Stabilized and accelerated speckle strain gauge

We have improved the stability and temporal response of the laser speckle strain gauge that detects strain from speckle displacement caused by surface deformation. In the gauge, a narrow laser beam is incident on a diffuse surface, and speckle patterns appearing in the light scattered through the symmetrical angles are detected by a pair of linear image sensors. Speckle displacement is derived by a real-time correlator as the peak of the cross correlation between the current frame and a reference frame of the sensor driven at 1000 frames per second. The reference frame is fixed until the correlation peak becomes lower than a threshold. Using a pair of image sensors, followed by the correlators, and a laser of output power of a few tens of milliwatts, we could measure dynamic strain of polymer films at the frequency between a fraction of hertz and 100 Hz. The resolution of the gauge, which only depends on geometry of the optical system and the sensor pitch, is a few tens of microstrains. The upper limit of the measurement can be extended arbitrarily because small incremental strain is integrated.