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2.6.3.5 Global Shutter External Exposure Triggering (Overlap Mode)
In overlap mode, every positive edge of an External trigger will trigger a signal frame read out and start a new
exposure. The period of External trigger pulse defines both the exposure time and cycle time. Note that when
an acquisition starts, the first positive edge of the trigger will initiate the first exposure but also output a frame
that has an incorrect exposure which is therefore discarded. The next positive edge of the trigger will end
the first exposure and start a new frame read out. From the figure below, it is can be seen that the minimum
exposure time is approximately two frame read out periods. The first read out period is used to read out the
“Signal” frame while the second is used to read out the “Reference” frame for the subsequent “Signal” frame.
If the period of the external trigger is less than the minimum period discussed above any positive edge occurring
during the “Signal” and “Reference” frames will be ignored. The ARM signal indicates when positive going edges
will be accepted.
Figure 32: Global Shutter External Exposure Triggering (Overlap Mode)
Table 29:
Global Shutter External Exposure (Overlap Mode) Timing Parameters
Parameter
Minimum
Maximum
Exposure (EXT Trig Period)
2 2 InterFrames
30 s
Cycle Time ( EXT Trig Period)
2 2 InterFrames
30 s
External Start Delay (1st Frame Only)
0
1 Row + Charge Transfer Time
External Start Delay (Frame #2... Frame #n)
Charge Transfer Time
Charge Transfer Time
EXT Trig Pulse Width
2 Sensor Speed Clock Cycles
-
NOTE: In global shutter external exposure mode, taking exposures in darkness of 270 ms or more will lead
to increased noise. A reference frame is taken at the start of the exposure and the camera then idles until
the end is signalled, which triggers the image readout. The increased noise is coming from the separation
of the reference frame and the image frame. This is less prominent with deeper cooling.
Summary of Contents for Zyla sCMOS 4.2 PLUS
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