|NDT.net - December 1999, Vol. 4 No. 12|
6th World Conference on NDT and Microanalysis in Diagnostics and |
Conservation of Cultural and Environmental Heritage, Rome, 1999 May.
Published by AIPnD, email: firstname.lastname@example.org
|TABLE OF CONTENTS|
A typical example of application of laser interferometry is the study of externally induced vibrations of frescoes for individuation of defects, layer-to-layer detachments, delaminations and surface cracks. On a smaller scale, holographic and interferometric techniques can be used for precise determination of surface profiles and study of small changes of the object in time.
In this paper we discuss the application of interferometric methods for reconstruction of surface profiles of paintings and statuettes and their changes under the effect of external perturbations. Results are also discussed on application of real-time speckle interferometry for the study of surface deformations under stress.
The main characteristics of laser sources is the coherence of light emitted, which is in turn associated with the possibility of producing interference pattern using different waves emitted from the same source or scattered and diffused by the object under study1. A typical interference experiment is sketched in fig.(1)
|Fig 1: Michelson-type interference experiment for measurement of surface profiles|
The above depicted scheme allows the measurement of surface profiles from the fringe pattern due to the interference of the reflected beams coming from the object and the reference mirror. According to this scheme, the spacing between two interference fringes corresponds to a variation of the profile of one half of the laser wavelength, i.e. about 0.3 (m using a typical laser source. However, this high sensitivity makes the simple scheme shown in fig.(1) unappliable for surface contouring of relievs and depths higher than a few microns, as well as for measurements of surfaces with microstructures of the same scale of the laser wavelength. We will see in the following that most of the methods currently used for surface profiling deals exactly with the reduction of the sensitivity of the system to values comparable with the actual scale of surface variations of the object under study. Before discussing some of these methods, let us briefly introduce the main characteristics of laser holography and holographic interferometry.
|Fig 2a: Simple set-up for hologram recording||Fig 2b: Hologram reconstruction|
The main and most impressive property of a hologram is the fact that it reproduces the exact shape the real object. Since the hologram carries information on both amplitude and phase of the scattered wave, the virtual image shows the same 3D structure of the real object; the realism of the reconstructed image may be astonishing. Moreover, colour holograms can be recorded whit the simultaneous use of three collinear lasers at wavelengths corresponding to the fundamental RGB colours.
In the field of Cultural Heritage, the applications of standard and colour holography might be very interesting, in view of possible 'virtual exhibitions' where holographic copies could be exposed instead of originals, thus saving the cost and danger of transporting precious and fragile piece of artworks. The possibility of holographic recombination of fragments of the same object, owned by different museums, has also been proposed2.
Since the holographic reconstruction of an object carries all the information of the real object, an interference pattern can be recorded taking two exposures of the same object at different times3. One can consider the first exposure as a reference hologram which interferes with the second virtual image; the result is a fringe pattern which gives information about all the changes of the object between the first and second exposure. This method, called Holographic Interferometry, is particularly useful for detecting stressed or damaged regions of the object under study when exposed to external perturbations (vibrations, shocks, etc.)4; it can be usefully applied, for example, for contactless early diagnostics of detachments and delaminations of frescoes from their substrate5. The drawback of Holography and Holographic Interferometry is the need for a special substrate (usually a holographic plate); in case of real time Holographic Interferometry the plate must be developed in situ and held in the same position with great precision. This is in general unpractical, so that, in general, different interferometric approaches are used when real time 'in field' analysis is required; we will discuss in the following one of this, called Electronic6 (or Computer Aided)7 Speckle Pattern Interferometry (ESPI/CASPI).
Since the speckle pattern is related to the profile of the scattering surface, the comparison of different images taken at different times gives information about the changes of the surface itself occurred in between the two exposures. The image processing procedure can be done using analogic electronic systems (ESPI) or it can be performed digitally by a computer (CASPI). In both cases, a real time sequence of images representing the interference patterns is generated, which can be continuously displayed on a TV set or on the computer monitor. In this way, it's particularly easy to investigate the response changes of the object under study as a function of the external perturbation parameters (for example, the frequency of an external acoustic perturbation). This technique could help in detecting particular zones of a fresco which can be resonantly excited at given acoustic frequencies, thus in danger of damage or detachment.
An alternative system for shape reconstruction, which is particularly useful for large and distant object, has been recently developed by **** 10. In this scheme, a solid state laser is intensity modulated at high frequency (around 100 KHz) and sent on the object; the reflected light is recollected through a large aperture telescope and compared in phase with the modulating signal. The phase shift between modulating and scattered signal is a measure of the distance of the object from the source. The advantage of this system is the high sensitivity even at long distances, an infinite depth of field of the resulting image and total absence of shadows. The intensity of the reflected light also gives a measure of the reflectivity of the object. On the other hand, the application of this method of artificial vision requires a scan of the laser over the whole surface of the object, which can be rather time consuming in case of large surface. Instrumental limitations on detection of very small phase shifts also make this system not practical for shape reconstruction of very small objects.
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