The duration of the illumination light pulse must be short enough that the motion of the particle is frozen during the pulse exposure in order to avoid blurring of the image. The light from each pulse scattered by the tracer particles is recorded by a CCD sensor on separate frames. The time delay between the illumination pulses must be long enough to be able to determine the displacement between the images of the tracer particles with suffcient resolution. It should be short enough to avoid particles with an out-of-plane velocity component leaving the light sheet between subsequent illuminations. Analyzing one image pair, it is possible to identify the path a particle has traveled. Knowing the time delay between the two pulses, velocity can be calculated. The time interval between two pulses has to adjusted according to the mean ow velocity and the magnication of the camera lens. The particle displacement must be small relative to the nest flow scale to be resolved.

After finding the displacement of each interrogation spot, the quantity is divided by and the magnification factor M of the image system to calculate the first order approximation of the velocity field as follows:

Hence