text: To perform PIV analysis on the flow, two exposures of laser light are required upon the camera from the flow.  Originally, with the inability of cameras to capture multiple frames at high speeds, both exposures were captured on the same frame and this single frame was used to determine the flow.  A process called autocorrelation was used for this analysis.  However, as a result of autocorrelation the direction of the flow becomes unclear, as it is not clear which particle spots are from the first pulse and which are from the second pulse. Faster digital cameras using CCD or CMOS chips were developed since then that can capture two frames at high speed with a few hundred ns difference between the frames.  This has allowed each exposure to be isolated on its own frame for more accurate cross-correlation analysis.  The limitation of typical cameras is that this fast speed is limited to a pair of shots.  This is because each pair of shots must be transferred to the computer before another pair of shots can be taken.  Typical cameras can only take a pair of shots at a much slower speed.  High speed CCD or CMOS cameras are available but are much more expensive.