Measurement of In-Plane Displacement in Two Orthogonal Directions by Digital Speckle Pattern Interferometry

Abstract

The measurement of in-plane displacement in two orthogonal directions is of considerable significance for modern industries. This paper reports on a spatial carrier phase-shift digital speckle pattern interferometry (DSPI) for the simultaneous measurement of in-plane displacement in two orthogonal directions. The object is illuminated from a single direction and observed from four symmetrical directions simultaneously. One pair of the four observation directions is sensitive to in-plane displacement in one direction, and the other pair is sensitive to in-plane displacement in the perpendicular direction, resulting in the displacement in two directions being measured independently. The polarization property of light is used to avoid cross-interference between the two pairs of beams. Spatial carrier frequencies are generated by aperture misalignment, and the displacement in two directions is modulated onto the same interferogram. With a spatial carrier phase-shift technique, the displacement can be separated in the frequency domain and the phase can be evaluated from a single interferogram in real time. The capability of DSPI is described by theoretical discussions and experiments. Appl. Sci. 2019, 9, 3882 2 of 10 (DSPI) [3,12]. The sensitivity of Moiré interferometry and DSPI is higher than that of DIC, and the set-up of DSPI is simpler than that of Moiré interferometry. Because of the high sensitivity and flexibility of DSPI, it has been well-accepted in experimental mechanics, displacement and displacement measurements, and strain measurements.In DSPI, in-plane displacement is measured by evaluating the phase difference of two recorded speckle interferograms under load. The surface under the test is illuminated from two symmetrical directions with a single direction of observation [13], or is observed from two symmetrical directions with a single direction of illumination [14]. The measurement of a surface displacement in two orthogonal directions is often required for the complete characterization of displacement. In traditional DSPI, in-plane displacement is measured in the single direction, depending on the illumination or observation direction. For example, if the surface is illuminated from two symmetrical directions, the measured displacement is in the vector direction, which is the subtraction of the vector directions of the two illumination beams. To measure the displacement in the perpendicular direction, the illumination beams must be rotated 90 degrees around the viewing axis. To make a system sensitive to displacement in two orthogonal directions, two pairs of beams, with different directions, should sequentially illuminate the surface. Thus, two separate interferograms can be recorded and combined vectorially [1,3,8,15]. To avoid cross-interference between the two pairs of beams, they should not illuminate the surface at the same time. Sequential measurement takes more time than usual, making the simultaneous measurement of transient or non-repeatable events, that require displacement in two orth...

Measuring material thickness changes through tri-aperture digital speckle pattern interferometry

“…Yan et al 12 reported the measurement of planar displacement in two orthogonal directions using DSPI with a spatial phase shift. In this case, a combination of four apertures is used where each of them coincides with the surface of a four-sided mirror that deflects the observation point.…”

“…Yan et al 12 . reported the measurement of planar displacement in two orthogonal directions using DSPI with a spatial phase shift.…”