Laser-speckles show displacement accompanied by structure change when diffuse objects generating them undergo displacement and/or deformation. This behaviour is theoretically investigated by calculating the cross-correlation functions of speckle intensities before and after the deformation. The dependences of speckle displacement on geometries of object illumination and speckle observation as well as on deformation parameters are clarified. The relations derived are interpreted physically by introducing the shift in diffraction rays due to displacement and deformation of a grating. Distributions of speckle displacement are calculated numerically and shown graphically.

Speckle Displacement and Decorrelation in the Diffraction and Image Fields for Small Object Deformation

New methods are introduced that utilize interferometric comparison of images of diffusely reflecting objects from different hologram plates that are sandwiched together. To make the plate positions optically and mechanically identical during reconstruction and exposure, they are also exposed in sandwiches. If the two sandwiched hologram plates are separated by a small distance a new method of fringe evaluation can be used. Fringes caused by object tilt between two exposures can be eliminated by an analogous, but much larger, tilt of the sandwich hologram during reconstruction. Even the direction of tilt, forward or backward, is found this way.

Sandwich hologram interferometry : a new dimension in holographic comparison

Publisher Summary This chapter discusses the nature of the speckle pattern and defines some of its basic characteristics. The speckle effect in optics is the generation of a random intensity pattern formed when coherent light is scattered by a rough surface or is diffused by a medium containing scattering centers. The chapter concentrates on the instrumental aspects of speckle interferometry and their applications rather than a detailed theoretical treatment. It is seen that a considerable degree of ingenuity has had to be employed to overcome the inherent difficulties that occur when interference takes place among randomly distributed phase fields rather than among smoothly varying ones. Although the aim of the research is to develop techniques useful to the engineer, developments are also being made in the use of speckle patterns for information processing, where the pattern is employed as an optical carrier.

IV Speckle Interferometry

Digital speckle correlation is based on a detailed analysis of changes in speckle images that are recorded from laser-illuminated rough surfaces. The two in-plane components are obtained by cross-correlation of corresponding subimages, a method also known as digital speckle photography. The local gradient of the hitherto inaccessible out-of-plane component is determined from the characteristic dependence of the speckle correlation on the spatial frequency. A detailed experimental study is carried out to analyze the new technique for systematic and random measuring errors. For moderate decorrelation the accuracy of the out-of-plane measurement is better than λ/10 and thus comparable with interferometric techniques. Yet the extremely simple and robust optical setup is suited to nondestructive-testing applications in harsh environments. The quality of the deformation maps is demonstrated in a practical application.

Three-dimensional deformation field measurement with digital speckle correlation

In line with the current interest in speckle motion in free-space propagation, it is proposed to investigate new aspects of the induced speckle motion due to rigid-body movements of a model using as a starting point the well-known concept of homology, first stated in holographic interferometry, the importance of which has only recently been realized in speckle metrology. At the outset, stating the homology conditions leads to a general expression relating the speckle shift to the geometrical parameters at the recording and to the six degrees of movement of the object surface. The expression presented in this explicit form is found to be true for both opaque and thin transparent diffusing models and for different conditions of illumination and observation. The study is then extended to (1) when the object is at rest, subjecting the illuminating source to a small displacement and (2) when the wavelength of the illuminating beam is changed between the two exposures. The case of the rigid-body displacement of the object is experimentally verified and the results are found to be in good agreement with theory. The viability of the method to make measurements is assessed, several applications are envisaged, and some advantages are pointed out.

Speckle motions induced by rigid-body movements in free-space geometry: an explicit investigation and extension to new cases

This paper describes the basic physical principles of defocused speckle photography, and how tilt (differential) topology variations on a scatter surface may be separated from linear displacements. Several rather special features about the scattering process have been revealed, such as the laser source distance from the scatter surface must be considered as well as the light incidence angle. Also, the surface must be treated as mirror facet discontinuities for linear displacements using ray and intensity analogies, and surface topology must be treated in terms of phase and pathlength changes, and a continuous smooth surface. The complementary relationship between focused speckle photography and defocused (topological) speckle photography is shown using specially developed separation techniques. This demanded a detailed look at the ‘speckle process’. The basic physical principles, together with an appropriate analysis technique for the developed speckle photographs, have enabled very small faults and indentations on engineering structures to be detected and measured.

Basic physical principles of defocused speckle photography: a tilt topology inspection technique

Defocused speckle photography is a new inspection technique for recording topological changes on engineering structures — giving information on bulges, plastic deformations, etc. Recording the photographs is a relatively simple technique. In this report we present theoretical information on an analysis technique for extracting the complex speckle information from the photographs. Partially coherent light is used to create virtual fringes on the phtographs, originating from Young's fringes. This analysis procedure is highly sensitive — probably better than holography — and also has a wide measurement range. We present a mainly theoretical description of the technique aimed at practical engineering applications.

D.A. Gregory

Papers

Basic physical principles of defocused speckle photography: a tilt topology inspection technique

Analysis of topological information from defocused speckle photographs