ICCD
compared to EMCCD
|
In
the
last years, electron multiplying CCD cameras, EMCCD, have become
very popular. In a variety of applications which do not require fast gating the EMCCD has started
to compete with the ICCD. In some specific
astronomical long time observations extremely cooled custom-built
EMCCD cameras yet are superior to ICCD camera systems.
Quantum
Efficiencies of ICCD and EMCCD
Since the back
illuminated EMCCD sensors were introduced to the market it can be heard
frequently, that the sensitivity of EMCCD cameras would outperform ICCD
systems and therefore EMCCD cameras would be the better choice in low
light
level applications. This point of view is mainly made up from
considering the
quantum efficiencies of both camera types.
As shown in the above diagram back illuminated EMCCD
chips reach a
quantum efficiency of up to 92% peak value. In contrast, third
generation image intensifier photocathodes reach 52% and second
generation image intensifier photocathodes mostly lie below 20%.
Indeed, it can be said that, as a spectral average, the quantum
efficiency of EMCCD cameras is by a factor 4 better than Gen II
equipped ICCD cameras and a factor 2 better than Gen III
systems.
|
|
The main functional
difference between ICCD and EMCCD cameras
But this is only one aspect, it is
the aspect which only concerns the
first process step of both camera types, the arriving-photon detection.
The second step, which comprises the amplification of the detected
light, is achieved in very different ways in both systems. The final
camera performance will of course result from the coaction of both
operation steps. For this reason, the two workig principals shall be
explained qualitatively to some extend in the following.
Let
us consider an arriving signal consisting of 4 photons per pixel
within the exposure time. As shown in the pictures below, maybe the
EMCCD collects all 2 photons due to its high quantum efficiency.
However, a Gen III equipped ICCD may only detect 1 out of these 2
photons. Whereas the EMCCD pixel now contains 2 electrons, the
appropriate ICCD pixel will be filled up with let's say 1 x 500 x 40% =
200 electrons as a result of the image intensification.
When the exposure time is expired the CCD sensors will be read out. The
readout process itself contributes with several noise sources to the
number of electrons actually stored in the pixel. When we consider an uncooled ICCD camera operated at maximum frame rate there maybe 50
noise electrons contributed to the already existing 200, resulting in a
ratio of 200 / 50 = 4. An EMCCD cooled down to 200 Kelvin and operated
at standard frame rates will only generate about 1 electron from charge
transfer, i.e. clock induced charge noise, and particularly from
readout darkcurrent. The ratio will in this case amount to 2/1 = 2.
|
Please
keep in mind that the comparison of the signal to noise ratios
of the different camera types is a complex issue. This simple
example shall only give you a more concrete idea of the principal
situation and the scale of the values. Despite the enormous number of
noise electrons generated in the ICCD camera compared to the EMCCD, it
nevertheless achieves the better ratio between the signal and the
technical noise contributions. It is a very important difference that
the ICCD amplifies the measurement signal prior to the readout process
and the large number of noise electrons are added to the even higher
number of signal electrons. This makes the ICCD system insensitive
against the sensor temperature and high readout frame rates.
However, the strongly cooled EMCCD system only adds an extremely small
amount of noise electrons to the measurement signal yet prior to the
amplification process. So, the noise contributions are amplified to the
same extent as the signal itself.
A detailed examination of this issue can be found here. |
|
|
|
|
