29 Laser Speckle in Dynamic Sensing Applications 345 is not perfect, there will inherently be a range of axial displacements where .z/ 1before and beyond the focal point. It is in these regimes where boiling does not dominate the coherence time and our experimental data shows closer agreement with the theoretical model. 29.5 Conclusions Our experiments have shown that reasonable lateral speed estimates may be produced from laser speckle in the case of pure lateral translation of a diffusing surface. Despite significant signal-to-noise limitations due to the use of a 5 mW laser, speed estimates were within20% of the ground truth for collimating and focusing probes operating below the photo detector bandlimit on surfaces that were far from the focal point. It is likely that lasers used in most PDV applications would produce improved results due to their increased power output. Axial displacement was found to significantly influence speed measurements in regimes where boiling is believed to dominate. By studying the behavior of both a focusing and collimating probe, we posit that the dimension of these boiling regimes may be related to the beam’s radius of curvature. To be more specific: boiling may dominate speckle dynamics when the beam radius of curvature is close to 1at the point of incidence. The details of such a relationship will be explored in future work. References 1. Briers JD (1996) Laser doppler and time-varying speckle: a reconciliation. JOSA A 13(2):345–350 2. Dolan DH, Lemke RW, McBride RD, Martin MR, Harding E, Dalton DG, Blue BE, Walker SS (2013) Tracking an imploding cylinder with photonic doppler velocimetry. Rev Sci Instrum 84(5):055102–055102 3. Goodman JW (1975) Statistical properties of laser speckle patterns. In: Laser speckle and related phenomena, pp 9–75. Springer, Berlin 4. Iwai T, Takai N, Asakura T (1981) The autocorrelation function of the speckle intensity fluctuation integrated spatially by a detecting aperture of finite size. J Mod Opt 28(10):1425–1437 5. Iwai T, Takai N, Asakura T (1982) Dynamic statistical properties of laser speckle produced by a moving diffuse object under illumination of a gaussian beam. JOSA 72(4):460–467 6. Moro EA, Briggs ME (2013) Note: Simultaneous measurement of transverse speed and axial velocity from a single optical beam. Rev Sci Instrum 84(1):016110–016110
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