Filter
Top Products
Chapter 6 Advanced Topics 71
71
The PreOpen {Open Before Trigger} mode is useful in cases where an External Sync
pulse cannot be provided 8 ms or ~ 20 ms (the length of time the 25 mm or 45 mm
mechanical shutter takes to open) 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 {Normal} 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.
Figure 31. Timing Diagram for External Sync Mode (+ edge trigger)
External Sync with Continuous Cleans {Clean Until Trigger}Timing
Another timing mode available with the PIXIS is called Continuous Cleans {Clean Until
Trigger}. In addition to the standard "cleaning" of the array, which occurs after the
camera is enabled, this mode will remove any charge from the array until the moment the
External Sync pulse is received.