Laser Scanners for High-Quality 3D and IR Imaging in Cultural Heritage Monitoring and Documentation

Journal of Imaging

Article Laser Scanners for High-Quality 3D and IR Imaging in Cultural Heritage Monitoring and Documentation

Soﬁa Ceccarelli 1, Massimiliano Guarneri 2,*, Mario Ferri de Collibus 2, Massimo Francucci 2, Massimiliano Ciafﬁ 2 and Alessandro Danielis 3

1 Industrial Engineering, University of Tor Vergata, 00133 Rome, Italy; soﬁ[email protected] 2 FSN-TECFIS-DIM, ENEA, 00044 Frascati (RM), Italy; [email protected] (M.F.d.C.); [email protected] (M.F.); massimiliano.ciafﬁ@enea.it (M.C.) 3 ENEA Guest, 00044 Frascati, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-06-9400-5553

Received: 24 August 2018; Accepted: 1 November 2018; Published: 5 November 2018

Abstract: Digital tools as 3D (three-dimensional) modelling and imaging techniques are having an increasing role in many applicative fields, thanks to some significative features, such as their powerful communicative capacity, versatility of the results and non-invasiveness. These properties are very important in cultural heritage, and modern methodologies provide an efficient means for analyzing deeply and virtually rendering artworks without contact or damage. In this paper, we present two laser scanner prototypes based on the Imaging Topological Radar (ITR) technology developed at the ENEA Research Center of Frascati (RM, Italy) to obtain 3D models and IR images of medium/large targets with the use of laser sources without the need for scaffolding and independently from illumination conditions. The RGB-ITR (Red Green Blue-ITR) scanner employs three wavelengths in the visible range for three-dimensional color digitalization up to 30 m, while the IR-ITR (Infrared-ITR) system allows for layering inspection using one IR source for analyses. The functionalities and operability of the two systems are presented by showing the results of several case studies and laboratory tests.

Keywords: cultural heritage; multiwavelength laser scanners; prototypes; 3D modelling; IR imaging

1. Introduction Artistic and cultural heritage (CH) is essential for present and future generations as a proof of human genius over the past eras, and its preservation has a key role in transmitting cultural and historical roots of nations around the world [1–3]. The safeguard and the communication of global CH are therefore more than an exclusively humanistic issue, being involved in many scientific activities of national and international institutions. The numerous variables involved in CH Conservation Science, such as the conservation status, material composition and the surrounding conditions, require the development of specific and multidisciplinary applicative solutions, both in terms of instrumentation and methodology [4]. In the actual digital era, imaging techniques and 3D (three-dimensional) modelling are widely employed in CH for recording and remote investigations, becoming common practice used in archaeological sites for contactless surveys [2,5–7]. The digital information achieved with these methodologies has an important usefulness, not only for historical reconstructions and dissemination purposes, but also for monitoring and diagnostic studies, allowing for investigations and comparisons of conservation status over time. Many methods are nowadays available for gaining these purposes and differ for factors, such as acquisition timing, resolution, accuracy and typology of achievable results [5,6,8–12]. Despite the variety of systems, functionalities and properties, it is well-known that one single method is insufficient for the complete 3D color digitalization of a

J. Imaging 2018, 4, 130; doi:10.3390/jimaging4110130 www.mdpi.com/journal/jimaging J. Imaging 2018, 4, 130 2 of 18 medium/large-sized object or site. Indeed, the complementary use of Image-Based Modelling (IBM) methods, such as photogrammetry, is a common practice with the Range-Base Modelling (RBM) ones, i.e., laser scanning, for the achievement, even at great distances, of accurate and detailed 3D textured models [9,10,13]. The general workflow of a 3D modelling process consists of the application of active methods for the object geometry acquisition (point cloud) and passive ones for color mapping (texture), in order to obtain photo-realistic 3D models, using an integrated approach between laser scanning and photogrammetry [14]. The resulting products are 3D textured reproductions of the object/site, where the color transposition is strongly dependent on the surrounding illumination conditions and the elaboration is often a time-consuming process [8,15–18]. 3D models are often combined to multispectral imaging for a deeper knowledge of the artefact in a visually appealing modality, aimed at the comprehension of artistic techniques, preparatory drawing, pentimenti or details regarding the supports through non-invasive layering inspections [19–23]. The systems used for imaging analysis can be divided into two main categories: cameras with external filters for obtaining multiple images [24–26] or laser-based instruments using one or more wavelengths for single layer inspections one at a time [27]. The choice of the approach depends on several factors, such as project requirements, parameters to analyze (underdrawings, superficial varnish, support), environmental working conditions and economic and human resources. The integrated approach between several technologies can be desultory, requiring often a greater amount of time than using a single technique, both in acquisition and in post-processing procedures. Furthermore, the dependence on external parameters, such as lighting conditions, can influence the reliability of acquired data. In this paper, we address these topics by illustrating the Imaging Topological Radar (ITR) technology, developed at the ENEA Research Center of Frascati (RM), as an acquisition method of highly detailed images for 2–3D color digitalization and layering analysis of medium–large targets, independently from the surrounding illumination conditions. The technology is exploited by two laser scanner systems, which use n-laser sources for contactless examination of several materials, from marble to wood and pigments, even at great distances without scaffoldings [28,29]. The technique and the post-processing data elaboration allow acquiring distance, color and underdrawings’ information from the backscattered optical signals from the scanned object. The ITR tools have been employed in different contexts with many conservative issues and/or necessities, such as nuclear inspections, post blasting simulations and CH analyses. Examples of applications on peculiar artworks will be shown with the aim to show the utility of ITR data in Conservation Science.

2. ITR Technology and Systems The DIM laboratory (Diagnostics and Metrology) of the ENEA Research Center has been working on laser applications for over two decades, developing several prototypes for many fields, from forensic science to CH studies [29,30]. In this paper, we present two laser scanners based on the use of n-laser stimuli as light sources for contactless acquisition of backscattered information: the RGB-ITR (Red Green Blue-Imaging Topological Radar) and the IR-ITR (Infrared-Imaging Topological Radar). The ITR systems are based on the double amplitude modulation (AM) range-finding technique, where the laser beam is exploited as the carrier wave of a radiofrequency signal, which modulates the beam intensity [31,32]. The double modulation of the laser sources combined with the lock-in technique allows estimating the amplitude and phase-shift information of the back-reflected signals from the target simultaneously. The estimation of the amplitudes’ information, proportional to the matter/laser interaction at a specific optical wavelength, contributes to the color (RGB-ITR version) or grey-level intensity (IR-ITR version) reconstruction of the target, while the phase-shift information to estimate the distance occurred between the scanner and the investigated surface. The amplitude and phase-shift information is actually stored point-by-point, collecting electronic signals in a range between 1 × 10−4 and 0.1 Volts. Structural digitalization of the surfaces is elaborated from the distance measurements, determined indirectly from the phase delay ∆ϕ of the collected back-reflected signal with respect to the reference modulating wave, as expressed by the formula [33]: J. Imaging 2018, 4, 130 3 of 18

ν ∆ϕ d = , (1) 4π fm where fm is the modulation frequency and ν is the velocity of light in the transmitting medium. For laser optical powers where the shot noise dominates over all other noise sources in the detection process (typically, a few nW at the output of the detection ﬁber), the accuracy of distance measurements can be shown to increase with the modulation frequency fm [33] according to the following expression:

