Flow measurement: using a chopper together with a CW laser to realize a frame straddling technique

A simple method that uses a chopper together with a CW laser to realize a frame straddling technique for flow measurement is introduced, in which a chopper with two windows is used to generate double pulses for exposure. By changing the size and position of each window, the time and interval of exposure can be adjusted easily. The light source is a CW semiconductor laser. The advantage of this method is in its being simple, cheap and compact. The method has been validated on a hydrodynamic flow involving a von K?rm?n Vortex Street.