How extensively can Andor's back-illuminated EMCCDs be used before they can no longer be recalibrated (EMCAL™) to factory EM gain settings?
One common concern associated with EM gain
ageing phenomenon and the associated EMCAL™
recalibration fix, is that of longevity. The clock voltage
setting which must be adjusted as part of the
recalibration routine will eventually reach a maximum
threshold value, after which further rescaling is not
possible and EM gain will then fall off irreparably upon
further extensive use of the camera. The question is,
when is this likely to happen under typical use?
Andor have run extensive testing on the iXon+ DU-897
camera in order to project the operational lifetime of
the 'gain register' (where signal amplification occurs
on-chip) in back-illuminated EMCCDs, the conditions
of testing described below:
• Overall duration of test: ~14 months
• Camera usage during this period: Continuous 24 hours per day, 7 days per week
• Frame rate: 30 frames / sec
• EM Gain setting: x1000
• Photons / pixel / frame: 90
• Number of pixels illuminated: ~ 200,000 (~ 75% of array)
• EMCAL: Applied once per day
Results
The clock voltage required to maintain the EM gain
calibration was measured once daily using Andor's
EMCAL™ routine and the plot shown was generated
by extrapolating from the equation derived from the
gain ageing trend. This shows that under the
aggresive test conditions employed, the EMCCD
calibration would be expected to reach only ~ 65% of
the available clock voltage scale after 7 years of
continuous operation! As such, calibration over the
‘full EM gain range’ can be expected for significantly
longer than 7 years.
Demanding test conditions
The combination of parameters employed in this test
represents quite aggressive acquisition conditions.
~ 1.1 billion images were recorded during this period,
with ~ 200,000 illuminated pixels per image,
corresponding to ~ 220,000 billion pixels being
amplified through the gain register with x1000 EM
amplification per pixel.
Most users would not subject the camera to 24/7
continuous acquisition at 30 frames/sec. We also
strongly recommend that, except for photon counting,
the EM gain setting is limited to no more than x300 for
the vast majority of applications, but x1000 was
chosen here as a more rigorous test condition. Finally,
it is quite rare that 75% off all pixels in the array will be
subjected to uniform signal of this magnitude, as was
imposed on the sensor here. In reality the light
emitting species of typical user samples will project
onto a much smaller fraction of pixels from frame to
frame.
EMCAL™ does NOT accelerate
EM gain ageing
It is important to recognize that the rate of ageing is
not accelerated by routine application of Andor's
EMCAL™ routine. The rate of ageing is determined by
the illumination and EM gain conditions that the
sensor is subject to through operation, irrespective of
routine recalibration using EMCAL™. If the 'previous'
mechanism of EM gain recalibration were to be used,
whereby the camera is shipped back to factory less
frequently for manual readjustment of the clock
voltage, the progress along the ageing curve would
not be any different from that shown here (adjusting
for the additional time that the camera would be out of
action).
How often to run EMCAL?
This is a very difficult question to give a definitive
answer to, because it depends very much on how
extensively images are acquired and how
'aggressively' it is used in terms of EM Gain and light
intensity across the sensor area. However, it is a
reasonable rule of thumb to recommend that it is
worth running EMCAL once per month. If the camera
is in a multi-user facility, users may wish to run EMCAL
more frequently for peace of mind - there is no
technical downside to this; it will NOT accelerate the
progress of the camera towards its EM gain end point.
Conclusion
For the vast majority of low-light applications and taking due care and attention to stay within
recommended operating conditions, applying EMCAL™ as required, the gain ageing phenomenon is not
considered to ever impose a restriction on the quantitative reliability of your Andor iXonEM+ camera.
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