1 σR ∝ . (2) m fmSNRi where σR is the “intrinsic” error (i.e., the minimum attainable error in optimal experimental conditions), m is the modulation depth and SNRi is the current signal-to-noise ratio: s Pητ SNR = , (3) i h f Γ which depends on the laser optical frequency f , the integration time τ, the detector’s quantum efficiency η, the overall optics merit factor Γ and the collected power P. Here, h stands for the Planck constant. Efficient noise rejection is obtained by using narrow field-of-view, interferential filtering and low-noise detection electronics. Because of phase periodicity, the AM technique is affected by the so-called “folding” ambiguity: the system returns the same distance value for target points, of which relative separation along the line of sight is a multiple of half the modulation wavelength. In order to overcome this problem, the laser probe is modulated at two different frequencies with values taken far apart from each other. The lower modulation frequency is calculated by the corresponding measurement range encompassing the whole scene of interest. Low-frequency measurements are then used to remove the ambiguity that affects the corresponding high-frequency, employed for more accurate measures. By using this technique of double AM, it is possible to achieve sub-millimetric accuracy on targets at distances or heights from a few tenths up to 30–35 m, without the need of scaffolding [28,31]. The ITR hardware is composed of two physically separate modules but optically connected by means of optical fibers: the optical head and the electronic components. In particular, the scanning module, i.e., the optical head, of the two instruments basically includes the launching and receiving optics, and it consists of a lens system for the collimation and focalization of the lasers to the center of a double-motorized mirror. The latter sweeps the beam onto the target according to a TV raster-like positioning, differently from the rotative movement of commercial scanners, and collects the back-reflected beam. The returning signals are detected by low-noise photodiode detectors and analyzed by means of lock-in amplifier units, which are also used to modulate the laser sources. The data are processed with custom software through MATLAB® interface (see Section 2.3), and the resulting models and images can be exported into the most common file formats, thanks to the feature of the software. Figure1 shows a schematic explanation of the ITR system operability. The parameters of the ITR scanners for defining their accuracy and resolution are: the laser spot size, which determines the minimum area of the collected pixel; the signal/noise ratio for estimating the distance measurement; the minimum angular step of the motorized mirror, responsible for the movement of the laser beam; the single pixel distance between the device and the target, which determines the spatial resolution between one point and the neighbors [34]. Due to construction limits of the optical heads of the systems, the actual rotation movement of the scanning mirrors is 1.4 × 5.4 rad (80◦ × 310◦), with a maximum point-to-point precision of 0.035 mrad. The pixel distance accuracy is 350 µm at 10 m of distance between system and target, while the spatial resolution is 700 µm. The average spot diameters measured at the same distance range are less than 1 mm for the RGB-ITR and 4–5 mm for the IR-ITR. The two ITR systems differ in number and wavelength of the laser sources, achieving two types of results: the RGB-ITR scanner (see Section 2.1) employs three J. Imaging 2018, 4, 130 4 of 18 wavelengths in the visible spectrum to obtain high-quality 3D color models of the object, while the IR-ITR instrument (see Section 2.2) uses a single infrared source for layering analyses of the surface. J. Imaging 2018, 4, x FOR PEER REVIEW 4 of 18

Target Laser Beam

Launching Fibre

Scanning Module

Receiving Fibre

Electronic components for laser generation, modulation and detection

Figure 1. Scheme of the ITR technology: by the use of custom software, the scanning process is FigureFigure 1. Scheme 1. Scheme of the of the ITR ITR technology: technology: by the by use the of use custom of custom software, software, the scanning the scanning process process is remotely is remotely controlled by a laptop connected to the electronic components. The laser beam (infrared (IR) controlledremotely bycontrolled a laptop by connected a laptop connected to the electronic to the electronic components. components The. laser The laser beam beam (infrared (infrared (IR) (IR or) in or in the visible range-RGB) is generated by laser sources, modulated and amplified by lock-in theor visible in the range-RGB) visible range is- generatedRGB) is generated by laser sources,by laser modulated sources, modulated and amplified and amplified by lock-in by techniques. lock-in techniques. Through optical fibers, the lasers are carried inside the optical head, where a system of Throughtechnique opticals. Through fibers, theoptical lasers fiber ares, carriedthe lasers inside are thecarried optical inside head, the where optical a systemhead, where of lenses a system collimates of lenses collimates and focalizes the ray on the center of a movable scanning mirror, which drives the andlens focalizeses collimates the ray and on focalizes the center the of ray a movable on the cent scanninger of a movable mirror, which scanning drives mirro ther, incidentwhich drives laser the beam incident laser beam on the surface’s object and collects the backscattered beam, elaborated with on the surface’s object and collects the backscattered beam, elaborated with custom software. custom software. 2.1. RGB-ITR Scanner 2.1. RGB-ITR Scanner The RGB-ITR scanner (Figure2) enables the simultaneous recording of range and color information The RGB-ITR scanner (Figure 2) enables the simultaneous recording of range and color by theThe phase-shift RGB-ITR between scanner the (Figure back-reflected 2) enables signal the simultaneous and the reference recording modulating of range wave, and andcolor the information by the phase-shift between the back-reflected signal and the reference modulating wave, elaborationinformation of by back-reflected the phase-shift RGB between triplets the from back the-reflected scanned signal object, and respectively.the reference modulating At the moment, wave the, and the elaboration of back-reflected RGB triplets from the scanned object, respectively. At the system uses three laser sources with wavelengths in the visible range corresponding to red, green and moment, the system uses three laser sources with wavelengths in the visible range corresponding to blue (660 nm, 517 nm and 440 nm), respectively [31]. red, green and blue (660 nm, 517 nm and 440 nm), respectively [31].

FigureFigure 2. 2RGB-ITR. RGB-ITR (Red (Red Green Green Blue-ImagingBlue-Imaging Topological Radar Radar)) scanner scanner prototype prototype in ina laboratory a laboratory environment,environment, set set upup with the the optical optical head head on ona tripod a tripod and the and electronic the electronic component componentss in the tower in the towerconfiguration. conﬁguration.

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Firstly, the three laser beams are overlapped by dichroic ﬁlters in a single white incident ray with a laser spot diameter of 1–2 mm at distances greater than 10 m and an overall power are calculated by the following formula:

mW Power density = Laser power/Spot area = 3 mW/0.785 mm2 = 3.821 mW/mm2, (4) mm2 where 3 mW correspond to the laser power of the three wavelengths in the visible range and the spot area is referred to a 1 mm-dimeter laser spot. A 40 mm-focus achromatic lens focuses the launching beam onto the target by means of a rotating mirror, while a 150 mm-focus lens is used for collecting and focusing the backscattered beams, split again into the three original red, green and blue components by means of an optical demultiplexer. The signals, in the ranges for each channel of about 0.1–100 mV, are detected by three low-noise avalanche photodiode detectors and analyzed by means of three lock-in amplifier units, which are also used to modulate the laser sources. Although all the beams are modulated, only the red and blue backscattered signals are exploited for high-frequency and low-frequency range determination, respectively. Typical modulation frequencies are 190–200 MHz for red, 1–10 MHz for blue and 3 MHz for green. The color information reconstruction is actually obtained by the use of just three narrow wavelengths, mainly due to the optical setup limit of the actual RGB-ITR scanner. For an accurate color estimation, an ad-hoc calibration procedure and several algorithms were implemented: as seen above, the amplitude information is stored in Volts, while the color spaces are usually described by non-dimensional normalized triplet of coordinates, like in the case of RGB, L*a*b* and HSV spaces. Moreover, all the optical signals collected by the scanner depend not only on the nature of the surface matter, but also by the quadratic dependency of the distance: to compensate these effects and normalize the data stored by the scanner, a calibration procedure is introduced. This calibration procedure consists of illuminating a certified white target by the three optical wavelengths, conveniently placed on a tripod and moved long all the linear range covered by the scanner for digitalizing the surface: every single calibration step is composed by moving the tripod every 0.5 m and collecting the back-reflected signals of the three wavelengths from the reference target. At the end of the procedure, the three distinct calibration curves (amplitude vs. distance) are used for normalizing the data collected on the investigated surface: the normalization coefficients are calculated by using, as a lookup-table, the three calibration curves interpolated by three distinct spline curves. The normalized amplitude values are calculated by dividing the single amplitude value for its equivalent normalization coefficient, also expressed in Volts, following Equation (5):

