Anna Pelagotti

Multispectral imaging of paintings

Identifying the materials of a painting is a crucial step in any conservation process. When the objective is to prepare an intervention plan, it is absolutely necessary to understand the matters the restorer is going to encounter. Also, when the aim is a better understanding of the artwork, and perhaps an authenticity check, it is highly relevant to know which materials were employed, since they may differ depending on the period of execution and on the specific artist as well. To identify materials on a paintings surface in a reliable manner, currently the most popular and trustworthy method is the analysis of microsamples of the paint layer. However, chemical analyses although reliable, have a number of drawbacks. The first is linked to the fact that this is an invasive method. The samples used need to be detached from the painting and will not be put back in place. Moreover, the achieved results are in principle - and often also in practice - valid only for that specific specimen and cannot generally be extended to neighboring areas.

Imaging live humans through smoke and flames using far-infrared digital holography

The ability to see behind flames is a key challenge for the industrial field and particularly for the safety field. Development of new technologies to detect live people through smoke and flames in fire scenes is an extremely desirable goal since it can save human lives. The latest technologies, including equipment adopted by fire departments, use infrared bolometers for infrared digital cameras that allow users to see through smoke. However, such detectors are blinded by flame-emitted radiation. Here we show a completely different approach that makes use of lensless digital holography technology in the infrared range for successful imaging through smoke and flames. Notably, we demonstrate that digital holography with a cw laser allows the recording of dynamic human-size targets. In this work, easy detection of live, moving people is achieved through both smoke and flames, thus demonstrating the capability of digital holography at 10.6 μm.

Optical reconstruction of digital holograms recorded at 10.6 μm: route for 3D imaging at long infrared wavelengths

We demonstrate the optical reconstruction in the visible range (0.532 microm) of digital holograms recorded at long IR wavelengths (10.6 microm) by means of a spatial light modulator. By using an integrated recording-reconstruction system, it is, in fact, feasible to achieve direct imaging of holograms acquired outside the visible range, i.e., in the IR spectrum. By choosing a Fourier recording configuration, the reconstructed image, obtained at about a 20 times shorter wavelength than the acquisition image, exhibits minor aberrations, which do not significantly affect the optical reconstruction. The high NA achievable at a long IR wavelength allows us to image large objects at reasonable distances.

An automatic method for assembling a large synthetic aperture digital hologram

A major issue so far for digital holography is the low spatial resolution generally achieved. The numerical aperture is limited by the area of currently available detectors, such as CCD sensors, which is significantly lower than that of a holographic plate. This is an even more severe constraint when IR sensors such as microbolometers are taken into account. In order to increase the numerical aperture of such systems, we developed an automatic technique which is capable of recording several holograms and of stitching them together, obtaining a digital hologram with a synthetic but larger numerical aperture. In this way we show that more detail can be resolved and a wider parallax angle can be achieved. The method is demonstrated for visible as well IR digital holography, recording and displaying large size objects.

Reliability of 3D Imaging by Digital Holography at Long IR Wavelength

Digital Holography (DH) in the infrared (IR) range presents some peculiar aspects compared with the more common DH in the visible range. The current major drawback is due to the size of the pixel pitch of presently available thermal cameras, which is rather large compared to what would be optimal, and what is possible with analog films. However, since the

An automatic registration algorithm for cultural heritage images

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Visible reconstruction by a circular holographic display from digital holograms recorded under infrared illumination

laser wavelength is 15 times longer than average visible wavelength, a much higher stability, a wider view angle, and shorter acquisition distances are achievable, allowing easier acquisition of large object holograms.

Infrared digital holography for large objects investigation

In art diagnostics it is often needed to compare and integrate different sets of information, coming from different sources, and stored in different images. In order to successfully integrate these data, images corresponding to the same areas need to be registered, that is a geometrical transformation that aligns points in one picture with corresponding points in another picture needs to be found and applied. In this paper we present an automatic registration technique for multispectral images, based on the computation of mutual information. We applied this method to obtain RGB images from multispectral data, and derive a (undersampled) spectral signature per pixel of the painting.

LAYERED MOTION ESTIMATION

A circular holographic display that consists of phase-only spatial light modulators is used to reconstruct images in visible light from digital holograms recorded under infrared (10.6 μm) illumination. The reconstruction yields a holographic digital video display of a three-dimensional ghostlike image of an object floating in space where observers can move and rotate around it.

Compression of digital holograms via adaptive-sparse representation.

In this work we show several acquisition setups and techniques which make it possible to obtain holographic recording and reconstruction of large objects by means of Infrared Digital Holography (IDH). In previous works it was demonstrated that, using the long wavelength coherent radiation produced by a CO2 laser instead of visible radiation, it is possible to obtain advantages in terms of larger field of view and lower seismic noise sensitivity. The only drawback using this wavelength is represented by the low resolution of current recording devices in this spectral region. The reported methods may have industrial applications where investigation of large dimension samples is needed.