Analyse the civ1 results with the uvmat programme (field velocity).
The correlation values are indicated by colors red, green, blue (with thresholds adjustable by sliders in the upper right). The correlation is poor for red vectors, and excellent for blue ones. Vectors with anomalies detected in the civ processing (vec_F different of 1) are indicated in black (if the default option show black is checked at the upper right) . You can check the values of vec_F for each vector by selecting it with the mouse: press the pushbutton select with the zoom option on). When selected the vector color becomes magenta and its properties appear in the text window at the upper right (index in the file, x, y coordinates, u, v, and flag values in case of anomaly ).
A common kind of anomaly is the systematic presence of black vectors in regions of large velocity, with values vec_F=-2. This indicates that the search box is too small, and CIV1 has to be run again with a larger search box (if the vector field looks good), or a smaller time interval dt (if many erratic vectors are observed).
After image quality and civ1 processing have been optimized, move to next step.
Note: The statistics on correlations vec_C provides in principle a convenient tool to optimize the time delay dt. For small dt, the correlations are only due to image noise (low luminosity) and possibly vibrations/fluctuations of the laser sheet. For larger dt, correlations decrease due to local turbulence and particle motion out of the laser sheet, as well as strain or rotation effects. The statistics on vec_C over a (rectangular) domain of interest, and time interval, can be obtained by the option stat_vs_time.