Aspecto físico espacial

55 In the past years, several types of scanners have

been proposed although for the purpose of this article, we will only briefly remind the working principles of the Image Plane Scanner (IPS) [9, 10]

and of the Moving Lens Scanner (MLS) [11].

In an IPS scanner, the detector array explores the image plane of a lens for large size photo‐

graphy, automatically driven by a mechanical precise XY axes system. As showed in Figure 1, the sensor is moved along rows and columns by steps equal to the width and height of the sensor respectively, and at each step an image is stored.

The movement is so precise that these images can be easily juxtaposed and mosaiced.

A MLS device (Figure 2) works similarly to the IPS except that here the lens moves instead of the sensor. The advantage of a MLS is that, for a given magnification, a larger area of the painting can be inspected at once [11]. This area is directly proportional to its magnification.

The Panoramic‐Head Scanner

A panoramic‐head is a photographic accessory that moves the camera with a rotation around its no‐parallax point (Figure 3).

Super wide angle scenes and even 360° panoramas are captured in this way. For a given type of pano‐

rama, single or multi‐raw, and its angular span, a motorized panoramic‐head computes the number of images to shoot, their amount of overlap and the necessary horizontal and vertical displacements according to the sensor dimension and lens type.

Images are then shot and stitched automatically.

Since the camera rotates around a point, an image of a sphere is recorded and the correct visualization of this panorama should require its projection on a spherical screen, as it happens in a planetarium (Figure 4).

From up to down:

Figure 1. Schematic representation of an Image Plane Scanner, the sensor explores the image plane.

Figure 2. Schematic representation of a moving lens scanner, scanning is performed moving the lens.

Figure 3. A panoramic head. The camera rotates around its no‐parallax point.

In order to assemble a flat “realistic” image, the stitching software must therefore be fed with the shooting parameters and the most appropriate projection type to compensate any distortions.

Since this automatic hardware and corresponding driving software are now commercially available at a reasonable cost, a panoramic shooting tech‐

nique seems an appealing possibility for infrared reflectography. Unfortunately, these systems cannot be used simply by equipping them with an InGaAs camera and modifying the driving soft‐

ware accordingly.

In fact, these devices and stitching software work quite well for distant landscapes but it is very difficult when imaging a painting at short range. In theory, the limited depth of field of photographic lenses enables recording of an acceptable image only for a small area around the point where the panoramic object sphere is tangent to the painting, as shown in Figure 5.

To verify if and up to which extent a panoramic system can be used for infrared reflectography, a simple scanning system was assembled in our Department workshop with a goniometric cradle and a small rotating table, both motorized with stepping motors. The system is driven by a dedi‐

cated software written in LabVIEW.

This “off‐the‐shelf” scanner moves a XenICs Xeva‐1.7‐320 InGaAs camera (spectral sensitivity 0.9 to 1.7 μm, 320 x 256 elements, 30 μm square).

The camera was equipped with a Tamron 500mm f/8 SP macro‐tele lens. This lens is quite compact for its focal length, has few glass elements and a minimum focal distance of about 1.7 m. At this distance the optical magnification is about 3:1 which means that the painting would be sampled at 280 pixels/inch at the center of the scanned area. Dioptric lenses were not considered because

Figure 4. For a panoramic scanning all images lay on a sphere.

The camera shoots from a fixed position moving around its non‐parallax point.

Figure 5. Equatorial section of figure 4. The yellow band represents the in focus zone (depth of field). In red the out of focus for the spherical object plane to the painting is represented.

of their remarkable weight and high number of infrared absorbing glass elements. On top of this, the antireflection coating of each element is optimized for the visible spectral range.

MARCO GARGANO & DUILIO BERTANI

e‐conservation 57 The system was tested on the panel painting of

St John the Baptist (1502‐1507) by Bernardino Zenale in the Bagatti Valsecchi Museum in Milan, as shown in Figure 6.

In a first test, the camera was set perpendicular to the painting at a minimum shooting distance (2 m) to scan an area of about 26 x 90 cm. The final composite reflectogram was stitched with Microsoft’s Image Composite Editor and rectified with Adobe Photoshop, as shown in Figure 8.

A visual inspection of the reflectogram confirmed that the image was acceptably in focus within a circle of about 20 cm radius and that, as expected, the sharpness rapidly decayed outside this circle.

The spatial resolution was 230 pixels/inches and details of the central and boundary parts of this reflectogram are shown in Figure 9.

Figure 6. Panoramic scanner in the Bagatti Valsecchi Museum in Milan.

Though the reflectograms, which were obtained with a simple panoramic scanner, can be consid‐

ered acceptable in most cases, especially for routine inspections, we have investigated which improvements could be implemented to consid‐

erably extend the scanned area of the painting.

The goal is to demonstrate the feasibility of a prototype performing as well as an IPS or a MLS scanner.

Camera Refocusing

IPS and MLS scanners are designed to shoot reflec‐

tograms orthogonally pointing to the painting.

