Resolution Considerations For Structured Illumination Microscale Particle Tracking Velocimetry

In microscale particle velocimetry, the spatial resolution of velocity measurements along the optical axis is often degraded by signal from tracer particles outside the focal plane. Structured illumination microscopy particle tracking velocimetry (SIM PTV) can eliminate most of this out-of-focus signal by using illumination with a non-zero spatial frequency to preferentially illuminate the in-focus particles. Two such (raw) images can then be combined to eliminate the background signal and demodulate the image. T he objective of this study was to quantify and optimize the spatial and temporal resolution of SIM PTV based upon Poiseuille flow in a microchannel at Reynolds numbers Re 0.02. The axial spatial resolution, estimated for this known velocity profile from the standard deviation of the velocity measurements, is improved (i.e., reduced) by at least a factor of 2 compared with the results for a uniformly illuminated flow. This axial spatial resolution, which is given by the point spread function of the imaging system, is in good agreement with theory. The spatial frequency of the illumination that optimizes the spatial resolution is a function of the scattering area of the tracer particles. In terestingly, increasing the number of raw images does not appear to improve the axial resolution. Finally, the temporal resolution of SIM PTV is estimated based upon both image and velocity acquisition times.