Specifications

Version 1.1 rev Jan 2013
Page 69
iXon Ultra
, Features and Functionality
3.5.1.3 - External Exposure (FT)
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 EMCCD is placed in thekeep clean mode. This keep clean mode uses the feature,
Global Clear. 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 27: External Exposure Trigger in Frame Transfer mode