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Features & Functionality
3.5.1.3 - External Exposure (FT)
External Exposure combined with Frame Transfer readout is available only on iXon+ 885 cameras. It is not
available for other iXon3 models, since this mode requires an 885 Global Clear function (please see page 57). This mode
is distinct from the triggering modes discussed previously in that the exposure period is fully controlled by the width
of the external trigger pulse. The exposure period starts on the positive edge and concludes on the negative edge. As
illustrated in the timing diagrams below, the positive edge can occur either after the previous image has been completely
read out or while it is still being read. The ability to overlap the readout with the exposure period allows for very high
frame rates. In order to ensure that light falling on the Image area before the start of the exposure does not contribute to
the measured signal, the CCD is placed in a special keep clean mode. This keep clean mode uses the feature, Global
Clear, which is only available on a limited range of CCD sensors and hence not available on all iXon3 cameras. Although
the start of the exposure can overlap the read out phase of the previous image, the end of the exposure cannot. This is
because the end of the exposure is marked by shifting the Image area into the Storage area. It is not possible to use the
same feature as is used to prevent light that fell before the exposure starts from contributing to the measured signal as
this would cause the already accumulated charge to be cleared. NOTE: If the falling edge occurs during the read out
phase it will be ignored and the next falling edge will terminate the exposure.
Figure 31: External Exposure Trigger in Frame Transfer mode (885 model only)
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