The synchronizer acts as an external trigger for both the camera(s) and the laser.  While analogue systems in the form of a photosensor, rotating aperture and a light source have been used in the past, most systems in use today are digital.  Controlled by a computer, the synchronizer can dictate the timing of each frame of the CCD cameras sequence in conjunction with the firing of the laser to within 1 ns precision.  Thus the time between each pulse of the laser and the placement of the laser shot in reference to the cameras timing can be accurately controlled.  Knowledge of this timing is critical as it is needed to determine the velocity of the fluid in the PIV analysis. Stand-alone electronic synchronizers, called digital delay generators, offer variable resolution timing from as low as 250 ps to as high as several ms. With up to eight channels of synchronized timing, they offer the means to control several flash lamps and Q-switches as well as provide for multiple camera exposures.