In this study, light emitted from red, green, and blue LEDs is adopted as the light source of an alternating-color micro-PIV system for micro flows measurement. The strobe frequency of the LED can easily reach 20,000Hz, which is high enough to provide the time resolution required for most micro flows measurement. A cardioid annular condenser is adopted in the micro-PIV system so that the incident light from the LED is redirected and only the light emitted from fluorescent particles reaches the object lens of the microscope. The image quality is significantly improved. Clear dark background particle images of micro flows are recorded from the proposed LED micro-PIV system. In addition, the diffraction limit of the microscope is improved from half of the wavelength to one-fifth of the wavelength. For alternating-color multiple-exposure image application, a triple-exposure alternating-color image — sequentially illuminated by red, green, and blue LED light — is recorded on a single frame by a color CCD camera. Three unique color images — a blue, a green, and a red image respectively — are obtained from separating the triple-exposure image. With three sequential images available, the velocity, acceleration distributions of micro flow, and different phases of the multiphase flow can be measured from these unique color sequential images. The alternating-color micro-PIV system is then applied to measure micro flow past a cylinder, circulation in a micro-droplet, hydrodynamic focusing sheath flow, and two-phase flow in micro channels. Satisfactory results are obtained for all the flows measured.

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