, C. De Deyne, M. Picard, R. Pillay, G. Godin: The 2D multi-spectral digitisation and the 3D modelling of easel paintings. to be presented, ICOM-CC, 14th Triennal Meeting ICOM-CC, 12-16 Sept. 2005, The Hague, The Netherlands.

J. Taylor, J.-A. Beraldin, G. Godin, L. Cournoyer, R. Baribeau, F. Blais, M. Rioux, J. Domey: NRC 3D Technology for Museum and Heritage Applications. The Journal of Visualization and Computer Animation 14/3 (2003).

3.2.5.2 Useful Websites CNR – INOA, Gruppo Beni Culturali: http://arte.ino.it

For more links see Chapter 3.1.

3.2.6 Contact Information Luca Pezzati CNR – INOA Istituto Nazionale di Ottica Applicata Largo E. Fermi 6 I - 50125 Firenze Italia E: luca@ino.it W: http://arte.ino.it Raffaella Fontana CNR – INOA Istituto Nazionale di Ottica Applicata Largo E. Fermi 6 I - 50125 Firenze Italia E: lella@ino.it W: http://arte.ino.it