scene_amplitude_in_Volts R(λ ) = (5) i normalization_coef ficients_in_Volts

The color information is also affected by the angular effect between the incident laser beams and every single illuminated portion of the surface: to balance this effect, the normalized color information is weighted by the equivalent cosine of the angles between the incident laser beam and surface normal versors (Lambert’s Law), following Equation (6) [35,36]:

R(λi) R(λi) = (6) cos ϑi

Finally, the single point RGB triplet is obtained by using Equation (7):

Y = K· ∑ S(λi)·R(λi)·ycm f (λi)∆λ (7) λ where S(λi) is a CIE illuminant, at the moment, set as energy equivalent and weighted by experimental values, R(λi) is the normalized reﬂectance factor, ycm f (λi) is one of the three CIE standard observer color-matching functions (xcm f (λi), ycm f (λi), z(λi)), Σλ represents summation across wavelength λ, J. Imaging 2018, 4, x FOR PEER REVIEW 6 of 18

= . (8) ∑ ()∙()∙∆ Finally, the calibrated RGB data is merged with range information converted into Cartesian coordinates expressed in physical units and the geometrical information is rearranged. The post- processingJ. Imaging elaboration2018, 4, 130 is carried out through the ITR software in order to produce high-quality6 of 18 3D digital models of the scanned surface [34].

2.2. IR∆λ-ITRis the Prototype measurement wavelength interval, and K is a conventional normalizing constant deﬁned as [35,37]: The ITR monochromatic laser scanner (Figure 1003a) is employed for inspections under a superficial K = . (8) layer with a thickness of up to some hundreds∑λ S(λ ofi)· ymicroncm f (λi)s· ∆byλ the processing of backscattered data. This system uses an IR source with a wavelength of 1550 nm and a power of some tenths of mW/mm2. Finally, the calibrated RGB data is merged with range information converted into The opticalCartesian setup coordinates of this system expressed consists in physical of an unitsaspherical and the lens geometrical for focusing information the launching is rearranged. laser beam and Thea receiving post-processing doublet elaboration lens (50 ismm carried in diameter, out through and the ITR150 softwaremm in focal in order length) to produce for the high-quality detection of receiving3D digital signals models from of up the to scanned 10 m of surface distance [34]., with a laser spot size of 4–5 mm and a power density of less than 1 mW/mm2, as can be calculated by Equation (4): 2.2. IR-ITR Prototype mW = = 10 mW = 0. 796 mW (4) The ITR monochromaticmm laser scanner (Figure 3a) is employed12 for.56 inspectionsmm under a superﬁcialmm layer with a thickness of up to some hundreds of microns by the processing of backscattered data. ThisThe systemmodulation uses an frequency IR source with in this a wavelength case is 3 ofM 1550Hz and nm andthe aprinciple power of someof operation tenths of mW/mmis the same2. as that Theof the optical RGB setup-ITR ofscanner. this system In this consists case, of a ann IR aspherical detector lens (Licel for focusing APD Inasga the launching, Figure laser3b) is beam directly connectedand a receivingwith the doubletoptical lensreceiving (50 mm channel in diameter, and the and lock 150 mm-in amplifier in focal length). The Licel for the APD detection (Avalanche of PhotoDiode)receiving signalsmodule from combines up to 10 a mSi of A distance,PD, TE cooler, with a lasertemperature spot size ofcontroller, 4–5 mm and low a- powernoise amplifier density of and XYZless positioner than 1 mW/mm in a single2, as can compact be calculated module. by Equation The laser (9): control is carried out through a laptop equipped with suitable software mW, while the scanning procedure is remotely commanded with ITR Power density = Laser power/Spot area = 10 mW/12.56 mm2 = 0.796 mW/mm2 (9) custom software. mm2

(a) (b)

FigureFigure 3. IR 3.-ITRIR-ITR scanner scanner prototype: prototype: (a ()a the) the optical optical head head and thethe electronic electronic elements elements in thein the background; background; (b) a detail of the launching fiber, directly connected to the IR laser source, and the IR detector. (b) a detail of the launching fiber, directly connected to the IR laser source, and the IR detector. The modulation frequency in this case is 3 MHz and the principle of operation is the same as that 2.3. Customof the RGB-ITR Software scanner. In this case, an IR detector (Licel APD Inasga, Figure3b) is directly connected withThe the scanning optical receiving procedure channel and andthe the post lock-in-processing amplifier. elaborations The Licel APD are (Avalanche controlled PhotoDiode) by the use of module combines a Si APD, TE cooler, temperature controller, low-noise amplifier and XYZ positioner custom software developed in MATLAB® framework: ScanSystem and itrAnalyzer. The features of in a single compact module. The laser control is carried out through a laptop equipped with suitable these tools allow setting scanning parameters based on working needs and, on the other hand, to software, while the scanning procedure is remotely commanded with ITR custom software. obtain high-quality images from data elaboration. 2.3. Custom Software 2.3.1. ScanSystem The scanning procedure and the post-processing elaborations are controlled by the use of custom softwareThe ITR developeddata acquisition in MATLAB process® framework: is driven by ScanSystem the custom and ScanSystem itrAnalyzer. tool, The whi featuresch allow of theses setting scanningtools allow parameters, setting scanningacquire parameters calibration based data on and working check needsing the and, functionality on the other hand, of the to obtainelectronic componenthigh-quality. With images a motion from controller, data elaboration. the software enables the movement of the laser spot onto the

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2.3.1. ScanSystem The ITR data acquisition process is driven by the custom ScanSystem tool, which allows setting scanning parameters, acquire calibration data and checking the functionality of the electronic component. With a motion controller, the software enables the movement of the laser spot onto the surface in order to fix vertical and horizontal limits and arranges the area to be scanned. Among the parameters controlled by the software, there are acquisition velocities (vertical and horizontal), angular resolution (calculated at each scan), number of points in terms of pixels per row. Furthermore, a single scan can be divided into multiple scripts, in order to reduce the angular span for construction limits of a scanning module or logistic reasons. The same software is used to achieve calibration data, recording reflectance signals from a white target reference at each fixed step along the distance range. The file extension of a scan project is .prj (Simulink Project), elaborated with itrAnalyzer in MATLAB® environment, while the calibration dataset is saved as Excel file (.xls).

