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54 PI-MTE System Manual Version 4.C
Shutter opens
Shutter remains open
for preprogrammed
exposure time
System waits while
shutter closes
Shutter opens
Controller waits for
External Sync pulse
Controller waits for
External Sync pulse
(shutter preopen) (shutter normal)
Figure 17. Chart Showing Two External Sync Timing Options
The PreOpen mode is useful in cases where an External Sync pulse trigger edge cannot
be provided 5–28 msec (shutter open time) before the actual signal occurs. Its main
drawback is that the CCD is exposed to any ambient light while the shutter is open
between frames. If this ambient light is constant, and the triggers occur at regular
intervals, this background can also be subtracted, providing that it does not saturate the
CCD. As with the Normal Shutter mode, accurate background subtraction may not be
possible for the first frame.
Also note that, in addition to signal from ambient light, dark charge accumulates during
the “wait” time (t
w
). Any variation in the external sync frequency also affects the amount
of dark charge, even if light is not falling on the CCD during this time.
Note: If EXT SYNC is still active (in Figure 18, this means that if it is still LOW) at the
end of the readout, the hardware may interpret this as a second sync pulse, and so on.
Figure 18. Timing Diagram for External Sync Mode (- edge trigger)