Displacement Measurement from Double-exposure Laser Photographs

TLDR

Surface displacement is measured by recording a double‑exposure laser photograph and processing the speckle pattern. The method analyzes the speckle image either point‑by‑point or via spatial filtering to resolve motion orthogonal to the line of sight, can determine surface tilt from a defocused double exposure, and can be extended to vibration measurement using a time‑averaged exposure and fringe‑pattern analysis. The technique’s usable range for lateral translations and rotations was theoretically and experimentally bounded, with lens aberrations and surface scattering identified as key limiting factors.

Abstract

Surface displacement can be measured by recording a double-exposure photograph of the laser-illuminated object, followed by optical processing of the recorded speckle-pattern image. The analysis can either be on a point-by-point basis, or by a spatial filtering technique which resolves the motion in any desired direction orthogonal to the line of sight. The limits within which the technique may be used to measure lateral translations and rotations of the surface are examined theoretically and experimentally, and the effect of lens aberrations and surface scattering properties are discussed. Surface tilt may also be measured by recording a defocused double exposure image and analysing its optical transform. The photographic techniques described can be extended to measuring surface vibration, by recording a single 'time-averaged' exposure and examining the modified optical transform fringe pattern.