Deep Vacuum TE Cooling and EMCCDs
TE Cooling and darkcurrent elimination
On harnessing EMCCD technology, darkcurrent is an absolutely critical parameter to
minimize, more so than in a standard sensitive CCD. The reason for this is that thermally
generated electrons are amplified by EMCCD just as photon-generated electrons (signal)
For optimal sensitivity in EMCCDs, thermoelectric cooling of
the sensor must be deep enough that this noise source is virtually
eliminated. It is important to recognize that this point applies very
much to short exposure operation also; the sensor readout process
alone results in significant darkcurrent production if left untreated.
The above point is demonstrated by the following simple test:
- Figure 1(A) shows two dark images taken with a 512 x 512
back-illuminated sensor (from E2V) at two different cooling
temperatures; -80°C and -30°C.
- All are in frame transfer mode at the maximum frame rate of 34
full frames per second, i.e. ~ 29 ms exposure.
- EM gain is set at X1000; at such an EM gain setting the vast
majority of darkcurrent and CIC events will be exposed.
The speckled 'salt and pepper' noise pattern that is quite obvious in
the -30°C condition is due almost entirely to amplified darkcurrent
electrons. Note that each of these images has ALREADY been
optimized for minimal CIC, so only the effect of cooling is being
demonstrated. If CIC had not been minimized, the -30°C situation
would have appeared bleaker still, with an extremely dense 'EMamplified'
As an alternative view Figure 1(B) shows a line intensity plot from a
single row from each image; the readout noise is visible as the fuzzy
baseline of each trace with the darkcurrent 'spikes' sticking out of it;
this is just what we expect. These spikes, or background events, are
what set the remaining detection limit of the camera, not the readout
It is instantly clear that cooling is beneficial. The performance at
–80°C is by far the best and there is no way that this level of low noise
detection limit can be achieved at -30°C, under any circumstances. It
is important to note that even with a short exposure time, the -30°C
background events are predominantly from darkcurrent. As such
,we are clearly better off with much deeper cooling, regardless of
Single thermal electrons are amplified by the EMCCD
gain mechanism. Deep vacuum TE cooling is critical
to optimize the sensitivity performance of backilluminated
EMCCD sensors, otherwise the raw
sensitivity will be compromised, even under conditions
of short exposures.
Deep TE cooling can also make a tangible difference to signal to
noise ratio for longer exposure conditions. Figure 2 shows extremely
low light images, recorded at -70°C and -95°C with the iXon 888 on a light tight imaging chamber using weak LED illumination through pinholes. The light levels used are typical of an experiment
involving imaging of weak luminescence signal. The photon flux is
so low, that a two minute exposure is required in order to visualize the
pinhole signals. It is clear from the significantly improved SNR, and
therefore contrast, at -95°C cooling, that such extremely low cooling
temperatures are recommended for longer exposure acquisitions.
1 MHz EM mode readout limits background of long exposures
When back-illuminated EMCCD sensors from E2V are read out
at faster horizontal speeds, such as 10 MHz, the EM-amplified
background becomes notably higher over longer exposures. A slower
1 MHz readout mode is required to minimize background under these
This is not so apparent for shorter exposure times (e.g. < 1 sec) but
when you go towards multiple secs to mins it is certainly apparent.
Some example dark noise images are show in Fig 1 and Fig 2 for two
different cooling temperatures, taken with the iXon 888 camera at
x1000 EM gain.
This basic trend can indeed be considered an oddity, because you
would intuitively imagine that the readout speed should not influence
the noise background that is built up during the exposure, but
nevertheless it is fact.
This means that we recommend using a readout speed of 1 MHz for
longer exposure times. This is not really disadvantageous since you
have already compromised the frame rate with the longer exposure
time. For shorter exposure times/faster frame rate measurements, it is
still best to opt for 10 MHz readout speed because darkcurrent (being
time dependent) is lower and CIC is the prominent contributor. Since
CIC is elevated by use of slower readout parameters, then are better
using 10 MHz. Also, you tend to need the faster readout speeds to
achieve faster frame rates.
Pixel Readout Speed
|< 1 sec
|< 10 sec
|< 1 sec
||< 10 sec
A slow readout 1 MHz mode is required to minimize EM-amplified background under longer exposure acquisition conditions.