Making Sense of Sensitivity
It is often questioned whether or not to use EMCCD gain or whether to use EM or
conventional CCD amplifiers (model dependent). The answer usually depends both on
required frame rate and on light levels. Plots of Signal to Noise ratio vs Signal Intensity
can be instructive in making such decisions. Here we introduce the concept of Signal to
Noise in EMCCDs and discuss such plots.
Part 1 - Understanding Noise Sources in EMCCDs
Read Noise in many instances can be considered the true CCD
detection limit, particularly the case in fast frame rate experiments
because, (a) short exposures combined with low darkcurrent make the
darkcurrent contribution negligible and (b) faster pixel readout rates,
such as 5 MHz and higher, result in significantly higher readout noise.
The fundamental advantage of EMCCD technology is that gains are
sufficient to effectively eliminate readout noise, therefore eliminating
the detection limit.
This noise source is only present in signal amplifying technologies
and is a measure of the uncertainty inherent to the signal multiplying
For example, during each transfer of electrons from element to
element along the gain register of the EMCCD, there exists only a
small probability that the process of impact ionization will produce an
extra electron during that step. This happens to be a small probability
but when executed over more than 590 steps, very large potential
overall EM gains result. However, the downside to this process results
from the probabilities. Due to this, there is a statistical variation in the
overall number of electrons generated from an initial charge packet
by the gain register. This uncertainty is quantified by a parameter
called "Noise Factor" and detailed theoretical and measured analysis
has placed this Noise Factor at a value of √2 (or 1.41) for EMCCD
technology. This is an additional form of noise that must be taken
into account when calculating Signal/Noise for these detectors. Note
that this noise source is significantly greater from the MCP of ICCDs
than from the gain register of the EMCCD. ICCDs have noise factors
typically ranging from 1.5 to >2.
However, one way to better understand the effects of this noise
source is in terms of an addition to the shot noise of the system. Extra
multiplicative noise has the same form as shot noise in that each noise
type results in an increase in the variation of number of electrons that
are read out of a CCD (under constant uniform illumination).
Indeed multiplicative noise can be thought to contribute directly to
the overall shot noise, in that one should multiply the Shot Noise by
the Noise Factor when calculating overall noise.
Simply put, multiplicative noise does not in any way reduce the
average signal intensity or reduce the number of photons that are
detected, it simply increases the degree of variation of the signal around the mean value, in addition to the variation that already exists
from the shot noise (variation from pixel to pixel or from frame to
Due to the effective cooling inherent to Andor's cameras, dark current
is minimized, and may often be considered practically negligible.
The extent of contribution is dependent on exposure time, since
the darkcurrent is quoted in electrons/pixel/sec. It is particularly
important to eliminate darkcurrent with EMCCD technology as even
single thermally generated electrons in the silicon will be amplified
in the gain register just as a single photoelectron, and will appear in
the final signal as a single noise spike. Fortunately, for fast frame rate
experiments combined with iXon very low darkcurrent, this noise
source may be ignored.
Spurious Noise appears in the form of Clock Induced Charge (CIC)
in EMCCD technology. CIC is independent of exposure time and
generally single electron events generated during charge shift (EBI is
the form of spurious noise in ICCDs and is exposure dependent). CIC
is generated in every CCD but is normally buried in the readout noise.
In the EMCCD however, these single electrons are amplified by the
gain register just as a single photoelectron would be. In the EMCCD,
CIC can in some ways be considered the true limit of detection, in
that at the single photon detection level, a single photon spike will be
indistinguishable from a CIC spike. Andor has specialized electronics
however, that enable this source of noise to be minimized. In practical
terms, ultra-weak signals of the single photon nature would be
distinguishable from CIC spikes in that one could generally expect
to see 'groupings' of photon spikes from adjacent pixels, even from
diffraction limited single molecule emissions.
Simplified consideration of EMCCD noise
From the above list it is apparent that in most uses of Andor’s iXon
cameras, since read noise and darkcurrent can be virtually eliminated
(i.e. the noise sources that would define the detection limit have been
rendered effectively negligible), the principal sources of noise that
must be considered are shot noise, noise factor (multiplicative noise)
and spurious noise.
It is easy to combine shot noise and multiplicative noise in an overall
noise equation, using:
Overall Noise = Shot Noise x 1.41
Shot Noise can be determined if the average signal is measured in
electrons - by measuring in electrons, the calculation is independent
of the sensor’s QE - i.e. the photons have already been converted to
photoelectrons so the QE corrected signal is being measured. If the
average signal in photons is already known (e.g. estimated from other
measurements with PMTs), the shot noise can be corrected for sensor
QE at that wavelength:
Since spurious noise is very different in nature to shot noise, it is
best to consider spurious noise separately. Each EMCCD will have a
measured figure for the levels of CIC spikes to be expected during a
readout. This will present a figure for the average number of random
spurious single electron spikes that will appear within the image.
If the measured signal is at the very low photon level (one or two
electrons per pixel), this noise source will be more significant. If
the signal is slightly more intense than this, it may become less of
an issue, and may even be filtered out. Note that often, the minimal
amount of spurious noise remaining from the iXon is minor compared
to the level of background photons in the image.
Part 2: Signal to Noise Plots
EM gain ON vs EM gain OFF (faster speeds)
Figure 1 shows Signal to Noise plots derived from the specs of the
back-illuminated iXon EMCCDs, read out at 10 MHz (for fastest
frame rates). A photon wavelength at which the QE of the sensor
is 90% is assumed. Such plots are very useful to gauge the signal
intensity at which it becomes appropriate to use EM gain to increase
S/N. It is clear that at 10 MHz readout, one needs to encounter
relatively intense signals of > 2900 photons / pixel before it becomes
advantageous to operate with EM gain off. Note that the ‘ideal’ curve
represents a pure Signal to Shot Noise and is shown for reference – if
the detector had no sources of noise, this is what the curve would
appear like. Even with EM gain turned on we encounter uniformly
lower signal to noise than the ideal curve. This is due to the influence
of EM Multiplicative Noise, which has the effect of increasing the
shot noise by a factor of √2 (~1.41). Note, Multiplicative Noise
(Noise Factor) is generally higher for ICCDs.
EM vs Conventional Amplifier (slower speeds)
Figure 2 shows Signal to Noise plots derived from the specs of
the back-illuminated iXon EMCCDs at 1 MHz (slower frame rate
operation), read out either with EM gain ON or alternatively through
the Conventional amplifier (i.e. standard CCD operation). Again, a
photon wavelength at which the QE of the sensor is 90% is assumed.
Specifically this figure applies to 897 and 888 models for which this
choice of amplifier is available.
Under slower speed operations, when one has the choice to read
out as a 'conventional CCD', it can often be advantageous to do so
in terms of achieving better signal to noise. Indeed the plots show
that the cross-over point is at ~ 42 photons / pixel, below which it is
advised still to readout through the EM amplifier with gain applied.