2.3.2. itrAnalyzer Data elaborations are carried out with itrAnalyzer software, which enables several operations on ITR data, imported in MATLAB® as matrices. The main features of this specially developed tool are 2–3D buildings of models, color and proﬁle analyses, registration of meshes and calibrations, while among the second-level functionalities, it allows for image enhancement for the study of hidden details, color correction and smoothing. This software can be considered as the link between RGB-ITR raw data and commercial software for 3D modelling and photo manipulation.

3. Case Studies The Imaging Topological Radar technology has been widely employed in CH investigations during the last twenty years with several different purposes. We report signiﬁcative applications carried out in the last three years both in situ and in laboratory case studies, showing the versatility of the techniques on different materials and situations.

3.1. In Situ Acquisitions Since 3D color digitalizations were considered among the requests and necessities, the RGB-ITR scanner has been employed. We show case studies where the scanner prototype has proved to be efﬁcient in terms of acquisition timing, quality results and good response to speciﬁc/critical conditions. In particular, the color laser scanner has been engaged in the following sites:

• Saint Brizio Chapel, Orvieto Cathedral (Viterbo, Italy); • Greek Chapel, Priscilla Catacombs (Rome, Italy); • Tower room, Saint Sebastian Gate, Aurelian Walls (Rome, Italy); • Egyptian wood sarcophagus (Milan, Italy).

3.1.1. Saint Brizio Chapel in Orvieto Cathedral In the right aisle of the Orvieto Cathedral, there is what is considered to be one of the jewels of the Early Renaissance: the Saint Brizio Chapel (Figure4a). This masterpiece dates back to the 15th century and shows the excellent skills of Italian painters, in terms of both painting techniques and realization of the topic [38]. The chapel was totally painted with the frescoes by Luca Signorelli [39], considered the master and precursor of Michelangelo. The peculiarities of the site are the high illumination due to the presence of three large windows and the gold glazing of some part of the paintings, factors that produce distortions in color transposition in most of the image-based acquisition methods [40]. The work inside the chapel was aimed at the 3D color digitalization of the frescoes, which was never done before with scanner techniques and important for structure and fresco monitoring over time. For the acquisition process, the RGB-ITR system (Figure4b) was set up in the center of the room, J. Imaging 2018, 4, 130 8 of 18

moving only the optical head in different stations in such a way as to allow the laser beam to scan the entire surface. The required time for data acquisition was about 26 h, comprehending night and day J. Imagingperiods, 2018 without, 4, x FOR interdiction PEER REVIEW to visitors. 8 of 18

(a) (b)

FigureFigure 4. The 4. The Saint Saint Brizio Brizio Chapel Chapel in in the the Orvieto Orvieto Cathedral Cathedral (Viterbo, Italy).Italy). ((aa)) PicturePicture of of the the Chapel Chapel fromfrom the the outside outside shot shot with with a a Canon Canon camera, camera, ( (bb)) the the RGB RGB-ITR-ITR system system working working inside inside the the Chapel Chapel (Photo:(Photo: S. Ceccarelli) S. Ceccarelli).. 3.1.2. Greek Chapel in Priscilla Catacombs 3.1.2. Greek Chapel in Priscilla Catacombs The funerary complex of Priscilla Catacombs, dating back to between the 2nd and the 5th centuries,The funerary extends complex for more of than Priscilla 8 km Catacombs, under Rome dating at several back levelsto between of depth the excavated 2nd and in the tuff 5th centurstones.ies, extends Around thefor more cemeterial than galleries,8 km under there Rome are severalat several important levels of iconographic depth excavate testimoniesd in tuff ofstone thes. Aroundpaleo Christianthe cemeterial age, from galleries inscriptions, there are and several paintings important to cubicula iconographic and small rooms. testimonies Among of the the latter, paleo Christianthe Greek age Chapel, from (Figureinscriptions5) is one and of paintings the most signiﬁcativeto cubicula and representations small rooms of. theAmong Testament the latter, inside the Greekthe complex.Chapel (Figure The little 5) is chapel, one of which the most takes significative its name from representation two Greek inscriptionss of the Testament inside it,inside stands the complex.out for The the coloredlittle chapel, decorations which andtakes religious its name scenes, from two unique Greek and inscriptions particular for inside the historical it, stands period out for theand color artisticed decorations skills [41]. The and hypogeum religious structures scenes, unique are critical and ambient particular for for the pictorialthe historical elements, period which and artisticcan beskills affected [41]. byThe biological hypogeum attack, structures saline efﬂorescence, are critical ambient material for swelling the pictorial or cracking elements [42,43, which]. Due tocan be affectedthe particular by biological thermo-hygrometric attack, saline conditions efflorescence, of the material Priscilla swelling catacomb or cracking environment, [42,43] such. Du ase to low the particulartemperature thermo (~14-hygrometric◦C) and high conditions humidity (~98%), of the procedures Priscilla catacomb of the monitoring environment, and documenting such as low temperatureof the state (~ of14 health °C) an wered high as importanthumidity as(~98%), urgent procedures in order to of keep the alterations, monitoring detachments and documenting and the of thegeneral state of conservation health were status as important under control. as urgent However, in order uncontrolled to keep alterations, microclimatic detachments parameters canand bethe generaldamaging conservation also to electronic status under components, control. builtHowever, for operation uncontrolled in ideal microclimatic thermo-hygrometric parameters conditions can be ◦ damaging(T~15 ÷ also35 toC, electronic RH-relative com humidity~35ponents, built÷ 75%) for operation [44]. In this in case,ideal thethermo solution-hygrometric to the hygrometric conditions (T~problems,15 ÷ 35 °C, i.e., RH condensation,-relative humidity was solved,~35 ÷ covering 75%) [44] the. In electronic this case, components the solution with to a PVCthe hygrometric (polyvinyl problems,chloride)-protective i.e., condensation, sheet. was solved, covering the electronic components with a PVC (polyvinyl Due to the several architectonic elements, the complete 3D digitalization of the room required chloride)-protective sheet. several stations, placing the electronic components in two positions and moving the scanning module along the chapel. For these reasons, the area was divided into multiple scans for a total of approx. 190 h.

Figure 5. The Greek Chapel inside the Priscilla Catacombs in Rome (Italy): photographic images of the RGB-ITR system during the scan (Photo: S. Ceccarelli).

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(a) (b)

Figure 4. The Saint Brizio Chapel in the Orvieto Cathedral (Viterbo, Italy). (a) Picture of the Chapel from the outside shot with a Canon camera, (b) the RGB-ITR system working inside the Chapel (Photo: S. Ceccarelli).

3.1.2. Greek Chapel in Priscilla Catacombs The funerary complex of Priscilla Catacombs, dating back to between the 2nd and the 5th centuries, extends for more than 8 km under Rome at several levels of depth excavated in tuff stones. Around the cemeterial galleries, there are several important iconographic testimonies of the paleo Christian age, from inscriptions and paintings to cubicula and small rooms. Among the latter, the Greek Chapel (Figure 5) is one of the most significative representations of the Testament inside the complex. The little chapel, which takes its name from two Greek inscriptions inside it, stands out for the colored decorations and religious scenes, unique and particular for the historical period and artistic skills [41]. The hypogeum structures are critical ambient for the pictorial elements, which can be affected by biological attack, saline efflorescence, material swelling or cracking [42,43]. Due to the particular thermo-hygrometric conditions of the Priscilla catacomb environment, such as low temperature (~14 °C) and high humidity (~98%), procedures of the monitoring and documenting of the state of health were as important as urgent in order to keep alterations, detachments and the general conservation status under control. However, uncontrolled microclimatic parameters can be damaging also to electronic components, built for operation in ideal thermo-hygrometric conditions (T~15 ÷ 35 °C, RH-relative humidity~35 ÷ 75%) [44]. In this case, the solution to the hygrometric problems,J. Imaging 2018 i.e., 4, 130condensation, was solved, covering the electronic components with a PVC (polyvinyl9 of 18 chloride)-protective sheet.

