The frame grabber needs 40 ns to lock onto the trigger signal. Afterwards, the control data can be transferred to the camera. The exposure time is controlled by the external trigger from the synchronizer in a user-defined range between 100 ns and 1 ms. Before the second exposure, the camera has a frame straddling time of 200 ns or 1 which depends on the parameter settings of the cross correlation function. Before the next double exposure can be started, data of the first image pair is transferred to the frame grabber.

The laser must be synchronized to the double exposure mode of the camera. For emitting a laser pulse, a high energy must be generated in the laser cavity. The laser cavity has a Nd:YAG rod that is pumped with energy from a flash lamp. There is a nonlinear relation between the time the cavity is pumped and laser power emitted. During the pumping procedure, the mirror at the far end of the cavity is closed by a Q-switch. The success of PIV measurements depends crucially on the time correlation between laser pulse generation and camera recording achieved by the synchronizer unit. Figure 3.16 shows the timing diagram for the pulsed laser with double shutter CCD camera.

Fig 3.16: Timing diagram for CCD camera and double pulsed laser (PIV Manual, Oxford Lasers).