Unravelling Sensitivity, Signal to Noise and Dynamic Range – EMCCD vs CCD
3 Dynamic Range
3.1 Dynamic Range vs EM Gain Plots
Dynamic Range (DR) is given by:
Calculating Dynamic Range in an EMCCD camera is a slightly more complicated story
than for conventional CCDs. This is because of the favourable effect of EM gain
on the detection limit vs. the limiting effect of EM gain on the full well capacity.
The easiest way to address this is to first take each parameter separately:
Detection Limit and EM Gain –
The main function of EMCCD is to eliminate the read noise detection limit and enable
detection of weak photon signals that would otherwise be lost within this noise
floor. With EM gain, the detection limit is given by the ‘Effective Read Noise’,
i.e. the read noise divided by the gain multiplication, down to one electron! Why
never less than one? This stems from the definition of detection limit, which is
essentially “the signal equal to the lowest noise level”. Since you can’t get a
signal less than one photon, then the detection limit should never be taken as less
than one electron.
For example, the iXonEM+ DU-897 has a read noise of ~49 electrons @10MHz
with EM Gain off. At EM gain x2, the new detection limit can be considered to be
25 electrons effective read noise, at x5 it will be 10 electrons, at x49 it will
be 1 electron. At x100, the Effective Read Noise will be 0.4 electrons, but as far
as the Detection Limit is concerned, this must still be taken as 1 electron!
Full Well Capacity and EM Gain -
One might imagine that applying EM gain will decrease the full well pixel capacity
proportionally. This is indeed the case, but a buffer has been built into EMCCD
cameras to enable at least some EM gain to be applied while maintaining the original
well capacity. This buffer is in the form of a higher capacity in the gain register
pixels, where the multiplication actually takes place. So, the true capacity is
given by the capacity of the pixels of the sensor, but as you apply EM gain this
holds only up until the point where the larger capacity of the gain register pixels
also become saturated by applied EM gain. After that point, you have to correct
the ‘effective’ full well of the sensor to be equal to the full well of the gain
register divided by the gain.
Dynamic Range and EM Gain -
These above factors combined means that as EM gain is increased, Dynamic Range will
increase with gain to a maximum, level off and then reach a point at which it begins
to deplete again with further gain. This can seem complicated, but fortunately these
DR vs EM gain relationships can be readily plotted out and visualized in graphical
form, as exemplified in the figure below.
There are a number of interesting points to note from these plots:
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1. The rationale behind offering readout speeds slower than 10MHz through the EM-amplifier
is so that frame rate can be traded of against dynamic range. You can see that the
highest dynamic range through an EM amplifier comes from the slowest 1MHz readout
speed.
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2. At any readout speed through the EM-amplifier, the best combination of dynamic
range and sensitivity can be obtained at a EM gain setting equal to the readout
noise at that speed. At this point the DR is at maximum and the effective readout
noise is 1 electron (i.e. just on the verge of single photon sensitivity).
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3. At x1000 EM gain the dynamic range is only 400:1. Excessively high EM gain can
also accelerate EM gain ageing in back-illuminated EMCCDs (see section 7). EM gains
of x300 or less are more than sufficient to optimize sensitivity, while ensuring
dynamic range is not excessively compromised. The only occasions when Andor recommend
extending EM gain to x1000, is for single photon counting experiments.
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4. The highest dynamic range is through the conventional amplifier at 1MHz.
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5. It is clear that the actual sensor dynamic range only exceeds 14-bits @ 1MHz,
through either EM or conventional amplifier. Therefore, it is at 1MHz that we require
an option to match this higher dynamic range output with a scientific grade, noise
free 16-bit A/D digitization. Andor’s iXonEM+ is uniquely designed to
do just that, making use of a real scientific grade A/D that is optimized for 1MHz
readout.
Note: There is a direct relationship between readout noise and maximum dynamic range
at a given readout speed. Lower readout noise affords higher dynamic range. The
readout noise specification used in calculating dynamic range must be with EM gain
turned off, as quoted in all iXonEM+ spec sheets.
Dynamic Range vs EMCCD Gain for iXonEM+ DU-897. Shown for EM amplifier @ 10, 5 and
1MHz readout speed and for Conventional amplifier at 1MHz readout speed. Well capacities
used in DR calculation are characteristic of the CCD97 512x512 back-illuminated
L3 sensor from E2V. Dynamic range only exceeds 14-bits max @ 1MHz, through either
amplifier
4. Effects Of Background Photons On Detection Limit