For large paintings a certain number of reflec‐

tograms must be recorded, moving the scanner along an XY path in front of the painting. Each of these images is first corrected to compensate any lens distortions and then stitched using

INFRARED REFLECTOGRAPHY

MARCO GARGANO & DUILIO BERTANI

e‐conservation 59 Previous page:

Figure 7 (left). St John the Baptist, Bernardino Zenale, 1502‐1507, tempera and oil on panel, courtesy of the Bagatti Valsecchi Museum in Milan.

Figure 8 (right). Reflectogram of the panel painting recorded with the panoramic scanning technique.

Figure 9. Details of the central (a) and boundary parts (b,c) of the reflectogram. The sharpness rapidly decay moving to the boundary parts of the scanned area.

A

B

C

Figure 10 (upper). Scheme of a panoramic head: a linear refocusing movement is added.

Figure 11 (lower). After each rotation around its no parallax point, the camera is moved along its optical axis to obtain an in focus image all over the scanned area without affecting the magnification.

INFRARED REFLECTOGRAPHY

the traditional orthogonal‐stitching‐mode to get a composite reflectogram of the entire painting.

IPS scanner and MLS scanner are both well performing systems but rather big and heavy.

In order to get a result comparable with an MLS or IPS system, a panoramic scanner should examine an area much larger than that allowed by the shallow depth of field. To solve this problem we

added a small linear translation stage to refocus the camera on the painting after each displace‐

ment. In order to leave the magnification unal‐

tered, refocusing was done moving the entire camera along its optical axis of the exact local distance from the sphere to the panel.

Preliminary tests to simulate this technique were carried out mounting a photographic camera on a Manfrotto QTVR spherical panoramic head, equipped with a linear stage. The scanning move‐

ments are schematically shown in Figure 10.

The scheme of the refocusing method is shown in Figure 10. As the camera rotates around the non parallax point, a calibrated displacement is applied by the linear translation stage. In this way, the optical magnification does not change as it would for refocusing by moving the lens.

To evaluate the amount of the distortions of the final mosaic, intrinsic in this type of scanning, a regular grid pattern was printed and shot.

Since the scanning method has a circular sym‐

metry, to simplify the test we placed this grid as shown in Figure 12 to examine only one quadrant of the entire area. As expected, the distortions are larger in the boundary regions, as can be seen in Figure 12 where an enlarged detail of upper right corner is shown.

As an example, the local amount of these distor‐

tions is visualized in Figure 13 where the same detail of Figure 12 is overlaid to the reference grid, in red. This calibration should be carried out for any of the shooting distance. Since the resolution decreases as this distance increases, it is advisable to always shoot at the minimum distance and therefore calibrate the system for this maximum resolution. An alternative method is the image‐to‐image registration of the reflec‐

Figure 12 (upper). A reference regular grid is placed in the upper right quadrant of the considered area. Distortions increase from the center outward as shown in the enlarged detail.

Figure 13 (lower). To visualize the distortions, the detail of figure 12 is overlaid with the reference grid, in red.

MARCO GARGANO & DUILIO BERTANI

e‐conservation 61 Figure 15 (above). Details of the central (A) and boundary parts (B, C) of the reflectogram. With the refocusing technique the sharpness cover the whole scanned area.

A

B

C

Figure 14 (left). Reflectogram recorded with the panoramic scanning with the refocusing technique.

INFRARED REFLECTOGRAPHY

togram with a high resolution photograph of the painting as a reference. This post processing can be done with a geospatial image processing software such as Erdas Imagine.

Since the distance panel‐camera changes after each displacement, in order to keep this distance unaltered, the camera was mounted on a linear translation stage to move back to the correct focus distance. The driving software was updated with an automatic refocusing algorithm.

The system was tested on the St John the Baptist panting and a sharp reflectogram of 90 x 26 cm was successfully recorded (Figure 14). Details of the central and boundary parts of this reflecto‐

gram are shown in Figure 15.

Conclusions

The preliminary tests carried out in laboratory and in situ proved that a panoramic scanning technique is a reliable method for infrared reflec‐

tography, provided that the infrared camera is mounted on a linear stage for refocusing. With such a device a reflectogram up to 1 x 1 m with a resolution of 280 pixels/inch can be recorded.

The system is comparable with other high resolu‐

tion scanning devices, with the advantage of a better portability and a lower cost. The complete reflectographic system weights about 6 kg and can be built with less than€20,000.

In the near future, we plan to consider commer‐

cial mechanical scanning systems such as gimbals, U‐fork mount, goniometric cradle/rotary table to find the best set‐up in terms of cost and perfor‐

mance. A dedicated shooting and stitching soft‐

ware will be written taking into account the mecha‐

nical characteristics of the chosen approach.

Nevertheless the successful outcome of this approach, this refocusing panoramic shooting technique requires further testing with different cameras for other diagnostic imaging techniques such as infrared False Color, UV photography and thermography.

Acknowledgments

The authors are very indebted to L. Pini, Director of the Bagatti Valsecchi museum for granting permission to examine the Bernardino Zenale’s painting and to the staff of the museum for the kind collaboration. F. Cavaliere and D. Viganò of the mechanical workshop of the Physics Depart‐

ment of the Università degli Studi di Milano are thanked for providing essential help in assembling the motorized scanning prototypes. Figures 3, 4 and 10 were drawn using Google Sketchup and the models are present in its web‐database.

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