J. Imaging 2018, 4, x FOR PEER REVIEW 9 of 18

Due to the several architectonic elements, the complete 3D digitalization of the room required several stations,Figure placing 5.5. The the GreekGreek electronic ChapelChapel component inside inside the the Priscilla Priscillas in two Catacombs Catacombspositions in and in Rome Rome moving (Italy): (Italy): the photographic pscanninghotographic module images image of alongs the of the chapeltheRGB-ITR. For RGB these-ITR system reasons,system during during the the area scanthe was scan (Photo: divided (Photo: S. Ceccarelli). S.in Ceccarelli)to multiple. scans for a total of approx. 190 h.

3.1.3.3.1.3. Tower RoomRoom in Saint Sebastian Gate ofof thethe AurelianAurelian WallsWalls

The city city of of Rome Rome has has many many remains remains of ancient of ancient protection protection walls, walls dating, dating back to back the Roman to the imperial Roman age.imperial This age. important This important proof of the proof military of the strength military of the strength Roman of army the surroundsRoman army several surrounds areas of several the city andareas comprehends of the city and monumental comprehends gates, monumental such as the gates Saint, Sebastiansuch as the one Saint (Figure Sebastian6a). The one architecture (Figure 6a is). typicalThe architecture of this type is oftypical structure, of this even type if itof has structure, undergone even several if it has remakes undergone over several the centuries: remakes one over central the archcenturies: between one two central lateral arch towers between was developedtwo lateral into towers three was ﬂoors developed [45,46]. Theinto room three of floors the right [45,46] tower. The is particularlyroom of the right interesting tower fromis particularly a historical interesting and artistic from point a historical of view, becauseand artistic of the point presence of view of, charcoalbecause drawingsof the presence and inscriptions of charcoal asdrawing testimoniess and inscriptions of soldiers’ as lives testimon in a militaryies of soldiers’ establishment lives in ina military the late 19thestablishment century (Figure in the late6b). 19 Theseth century inscriptions, (Figure illegible6b). These from inscriptions, the ground, illegible have from been the the ground, main object have ofbeen the the ENEA main work, object through of the ENEA 3D modelling work, through and image 3D modelling enhancement and image of RGB-ITR enhancement data for of the RGB content-ITR extrapolationdata for the content and reconstruction. extrapolation and The reconstruction. data acquisition The process data acquisition was carried process out in was 10 days,carried moving out in the10 days, system moving in seven the different system stations in seven for different the complete stations digitalization for the complete of each architectural digitalization structure of each (columns,architectural vault). structure (columns, vault).

(a) (b)

Figure 6 6.. ((aa)) Saint Saint Sebastian Sebastian monumental monumental gate gate,, which which is ispart part of ofthe the Aurelian Aurelian Wall Wall and and the the location location of theof the Walls Walls Museum Museum in Rome in Rome (Italy) (Italy).. The The red reddashed dashed square square indicates indicates the thedigitalized digitalized tower tower room room;; (b) (detailb) detail of the of theinteriors interiors of ofthe the right right tower tower with with the the RGB RGB-ITR-ITR in in working working position position.. (Photo: (Photo: S. S. Ceccarelli) Ceccarelli)..

3.1.4.3.1.4. Egyptian W Woodenooden SarcophagusSarcophagus The Egyptian Egyptian anthropoid anthropoid sarcophagus sarcophagus of Peftjauyauyaset of Peftjauyauyaset (Figure (Figure7a) belongs 7a) belongs to the XXVI to the dynasty XXVI (7th–6thdynasty (7 centuryth–6th BC)century and isBC) precious and is precious evidence evidence of crafting of skillscrafting of the skills Late of Periodthe Late of Period ancient of Egypt. ancient Egypt. This fragile artefact, coming from the Archaeological Museum of Milan, is made of Lebanon cedar wood, covered with painted plaster and intaglio carvings. The sarcophagus is completely decorated, both internally and externally, with polychromatic drawings and hieroglyphics. The general conservation status is particularly exceptional for what concerns the wooden structure and the decorations. The 3D models of the two part of the sarcophagus were requested from the restorers as documentation material before restoration. The digitalization process had to deal with a set of problems, such as reduced distances between object and system (<3 m), complex structures made of concavities and convexities, multiple materials and several incisions. The RGB-ITR system was placed at a fixed station (Figure 7b), while the two parts of the sarcophagus were moved to three different positions in order to scan entirely the artwork.

J. Imaging 2018, 4, 130 10 of 18

This fragile artefact, coming from the Archaeological Museum of Milan, is made of Lebanon cedar wood, covered with painted plaster and intaglio carvings. The sarcophagus is completely decorated, both internally and externally, with polychromatic drawings and hieroglyphics. The general conservation status is particularly exceptional for what concerns the wooden structure and the decorations. The 3D models of the two part of the sarcophagus were requested from the restorers as documentation material before restoration. The digitalization process had to deal with a set of problems, such as reduced distances between object and system (<3 m), complex structures made of concavities and convexities, multiple materials and several incisions. The RGB-ITR system was placed at a ﬁxed station (Figure7b), while the two parts of the sarcophagus were moved to three different positionsJ. Imaging 2018 in,order 4, x FOR to PEER scan REVIEW entirely the artwork. 10 of 18

(a) (b)

Figure 7.7. Digitalization of Egyptian woodenwooden sarcophagussarcophagus of Peftjauyauyaset: (a)) the artefact in one of the three positions during the scan. The The decorations decorations are are clearly clearly evident evident both both internally internally and and externally, externally, where also also incisions incisions characterize characterize the the artefact; artefact (b); acquisition (b) acquisition conditions conditions with the with RGB-ITR the RGB laser-ITR scanner. laser (Photo:scanner S.. (Photo: Ceccarelli). S. Ceccarelli).

3.2.3.2. Laboratory Tests Due to its very prototypal status, the IR scanner was tested in a laboratory environment on two painted samples.samples. TheThe ﬁrst first canvas canvas (Figure (Figure8a) 8 presentsa) presents three three bands bands (blue, (blue, orange orange and light and green)light green) made withmade watercolor. with water Fromcolor a. naked-eye From a naked observation,-eye observation an underlying, an underlying drawing can drawing be noticed can although be noticed the differentalthough elementsthe different are not elements clearly visible.are not Theclearly second visible canvas. The (Figure second8b) canvas has a wave(Figure motif, 8b) withhas a colorwave gradientmotif, with from a color black gradient to greenish. from Theblack materials to greenish. are acrylics,The materials but from are theacrylics direct, but observation from the ofdirect the work,observation the painting of the work technique, the painting is not evident. technique The is laboratorynot evident. experiment The laboratory (Figure experiment8c) was carried (Figure out, 8c) illuminatingwas carried out with, illuminating IR-ITR scanner with one IR-ITR canvas scanner at a time one atcanvas two differentat a time distancesat two different (at 10 m distances (Figure8 (ata) and10 m at (Figure 5 m (Figure 8a) and8b)), at 5 in m order (Figure to compare8b)), in order the quality to compare of the the results quality at aof different the results distance at a different range. Bydistance using range a Thorlabs. By using detector a Thorlabs card [47 detector], the IR lasercard beam[47], the was IR visualized laser beam on was the visualized canvas and on the the scanning canvas areaand the was scanning deﬁned througharea was the defined ScanSystem through software. the ScanSystem software.

