Optical Tecniques for Cultural Heritage


Holographic and Speckle Interferometry

Image Decorrelation



 

Holographic and Speckle Interferometry


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A diagnostic centre, modelled along the lines suggested by the fundamental research in coherent optics, has been developed at the University of L'Aquila since '70s years. The aim was to define non destructive optical methodologies in the diagnostics of material characteristics. Holographic interferometry is a well known and established technique. It gives precise information about the location and size of a wide variety of defects through an image of the object under test covered with a quickly interpretable fringe pattern. A deformation of the fringes indicates the presence of a defect.

Figure 1
Figure 1 shows a famous interferogram realized by Prof. Gori and coworkers in the Laser Laboratory in the early '70s. In this "Santa Caterina" by Pier Francesco Fiorentino (38 cm x 49 cm x 4 cm, poplar wood, about 1450) detached regions are revealed.


Also holographic contouring techniques have been widely investigated.

Figure 2 shows contouring fringes obtained by sandwich holography on a marble head of roman age.
Figure 2
Figure 3
In figure 3 is shown holographic contouring test on a wooden carving of the XIII century. In spite of these beautiful results, some shortcomings (mainly stringent stability requirements and high costs) prevented a significant transfer of holographic techniques from research laboratories. In recent years, the technique of Electronic Speckle Pattern Interferometry (ESPI) proved to be a valid alternative. The ESPI system makes finally possible to perform measurements in situ.
Figure 4 shows a portable ESPI system working on the mural fresco "Incoronazione dell'Assunta" (XV century) inside the Cathedral S. Maria di Collemaggio (L'Aquila - Italy).
Figure 4
Figure 5
In figure 5 we can see another example of in situ measurement. The panel under test was the "Madonna col Bambino, Santi ed Angeli Musicanti" of the Antonio Solario's school (about 1525) in the Museo Capitolare di Atri (Italy). An electronic speckle pattern interferometer can be assembled from commercially available components. The theory and practice of fringe interpretation are well established. Decreasing costs of digital equipment have made it possible to carry out highly automated and sophisticated analysis of interferograms for a quantitative evaluation of the defects.
Figure 6 shows the fresco "Adorazione dei Magi" by Farrelli (1667) in the church of S. Maria della Croce in Roio (L'Aquila - Italy).
Figure 6
Figure 7 and figure 8 show the particular area of the fresco highlighted in figure 6 before and after restoration of the fresco itself.
By image elaboration a precise location of the defect is possible.
Figure 7
Figure 8

 


D.PAOLETTI and G.SCHIRRIPA SPAGNOLO
"Interferometric methods for artwork diagnostic"
Progress in Optics XXXV, 197-255, (1996)

G. SCHIRRIPA SPAGNOLO
"Electronic Speckle Pattern Interferometry: an aid in cultural heritage protection"
Trends in Optics vol 3, 299-326, (1996)

S.AMADESI, F.GORI, R.GRELLA and G.GUATTARI
"Holographic methods for painting diagnostics"
Applied Optics 13, 2009-2013, (1974)

S.AMADESI, A.D'ALTORIO and D.PAOLETTI
"Sandwich holography for painting diagnostics"
Applied Optics 21, 1889-1890, (1982)

D.PAOLETTI, G.SCHIRRIPA SPAGNOLO and A. D'ALTORIO
"The state of art of holographic non destructive testing in work of ar t diagnostics"
Revue Phys. Appl. 24, 389-399, (1989)

P.CARELLI, D.PAOLETTI, G.SCHIRRIPA SPAGNOLO, A.D'ALTORIO
"Holographic contouring method: application to automatic measurements of surface defects in artworks"
Optical Engineering 30, 1294-1298, (1991)

D.PAOLETTI, G.SCHIRRIPA SPAGNOLO, M.FACCHINI and P.ZANETTA
"Artwork diagnostics with fiber-optic digital speckle pattern interfer ometry"
Applied Optics 32, 1-6, (1993)



 

Image Decorrelation


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Holographic interferometry is a well known tool in artwork diagnostics. However, it can not easily be used in routine analysis. Electronic Speckle Pattern Interferometry (ESPI) represents a significant improvement. Interesting results can also be obtained by digital speckle pattern correlation. This technique permits the developments of portable, safe, and easy-to-use instruments for out-of-laboratory diagnostics of wooden artifact.

The principle of the method consists of the evaluation of a local parameter that estimates the decorrelation of speckles after any modification of the test object. It can be shown that this method is suited to the in situ detection of subsurface detachments or surface alterations of wooden artifacts. In practice, two speckle pattern images are acquired and stored. A digital subtraction is performed and displayed after a square-law operation. Then, a convolution with a small matrix performs an ensemble average via an average in the neighborhood of image points. Where decorrelation occurs, bright areas become visible.

An application of the method is showed on Applied Optics cover. On the left we can see a wooden statue by Pietro Alemanno (15th century). This statue, height about 20 cm and covered by a gold paint, is believed to represent a young girl. On the right we can see the comparison between holographic and correlation methods.

 

As another example, we inspect an icon of Byzantine age. The (b) image shows a 5-day growth of mold on the surface as detected by the correlation method.
Icona1.JPG (268667 byte) Icona2.JPG (105062 byte)

 


G.SCHIRRIPA SPAGNOLO, D. AMBROSINI, D.PAOLETTI
"Image decorrelation for in situ diagnostics of wooden artifacts"
Applied Optics 36, 8358-8362, (1997).

G.SCHIRRIPA SPAGNOLO, D.PAOLETTI, D. AMBROSINI, G. GUATTARI
"Electro-optic correlation for in situ diagnostics in mural frescoes"
Pure and Applied Optics 6, 557-563, (1997).