Here, C is the tracer particle concentration light sheet thickness image magnification, particle image diameter [m] and is the interrogation spot diameter [m]. The source density represents the type of image that is recorded. A source density larger than unity means that the image is a speckle pattern and when the source density is less than unity, the image consists of individual particle images. The image density represents the mean number of particle images in an interrogation region. It should larger than 10-15 for a good PIV measurement.

The optimal pulse separation between two images is influenced by a number of parameters. Two main types of error affect the choice of pulse separation () very much. These are random error and acceleration error. Random error arises from noise during recording of images and subsequent interrogation of the particle images. Acceleration error arises from approximation of Lagrangian motion of tracer particle to local Eulerian velocity based on small particle displacement. These two kinds of errors contradict the selection of pulse separation. The random error contribution can be reduced by increasing but acceleration error increases as increases. At some intermediate value of , the total error should be a minimum. The optimum separation can be derived by considering these two errors as