(a) (b) (c)

Figure 8. Laboratory test with the IR-ITR scanner on painted samples: (a) canvas painted with three watercolor bands. The presence of drawings under the pictorial film is evident, but not clearly discernible; (b) acrylic painting on canvas, where the shading technique is not immediately evident; (c) laboratory conditions for tests on the watercolor canvas placed at a 10 m distance from the IR-ITR scanner. (Photo: S. Ceccarelli).

J. Imaging 2018, 4, x FOR PEER REVIEW 10 of 18

(a) (b)

Figure 7. Digitalization of Egyptian wooden sarcophagus of Peftjauyauyaset: (a) the artefact in one of the three positions during the scan. The decorations are clearly evident both internally and externally, where also incisions characterize the artefact; (b) acquisition conditions with the RGB-ITR laser scanner. (Photo: S. Ceccarelli).

3.2. Laboratory Tests Due to its very prototypal status, the IR scanner was tested in a laboratory environment on two painted samples. The first canvas (Figure 8a) presents three bands (blue, orange and light green) made with watercolor. From a naked-eye observation, an underlying drawing can be noticed although the different elements are not clearly visible. The second canvas (Figure 8b) has a wave motif, with a color gradient from black to greenish. The materials are acrylics, but from the direct observation of the work, the painting technique is not evident. The laboratory experiment (Figure 8c) was carried out, illuminating with IR-ITR scanner one canvas at a time at two different distances (at 10 m (Figure 8a) and at 5 m (Figure 8b)), in order to compare the quality of the results at a different distanceJ. Imaging 2018 range, 4,. 130 By using a Thorlabs detector card [47], the IR laser beam was visualized on the canvas11 of 18 and the scanning area was defined through the ScanSystem software.

(a) (b) (c)

Figure 8 8.. LLaboratoryaboratory test test with with the the IR- IR-ITRITR scanner scanner on painted on painted samples samples:: (a) canvas (a) canvas painted painted with three with waterthreecolor watercolor bands. bands. The presence The presence of drawings of drawings under under the the pictorial pictorial film ﬁlm is is evident, evident, but but not not clearly discerniblediscernible;; ( b)) acrylic painting on canvas canvas,, where where the shading techn techniqueique is is not not immediately immediately evident; evident; (c) l laboratoryaboratory conditions for tests on the water watercolorcolor canvas placed at a 10 m di distancestance from the IR IR-ITR-ITR scanner. (Photo: S. Ceccarelli).

4. Results The reported results were achieved using the instruments for a minimum of one hour to a maximum of one week, with post-processing phases duration from less than hours for preliminary 3D modelling up to several days for the complete merging of multiple scans or image enhancement. In Table1, the main scanning parameters and difﬁculties of each case study reported in this paper are summarized.

Table 1. Scanning parameters and critical issues of the case studies considered in this work.

Acquisition Minimum Angular Distance Target Critical Issues Time Step Target/System The San ~26 h 0.14 mrad 4 ÷ 9 m Excessive brightness Brizio Chapel (day and night) ~190 h High humidity and reduced The Greek Chapel 0.07 ÷ 0.035 mrad 3 ÷ 6 m (day and night) working space The Saint ~130 h Excessive brightness and 0.14 ÷ 0.07 mrad 5 ÷ 11 m Sebastian Gate (day and night) small inscriptions Egyptian ~70 h Irregular surface and 0.07 mrad Less than 3 m sarcophagus (day and night) reduced distance Unknown layering Watercolor canvas 1 h and 45 min 0.035 mrad 10 m and watercolor Unknown layering and Acrylics canvas 40 min 0.035 mrad 5 m acrylic colors

4.1. 3D Modelling and Image Enhancement The digitalization of the San Brizio Chapel was carried out during day and night hours, independently from the high illumination that characterizes the room and strongly defeats photographs, as can be noticed in Figure9. J. Imaging 2018, 4, x FOR PEER REVIEW 11 of 18

4. Results The reported results were achieved using the instruments for a minimum of one hour to a maximum of one week, with post-processing phases duration from less than hours for preliminary 3D modelling up to several days for the complete merging of multiple scans or image enhancement. In Table 1, the main scanning parameters and difficulties of each case study reported in this paper are summarized.

Table 1. Scanning parameters and critical issues of the case studies considered in this work.

Acquisition Minimum Angular Distance Target Critical Issues Time Step Target/System The San Brizio ~26 h 0.14 mrad 4 ÷ 9 m Excessive brightness Chapel (day and night) ~190 h High humidity and reduced The Greek Chapel 0.07 ÷ 0.035 mrad 3 ÷ 6 m (day and night) working space The Saint Sebastian ~130 h Excessive brightness and small 0.14 ÷ 0.07 mrad 5 ÷ 11 m Gate (day and night) inscriptions Egyptian ~70 h Irregular surface and reduced 0.07 mrad Less than 3 m sarcophagus (day and night) distance Unknown layering and Watercolor canvas 1 h and 45 min 0.035 mrad 10 m watercolor Unknown layering and acrylic Acrylics canvas 40 min 0.035 mrad 5 m colors

4.1. 3D Modelling and Image Enhancement The digitalization of the San Brizio Chapel was carried out during day and night hours, independentlyJ. Imaging 2018 , 4 from, 130 the high illumination that characterizes the room and strongly12 of defeats 18 photographs, as can be noticed in Figure 9.

FigureFigure 9. The 9. The San San Brizio Brizio Chapel Chapel:: photograph photograph acquired acquired withwith a a Canon Canon digital digital camera. camera The. T picturehe picture is is highlyhighly illuminated illuminated due due to tothe the presence presence of of three three wide wide windows inside inside the the chapel. chapel (Photo:. (Photo: S. Ceccarelli). S. Ceccarelli).

The ﬁnal 3D model was built from the merging of multiple scans acquired with a very partial The final 3D model was built from the merging of multiple scans acquired with a very partial overlapping along the edges, in order to give reference points during the registration process, overlapping along the edges, in order to give reference points during the registration process, carried J. Imagingcarried 2018 out, 4, x by FOR picking PEER REVIEW the same points in the two textures to register, as shown in Figure 10. 12 of 18 out by picking the same points in the two textures to register, as shown in Figure 10.

FigureFigure 10. Procedure 10. Procedure for for the the registration registration of of textures textures byby pickingpicking similar similar points points for for the the elaboration elaboration of of the 3theD model 3D model of the of the Sa Sann Brizio Brizio Chapel. Chapel. (Elaboration (Elaboration carried carried out out by by ENEA ENEA team team with with the customthe custom softwaresoftware itrAnalyzer). itrAnalyzer).

Figure 11 shows the complete model of the chapel, providing a truthful transposition of colors in Figure 11 shows the complete model of the chapel, providing a truthful transposition of colors a 3D reconstruction without saturations or shadows. in a 3D reconstruction without saturations or shadows.

Figure 11. 3D model of the San Brizio Chapel obtained with the RGB-ITR scanner. The colors are uniform without shadows, unlike photographs (Figure 9). (Elaboration carried out by ENEA team with the custom software itrAnalyzer).

