Recording of In-plane Surface Displacement by Double-exposure Speckle Photography

Abstract

Local displacements in the plane of an object surface illuminated by coherent light may be measured by recording double-exposure photographs of it. When the surface is illuminated symmetrically by two oblique beams, 'speckle correlation fringes' appear in the doubly exposed negative, due to the nonlinear nature of the photographic recording. Factors affecting the visibility of these fringes and the range of displacement which can be measured are discussed. The fringe visibility falls to zero for displacements larger than a speckle width, but measurements can then be performed upon the optical transform of the negative image. With uni-directional object illumination, the in-plane displacement can be measured on a point-by-point basis in magnitude and in direction from Young's fringes observed. With two symmetrical oblique illuminating beams the effect of a small surface strain is displayed, even if large lateral displacement has occurred, by using spatial filtering when viewing the photographic image.

Application of Speckling for In-plane Vibration Analysis

“…The Fourier transform of the doubly exposed speckle pattern, which may be registered on a photographic emulsion, reveals interference fringes in the Fourier plane which are a measure for the lateral displacement in the object plane. This was also found by other authors [9][10][11]. In the work to be reported, the speckling was analysed for the study of the mechanical oscillations of a tuning fork of a new electronic watch.…”

“…The other terms are those explained in expression (11). Llu' is the displacement in the image space in the appropriate coordinates.…”

Distance measurements by speckle correlation of objective speckle patterns, structured by the illumination

“…The dynamics of speckles have found several applications within fields such as speckle photography, speckle interferometry, and spatial filtering velocimetry. Typical applications of speckle photography, where speckle pattern intensities are recorded only, are measurements of in-plane displacement/velocity [2], angular displacement/velocity [3], and in-plane object deformation [4] due to, e.g., strain, torque [5], and temperature [6]. In speckle interferometry, interferograms are recorded [7] as they are applied to obtain two-and three-dimensional deformation fields [8] of a deforming object.…”

Multiple rotation assessment through isothetic fringes in speckle photography

“…Double-exposure speckle photography consists of recording a scattering object before and after deformation whose surface is illuminated by a laser beam. 6,7 It is also possible to use white-light speckles to study object deformation. 8 When the two laser speckle patterns are superimposed, no apparently discernible interference pattern results.…”