In µPIV, ensemble averaging is often used to reduce errors due to noise from out-of-focus particles. However, unsteady non-periodic flows require two-frame correlations which can be difficult to achieve with acceptable accuracy using µPIV. The confocal microscope uses pinhole spatial filtering to remove much of the light originating from outside the focal plane, reducing light from out-of-focus particles and improving the accuracy of two-frame cross correlation µPIV. Improvements to two-frame PIV correlations provided by the confocal system are evaluated using the correlation peak signal to noise ratio and universal outlier detection in steady Poiseuille flow as a function of particle volume fraction and focal depth into the channel. We find that the confocal system increases the mean correlation signal to noise ratio for all cases and reduces the fraction of erroneous vectors under conditions where there is a large number of out-of-focus particles. Time-resolved high-speed PIV is demonstrated through the measurement of an example unsteady flow created by an electrokinetic instability.
