Axial and temporal resolution of structured illumination microscopy particle tracking velocimetry

In microscale particle velocimetry, particles outside the focal plane can add noise and increase measurement error. Structured-illumination microscopy particle tracking velocimetry (SIM PTV) uses spatially varying illumination to enhance the signal from the tracer particles in the focal plane of the imaging system.  A pair of “raw” images are acquired of the flow with a phase shift in the sinusoidally varying illumination. A single PTV image is then reconstructed from these raw images via a Hilbert transform, eliminating most of the signal outside the focal plane.  Experimental SIM PTV results are presented for microchannel Poiseuille flow at flow speeds of O(0.1 mm/s), or Re = 0.1, using 3 μm tracer particles.  Improved reconstruction enables sub-pixel location of isotropic particle images. The effect of illumination spatial frequency upon axial spatial resolution (i.e., the thickness of the imaged slice) and the number of velocity samples is characterized and compared with previous theory.  Results are presented that illustrate how the axial spatial resolution of SIM depends upon the object characteristics.  Finally, the double-exposure SIM approach used here is compared with other SIM techniques using more raw images.