Project introduction  (Page 3/3)

Portability is a valuable and highly preferred feature for underdrawing inspection. For cumbersome systems, or systems that aredifficult to relocate, artwork can be moved to the location of the camera for inspection. However, this need for transportation puts theoriginal artwork at risk for damage. Since such great care is needed to move the artwork, a portable camera would be a much better solutionsince it removes this potential hazard. Reducing the risk of damage to the original artwork is critical because, while it is generallypossible to replace a broken camera, it is impossible to replace the artwork.

While analyzing underdrawings does not directly mandate a specific image resolution, one must be able to make out thevarious lines with reasonable clarity in order to derive meaningful results. By image resolution, we mean a pixels / mmspecification for the final reconstructed image of the underdrawing. A camera with a low image resolution could beused, but one would need to take many pictures by zooming in on small parts of the target. These pieces would later need to bealigned and stitched together to view the entire underdrawing, a process which is time consuming and could be quite involved forthe user. A better approach is to use a camera with a larger image resolution which can capture the entire underdrawing orareas of interest with only one or several pictures. According to our initial research, a system that can provide at least 2pixels for each mm of target length should be sufficient . We expect to use our design with small to medium paintings, as indicated by the average target size of 0.33 m x0.33 m. This assumes our system will have a field of view (angular observable range) comparable with the largest artworksize.

Existing technologies

There are a variety of systems that can be used to image underdrawings. Most contain as their central component cameras suchas the two listed in the table below.

Five alternatives for the detector component of such cameras were surveyed by Gargano, Ludwig, and Poldi in their recent comparison ofIR reflectographic systems . They analyzed CCD Si, FPA InGaAs, FPA HgCdTe, InSb detectors, and vidicon tubes. The respective upperspectral limits for these sensors are 1050 nm, 1700 nm, 2500 nm, 5 nm, and 2 nm. Using pigments modeled after those used in the 15-16thcenturies, they measured the pigment transparency that could be achieved with each sensor.

Their conclusion is that InGaAs sensors are best, as they achieved high transparency values and exhibited a large gray range. However,this result only holds for underdrawings make with black, carbon-based drawing implements; since black pigments have only 2% transmittancein the NIR range, they will not be transparent for any of the five devices analyzed. For underdrawings made with iron gall inks, redcrayons, or grayish inks, CCD Si cameras are probably best, since these substances are usually completely transparent over 1200 nm.Our project would focus on providing better imaging for black underdrawings only, since, for imaging other types, the bestsystem, CCD Si, is already relatively low-cost.

Current underdrawing imaging systems that use sensors other than the cheap CCD Si require image scanning to achieve adequate imageresolution. This is because their sensor arrays are small, on the order of 320 x 256 pixels; it is not economically feasible tobuild larger arrays. Over the past decades, scanning systems have made advances in precision of motion control and image assembly . However, in the image reconstruction process, they still face difficulties withperfecting image mosaicing.

The best systems to date resemble the CPS 200E positioning system, which moves the camera while the painting stays stationary . This is advantageous from an art conservation standpoint, in that it reduces wear on the painting. The CPS200E device achieves very accurate motion. However, accurate motion alone is not enough to ensure high image quality. Thecomplexity of the image mosaicing task also requires very precise control of lighting and knowledge about the geometric distortionproduced by the exact positioning of the camera in the scanning frame. Lighting control can be achieved by imaging anillumination control card to detect inhomogeneity. Geometric distortion can be measured by imaging graph paper. Theinformation from these two images is then merged with that of the other images during the mosaicing process.

Aside from the camera and positioning system, the lighting mechanism is the most important element in modern underdrawingimaging systems. To reduce any damage to a painting, high-energy light, such as that from the visible and UV ranges, should befiltered out during imaging. However, the intensity of the light source must be high, since this value greatly influences the SNRof the detector. In one typical imaging setup, a pair of halogen bulbs, each with intensity in the 400 W is used . The light from these bulbs is passed through an RG 1000 filter and a BG 39 filter, which sufficientlyreduces high-energy radiation without damaging IR imaging potential.

The existing solutions to the various aspects of the underdrawing imaging problem - NIR detector, camera-painting positioning system,mosaicing techniques, and illumination schemes - are inadequate. They combine to make systems which, while cheaper than using asingle huge NIR sensor array to take a single image, derive their economical advantage partly at the expense of system portability, accuracy, and simplicity. We intend to implement a differentalternative to the single huge NIR array. Our alternative would improve upon the current solutions by reducing overall systemcost, perhaps not only in monetary terms, but also in terms of portability, accuracy, or simplicity. What current systems offerfor $50,000, we intend to match or beat for under $5,000 .