Low
light conditions: signal
to noise ratio
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| The
signal to noise ratio SNR is defined by the ratio of the incoming
signals average value to its standard deviation. The diagram below
shows the signal to noise ratio for image intensified CCD cameras and electron multiplying CCD
cameras at very low light conditions. |
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The EMCCD
cameras strong difference between frame adding and long frame
again results from the fact, that the EMCCD sensors readout noise is
amplified and accumulated with each added frame. So, for the EMCCD
camera type long frame operation should be preferred. |
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The
second diagram gives the necessary total observation time to yield a
unity signal to noise ratio. The total observation time may either
result from a long exposure time, long frame, or from a corresponding
accumulation of a series of shorter exposure times, frame adding.
For the ICCD camera type the readout noise is not amplified and no
significant difference is visible between both operation modi. This
predestines the ICCD camera type for frame adding operation which is
the precondition for the significant resolution enhancement attainable
from our Dynamic Range
Expansion
System.
The work from which the diagrams have been taken can be found here. |
Whereas
there is no cooling necessary for the ICCD camera, there is a
indispensable need for strong cooling of the EMCCD camera type.
This is due to the fact, that the EMCCD sensor also amplifies its own
darc current noise to the same amount as the wanted signal. However,
the ICCD camera first amplifies the wanted signal to a high level by
the image intensifier and then adds the comparativly marginal darc
current noise. So, it does not make visible difference if the CCD
sensor itself is cooled or not, as long as its temperature does not
exceed about 30° C.
An ideal cameras signal to noise ratio is just the ratio of the
light signals average value to its shot noise, without any additional
camera noise effects taken into account. The shot noise of the signal
in turn is derived as the square root from the signals average value.
Thus, the signal to noise ratio of every incoming light signal simply
equals the square root of its average value:
This ideal signal to noise
ratio is shown as black line in both diagrams and the hatched area is
therefore not accessible.
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