iXon Ultra and iXon3 Trigger Modes
Andor's EMCCDs offer a comprehensive range of internal, software and external trigger
modes. Furthermore, software and internal trigger modes avail of cutting-edge firmware
and SDK enhancements, delivering enhanced speed performance during complex software
acquisition protocols (iCam). On-head storage of multiple exposure times facilitates rapid
exposure time switching upon receiving a trigger.
Part 1 - External Trigger Modes
The iXon back-illuminated range of cameras have several different
external trigger modes:
External Trigger in Frame Transfer Mode (Simultaneous
Exposure and Readout)
In this mode, the camera sits in its 'External Keep Clean' cycle, which
can be interrupted by the external trigger with a jitter of only a couple
of microseconds (exact dependent on camera model). Upon receiving
a trigger, the camera stops all vertical clocking and waits for the
programmed user delay period before starting the read phase. During
the readout phase the Image area is transferred rapidly to the Storage
area. The Storage area is then read out in the normal way.
Once the read out is complete the camera continues to wait for the
next external trigger event. While the camera is waiting for the trigger
event the shift register is continually clocked but the Image and
Storage areas are not. On the next trigger the camera again waits for
the programmed delay before starting the read out phase. The camera
continues in this cycle until the number of images requested have
been captured. Since the Image area is not cleaned between trigger
events, the exposure time is defined by the time between trigger
events. This sequence is illustrated in Figure 1.
External Exposure combined with Frame Transfer Mode
(available only on iXon3 885 cameras)
This mode is not available for other iXon models, since it requires
an 885 Global Clear function, whereby the anti-bloom structure of
each pixel is used to drain the pixel of charge outside of the exposure
period. This mode is distinct from the trigger 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 (Figure 2) 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 iXon cameras. Although the start of the exposure can
overlap with 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.
'External Trigger' in Non-Frame Transfer Mode
In this mode, the camera is once again sitting in 'External Keep
Clean'. As can be seen from Figure 3, the External Keep Clean Cycle
runs continuously until the external trigger event is detected, at which
point the exposure phase starts. Once the exposure time has elapsed
the charge built up in the Image area is quickly transferred into the
Storage area. From the Storage area the charge is read out as normal.
At the completion of the read out the camera restarts the external keep
clean cycle and will perform a minimum number of cleans before
accepting the subsequent trigger event.
'Fast External Trigger' in Non-Frame Transfer Mode
This mode is for the most part identical to External Trigger Mode
and differs in only one respect: after an acquisition and readout, the
camera will not wait for a sufficient number of keep clean cycles
to have completed before allowing a trigger event to start the next
acquisition. As a result Fast External Trigger allows a higher frame
rate than standard external trigger mode. Fast External Trigger is
most useful in those cases where there is very little light falling on
the sensor outside of the exposure times and the user is looking for
fast frame rates.
'External Exposure' in Non-Frame Transfer Mode
Figure 4 shows a timing scheme for External Exposure mode, another
external trigger option for non-frame transfer readout. This mode is
similar to the external trigger mode discussed above, in that the type
of keep cleans are identical and the exposure is started by the positive
edge of the trigger pulse. Where these two trigger modes differ is in
how the exposure time is controlled. With standard External Trigger,
the user (via software) controls the exposure time. With External
Level Trigger mode, the level of the trigger pulse (i.e. the time spent
in the 'on' state of the TTL signal) controls the exposure time. The
exposure period starts on the positive edge of the trigger pulse and
stops on the negative edge. The exposure is physically ended by
shifting the Image area into the Storage area. The Storage area is then
read out in the normal manner. On completion of the readout, the
external keep clean cycle is started again.
Part 2 - iCam Fast Exposure Switching
iCam encompasses a set of unique innovations that empower Andor
iXon cameras to operate with complete acquisition efficiency through
Andor iQ multi-dimensional microscopy suite and other 3rd party
imaging software packages.
||Frame Rate (fps)
|iXon3 897 with iCam
Table 1 - Comparison of exposure switching speed in iXon3 897 vs.
competing EMCCD camera with same 10 MHz pixel readout. Freerun
imaging sequence involving dual channel acquisition protocol
using rapid toggle between 1 ms and 2 ms for each channel. Both
cameras have the same 512 x 512 back-illuminated sensor.
Imaging cameras in a heightened state of readiness...
Andor's iCam technology is a combined firmware and software
innovation, a highly efficient and performance-optimized solution
that is now integrated across all new Andor imaging cameras and
Andor's iQ and SDK software platforms. iCam functionality has
been integrated into a number of popular 3rd party software drivers,
including MDC Metamorph and Nikon NIS Elements. iCam offers
heightened EMCCD performance during tightly synchronized and
complex multi-dimensional microscopy experiments.
Using state of the art bi-directional communication between camera
and PC, iCam is particularly effective for multi-channel acquisitions
during which different exposure times are rapidly toggled between
channels, whether software triggered or hardware (externally)
triggered, with absolute minimal overheads.
- Enhanced software triggering during acquisition - highly
efficient upload of acquisition parameters from software to
camera with minimized overheads.
- Ring Mode - ultimate in exposure switching during multi-channel
protocols. Software pre-loads up to 16 acquisition channels onto
- Enhanced external trigger mode- optimized speed performance
across all of Andor’s comprehensive external trigger options.
- Asynchronous frame transfer mode (AFTM) - enhanced speed
and synchronization in overlapped/frame transfer
- Bi-directional communication - between PC and camera
- Further enhanced baseline stability - takes Andor’s marketleading
quantitative baseline stability to a whole new level.
- 3rd party software compatibility - most popular imaging suites
can take advantage of iCam.
Enhanced data exchange between camera and PC
iCam allows for faster frame rates in software by using dedicated
timing patterns that shorten unnecessary overhead times. These time
lags prevent other EMCCD and interline cameras on the market from
achieving fast frame rate during complex experimental protocols.
Furthermore, iCam's 'Ring Mode' offers the capacity to use up to 16
different timing patterns uploaded into the camera head, thus external
triggers can result in virtually instantaneous switching between
channels, facilitating unparalleled synchronization with other
peripheral equipment such as filter wheel, laser-AOTF or z-stage.