With the use of an epifluorescent microscope, microscopic flows can be analyzed.  MicroPIV makes use of fluorescing particles that excite at a specific wavelength and emit at another wavelength.  Laser light is reflected through a dichroic mirror, travels through an objective lens that focuses on the point of interest, and illuminates a regional volume.  The emission from the particles, along with reflected laser light, shines back through the objective, the dichroic mirror and through an emission filter that blocks the laser light.  Where PIV draws its 2-dimensional analysis properties from the planar nature of the laser sheet, microPIV utilizes the ability of the objective lens to focus on only one plane at a time, thus creating a 2-dimensional plane of viewable particles.
MicroPIV particles are on the order of several hundred nm in diameter, meaning they are extremely susceptible to Brownian motion.  Thus, a special ensemble averaging analysis technique must be utilized for this technique.  The cross-correlation of a series of basic PIV analyses are averaged together to determine the actual velocity field.  Thus, only steady flows can be investigated.  Special preprocessing techniques must also be utilized since the images tend to have a zero-displacement bias from background noise and low signal-noise ratios.  Usually, high numerical aperture objectives are also used to capture the maximum emission light possible.  Optic choice is also critical for the same reasons.