User`s manual

REDLAKE MASD, INC.
3.4.6 Triggered Double Exposure
Triggered double exposure is used in Particle Image Velocimetry (PIV) because of the short time
between successive exposures. A triggered double exposure starts with the user-supplied trigger
as illustrated in Figure 3-1 below.
Figure 3-1. Triggered Double Exposure Timing
Two images are captured in rapid succession by the camera in the triggered double exposure
mode. This is accomplished by capturing the first image in the photo diode array, transferring this
image to the CCD array, and then capturing a second image in the photo diode array. The first
image is transferred from the CCD to the frame grabber while the second image is being captured
by the photo diode array. The second image is then transferred into the CCD array and then on
to the frame grabber’s second image buffer.
Strobes have a light output delay that can be from zero to several microseconds. Obtain this
information from the manufacturer or determine the delay by experiment. This time plus
allowance for light duration must be programmed into the camera via the command TPD
(Transfer Pulse Delay).
The user must use the strobe output pulse, delayed through a pulse generator, to fire the second
strobe within 33 milliseconds (66 milliseconds in single channel mode) after the transfer pulse.
The first image has an exposure period from 1 to 33 milliseconds, as set by the TPD command
and that the second image has a fixed exposure period of 33 milliseconds. It is therefore
necessary to conduct the experiment in a dark environment where the strobe illumination
dominates.
The transfer pulse width internal to the camera is normally 5 microseconds. As the time between
exposures is reduced below 5 microseconds, the transfer pulse width must be reduced by the
same factor. The Transfer Pulse Width is set by the TPW command.
91000064-004 Revision A 3-4 8/29/01