The 3D modelling of the Greek Chapel required tens of scans, dividing the room into multiple sub-areas. The single scans were calibrated and then all registered with each other using itrAnalyzer software for the building of the final 3D model, partially shown in Figure 12.

J. Imaging 2018, 4, x FOR PEER REVIEW 12 of 18

Figure 10. Procedure for the registration of textures by picking similar points for the elaboration of the 3D model of the San Brizio Chapel. (Elaboration carried out by ENEA team with the custom software itrAnalyzer).

J. ImagingFigure2018, 114, 130 shows the complete model of the chapel, providing a truthful transposition of13 color of 18s in a 3D reconstruction without saturations or shadows.

FigureFigure 11.11. 3D3D modelmodel ofof thethe SanSan BrizioBrizio ChapelChapel obtainedobtained withwith thethe RGB-ITRRGB-ITR scanner.scanner. TheThe colorscolors areare uniformuniform without without shadows, shadows, unlike unlike photographs photographs (Figure (Figure9). (Elaboration9). (Elaboration carried carried out by out ENEA by ENEA team withteam thewith custom the custom software software itrAnalyzer). itrAnalyzer).

TheThe 3D3D modellingmodelling ofof thethe GreekGreek ChapelChapel requiredrequired tenstens ofof scans,scans, dividingdividing thethe roomroom intointo multiplemultiple sub-areas.sub-areas. TheThe singlesingle scansscans werewere calibratedcalibrated andand thenthen allall registeredregistered withwith eacheach otherother usingusing itrAnalyzeritrAnalyzer J. Imaging 2018, 4, x FOR PEER REVIEW 13 of 18 softwaresoftware forfor thethe buildingbuilding ofof thethe ﬁnalfinal 3D 3D model, model, partially partially shown shown in in Figure Figure 12 12. .

Figure 12. 3D partial model of the Greek Chapel reporting truthful colors. (Elaboration carried out by

ENEA team with the custom software itrAnalyzer). Figure 12. 3D partial model of the Greek Chapel reporting truthful colors. (Elaboration carried out by InENEA the case team study with the of custom the Saint software Sebastian itrAnalyzer). tower, in addition to the 3D modelling (Figure 13a), the post-processing phases on RGB-ITR data included the application of ‘afﬁne’ algorithm for letter detectionIn the and case background study of the removal Saint (FigureSebastian 13 b).tower The, in research addition has to provided the 3D modelling a partial reconstruction(Figure 13a), the ofpost the- inscriptionsprocessing phasescontents on through RGB-ITR the data extrapolation included of the single application letters [48 of ]. ‘affine’ Further algor elaborationsithm for letterare neededdetection for automaticand background word reconstructions removal (Figure based 13b). on The letter research recognition. has provided a partial reconstruction of the inscriptions contents through the extrapolation of single letters [48]. Further elaborations are needed for automatic word reconstructions based on letter recognition.

(a) (b)

Figure 13. Tower room of the Saint Sebastian Gate: (a) image of part of the 3D model of the room; (b) detail of background removal elaboration for the reconstruction of inscriptions’ contents [48]. (Elaboration carried out by ENEA team with the custom software itrAnalyzer).

The sarcophagus digitalization has represented an interesting instrumental challenge, both in terms of material and shape. The several concavities and convexities of the artefact requested different object positioning and multiple scans, translated into the registrations of several textures. Since the study is still ongoing, the results are not complete but promising, having achieved accurate digitalization of colors, materials and irregularities (Figure 14b), not well appreciable with laboratory artificial illumination (Figure 14a).

J. Imaging 2018, 4, x FOR PEER REVIEW 13 of 18

Figure 12. 3D partial model of the Greek Chapel reporting truthful colors. (Elaboration carried out by ENEA team with the custom software itrAnalyzer).

In the case study of the Saint Sebastian tower, in addition to the 3D modelling (Figure 13a), the post-processing phases on RGB-ITR data included the application of ‘affine’ algorithm for letter detection and background removal (Figure 13b). The research has provided a partial reconstruction of the inscriptions contents through the extrapolation of single letters [48]. Further elaborations are neededJ. Imaging for2018 automatic, 4, 130 word reconstructions based on letter recognition. 14 of 18

(a) (b)

FigureFigure 1 13.3. TowerTower room room of of the the Sain Saintt Sebastian Sebastian Gate Gate:: (a) (imagea) image of part of part of the of 3D the model 3D model of the of room the room;; (b) detail(b) detail of background of background removal removal elaboration elaboration for forthe the reconstruction reconstruction of ofinscriptions inscriptions’’ contents contents [48] [48.]. (Elaboration(Elaboration carried carried out out by by ENEA ENEA team team with with the the custom custom software software itrAnalyzer). itrAnalyzer).

TheThe sarcophagus sarcophagus digitalization digitalization has has represented represented an an interesting interesting instrumental instrumental challenge, challenge, both both in in termsterms of materialmaterial andand shape. shape. The The several several concavities concavities and convexitiesand convexities of the of artefact the artefact requested requested different differentobject positioning object positioning and multiple and multiple scans, translatedscans, translated into the in registrationsto the registrations of several of several textures. textures Since. Sincethe study the study is still is still ongoing, ongoing, the the results results are are not not complete complete but but promising, promising, havinghaving achieved achieved accurate accurate digitalizationdigitalization of of color colors,s, materials materials and and irregularities irregularities (Figure (Figure 1 144bb),), not not well well appreciable appreciable with with laboratory laboratory artificialartiﬁcialJ. Imaging illuminationillumination 2018, 4, x FOR (PEER (FigureFigure REVIEW 1 144a).a). 14 of 18

(a) (b)

FigureFigure 14. 1Egyptian4. Egyptian sarcophagus: sarcophagus: (a) photographic (a) photographic image of the image inferior of part the of theinferior anthropomorphous part of the sarcophagusanthropomorphous (Photo: S. sarcophagus Ceccarelli); ( b(Photo:) part of S. theCeccarelli) 3D model; (b of) part the ancientof the 3D wooden model artefact of the ancient achieved wood withen theartefact RGB-ITR achieved system with (Elaboration the RGB-ITR carried system out by(Elaboration ENEA team carried with theout customby ENEA software team with itrAnalyzer). the custom software itrAnalyzer).

4.2. Infrared Imaging The laboratory test on a three-color canvas has shown interesting details of the drawings underlying the superficial painting, barely visible to the naked eye (Figure 15a). Indeed, the IR image has clearly revealed different elements under the three watercolor bands (Figure 15b), providing an efficient means of investigation for preparatory drawings.

(a) (b)

Figure 15. IR-ITR experiment: (a) photographic image of a watercolor canvas (Photo: S. Ceccarelli); (b) IR-ITR image where the underdrawings’ details are clearly visible (dashed shapes indicate the underdrawings elements in correspondence of the visible image), (Elaboration carried out by ENEA team with the custom software itrAnalyzer).

J. Imaging 2018, 4, x FOR PEER REVIEW 14 of 18

(a) (b)

Figure 14. Egyptian sarcophagus: (a) photographic image of the inferior part of the anthropomorphous sarcophagus (Photo: S. Ceccarelli); (b) part of the 3D model of the ancient wooden J. Imagingartefact2018, 4, achieved 130 with the RGB-ITR system (Elaboration carried out by ENEA team with the custom 15 of 18 software itrAnalyzer).

