Optics InfoBase: Spotlight on Optics - Measurement of wide frequency range structural microvibrations with a pocket digital camera and sub-pixel techniques

Spotlight on Optics

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July 2012

Measurement of wide frequency range structural microvibrations with a pocket digital camera and sub-pixel techniques

Published in Applied Optics, Vol. 51 Issue 14, pp.2664-2671 (2012)
by David Mas, Julian Espinosa, Ana B. Roig, Belen Ferrer, Jorge Perez, and Carlos Illueca

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Spotlight summary: Non-destructive testing in large civil structures often requires the measurement of low-frequency sub-millimetric displacements. Few are the cases in which this can be achieved by the use of contact sensors, which typically need to be fixed both to the structure under examination and to a stable external structure, creating installation problems. In most cases, the points of the structure under investigation can be far (many meters away) from the observation point and difficult to reach (as in the case of bridges, towers, facades of buildings, etc.) if a contact sensor has to be applied. At present, (seismic) accelerometers are often used for such investigations. However, not only is direct installation of these devices on the structure necessary, but in order to have the displacement time history of the surface where they are applied, double numerical integration is required, which is often a cause of errors.

The authors of this article confronted the problem to perform remote sub-millimeter oscillation measurement of structures, in a contactless and cost-effective manner. The proposed approach is based on the use of a commercial pocket camera, a special target to be applied on the structure under investigation, and the development of a dedicated post-processing software able to efficiently reduce the noise. The proposed method effectively pushes the vision-based measurement approach in structural engineering to sub-pixel accuracy. No installation of transducers on the surface is needed, and the errors due to numerical integration are no longer present.

Even if this approach was demonstrated in only two simple cases, it is surely promising because it can be implemented with virtually any pocket camera without the need of specific software; moreover, if used in combination with high-end cameras characterized by higher spatial and temporal resolutions, it should allow the achievement of higher performances. Even if not directly reported, the simultaneous monitoring of more than one observation point on the scene acquired by the camera should be straightforward; this possibility could allow many simultaneous observations of different parts of the structure with one single measurement session (which in the testing of a large civil structure can be a considerable practical advantage). A possible future advancement of this method should be the possibility to operate without the need of applying the targets on the surface under investigation.

--Lorenzo Scalise

Technical Division: Information Acquisition, Processing, and Display
ToC Category: Image Processing

OCIS Codes:	(040.7290) Detectors : Video
	(100.2000) Image processing : Digital image processing
	(100.4999) Image processing : Pattern recognition, target tracking

Posted on July 10, 2012