4.2.4.2. Infrared Infrared Imaging Imaging TheThe laboratory laboratory test teston on aa three-colorthree-color canvascanvas has shown i interestingnteresting details details of of the the drawings drawings underlyingunderlying the the superﬁcial superficial painting, barely barely visible visible to the to thenaked naked eye (Figure eye (Figure 15a). Indeed,15a). Indeed,the IR image the IR imagehas clearly has clearly revealed revealed different different elements elements unde underr the three the three water watercolorcolor bands bands (Figure (Figure 15b), 15providingb), providing an anefficient efﬁcient means means of of investigation investigation for for preparatory preparatory drawings drawings..

(a) (b)

FigureFigure 15. 15IR-ITR. IR-ITR experiment: experiment:( a(a)) photographicphotographic image of of a a water watercolorcolor canvas canvas (Photo: (Photo: S. S.Ceccarelli) Ceccarelli);; (b)(b IR-ITR) IR-ITR image image where where the the underdrawings’ underdrawings’ detailsdetails are clearly visible visible (dashed (dashed shapes shapes indicate indicate the the underdrawingsunderdrawings elements elements in in correspondence correspondence ofof the visible image) image),, (Elaboration (Elaboration carried carried out out by by ENEA ENEA teamteam with with the the custom custom software software itrAnalyzer). itrAnalyzer). J. Imaging 2018, 4, x FOR PEER REVIEW 15 of 18 The scan on the acrylic canvas (Figure 16a) has provided details on the painting technique, The scan on the acrylic canvas (Figure 16a) has provided details on the painting technique, revealingrevealing how how the the nuances nuances of of the the blackblack waveswaves have been realized realized before before the the definitive deﬁnitive and and visible visible drawingdrawing (Figure (Figure 16 16bb).).

(a) (b)

FigureFigure 16. 1IR-ITR6. IR-ITR experiment: experiment: (a) photographic(a) photographic image image of an acrylic-colored of an acrylic-color canvased (Photo:canvas S.(Photo: Ceccarelli); S. (b)Ceccarelli) IR-ITR image; (b) IR where-ITR image the lines where and the shades lines and are shades different are fromdifferent the from visible the image, visible revealingimage, revealing changes fromchanges the original from the drawing original and drawing the realization and the realization technique technique (dashed (dashed shapes indicateshapes indicate the different the different elements inelements relation toin therelation visible to the image), visible (Elaboration image), (Elaboration carried out carried by ENEA out by team ENEA with team the with custom the custom software itrAnalyzer).software itrAnalyzer).

5. Conclusions Image techniques and 3D modelling are nowadays widely employed in the CH Conservation Science, which disclose a modern, non-invasive and engaging analysis tool. Several commercial systems are available on the market for 3D modelling, but they often require the complementarity with other techniques with limitation in terms of illumination conditions and spatial resolution. Likewise, close-range imaging, such as filter-wheel cameras, has distance and object size constraints. In this paper, two prototypal laser scanners using n-laser stimuli have been presented in order to demonstrate the quality, efficacy and the versatility of this methodology in this application field as a unique means for 3D digitalization and IR layering inspections. The color laser scanner, called RGB- ITR, has been used for 3D modelling and in-depth analyses from high-quality image enhancement, proving to handle critical surrounding conditions greatly, such as high humidity and great lighting, and irregular material, such as wooden anthropoid sarcophagi. The IR system, called IT-ITR, has been tested on laboratory samples, attesting validity for inspection under layers of different painting materials (acrylic and watercolor) at up to 10 m of distance from the system, contrary to close-range instruments. The results have shown the potentialities of the two laser scanners and their versatility in terms of environmental circumstances and materials, two important but unstable factors in the CH field. The ITR images can be used for conservation purposes for their high detailing and re-usability over time. Future plans include the implementation of laser sources, even using the same scanner, in order to achieve a multispectral scanner as a multipurpose system for applications in a wider range of materials and not exclusively limited to CH. The research group is working on a new version of this scanner, which will host 7 wavelengths—one in the near ultraviolet spectrum (380 nm), four in the visible spectrum (441, 530, 580, 660 nm) and two in the infrared (800, 1500 nm) spectrum—with the aim to develop a multispectral (up to several ten meters) high-density point cloud remote sensor

Author Contributions: Conceptualization, Methodology and Validation, M.G., M.F.d.C., M.F., A.D. and S.C.; Software, M.G.; Formal Analysis, M.G., A.D. and S.C.; Investigation, M.G., S.C., M.C.,

J. Imaging 2018, 4, 130 16 of 18

5. Conclusions Image techniques and 3D modelling are nowadays widely employed in the CH Conservation Science, which disclose a modern, non-invasive and engaging analysis tool. Several commercial systems are available on the market for 3D modelling, but they often require the complementarity with other techniques with limitation in terms of illumination conditions and spatial resolution. Likewise, close-range imaging, such as filter-wheel cameras, has distance and object size constraints. In this paper, two prototypal laser scanners using n-laser stimuli have been presented in order to demonstrate the quality, efficacy and the versatility of this methodology in this application field as a unique means for 3D digitalization and IR layering inspections. The color laser scanner, called RGB-ITR, has been used for 3D modelling and in-depth analyses from high-quality image enhancement, proving to handle critical surrounding conditions greatly, such as high humidity and great lighting, and irregular material, such as wooden anthropoid sarcophagi. The IR system, called IT-ITR, has been tested on laboratory samples, attesting validity for inspection under layers of different painting materials (acrylic and watercolor) at up to 10 m of distance from the system, contrary to close-range instruments. The results have shown the potentialities of the two laser scanners and their versatility in terms of environmental circumstances and materials, two important but unstable factors in the CH field. The ITR images can be used for conservation purposes for their high detailing and re-usability over time. Future plans include the implementation of laser sources, even using the same scanner, in order to achieve a multispectral scanner as a multipurpose system for applications in a wider range of materials and not exclusively limited to CH. The research group is working on a new version of this scanner, which will host 7 wavelengths—one in the near ultraviolet spectrum (380 nm), four in the visible spectrum (441, 530, 580, 660 nm) and two in the infrared (800, 1500 nm) spectrum—with the aim to develop a multispectral (up to several ten meters) high-density point cloud remote sensor

Author Contributions: Conceptualization, Methodology and Validation, M.G., M.F.d.C., M.F., A.D. and S.C.; Software, M.G.; Formal Analysis, M.G., A.D. and S.C.; Investigation, M.G., S.C., M.C., M.F.d.C., M.F. and A.D.; Data Curation, M.G., S.C. and A.D.; Writing—Original Draft Preparation, S.C.; Writing—Review & Editing, M.G. and M.F.d.C. Funding: Part of the research presented in this paper was performed within CO.B.RA. project and funded by regional grant (lr13/2008 project n. 1031). Acknowledgments: The authors sincerely thank to everyone who have contributed to the works reported in this paper, from the responsible managers to technicians, restorers, curators, archaeologist and collaborators. A particular appreciation goes to: Roberta Fantoni and Antonio Palucci as chiefs of the ENEA Division and Laboratory, Ersilia Maria Loreti as Director of the Walls Museum, l’Opera del Duomo di Orvieto as the association for the conservation of Orvieto Cathedral, Barbara Mazzei as archaeologist responsible for the Priscilla Catacombs; Emiliano Abrusca, Emiliano Antonelli and Barbara Bassignano (CROMA s.r.l.) as restorers working on the Egyptian sarcophagi. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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