Appendix
pco.edge User Manual V1.03 © PCO AG, Germany
71
A11 Binning in CMOS sensors
Binning describes the summation of single pixels to form larger pixels and
thereby improving the signal-to-noise ratio (SNR).
Binning in CCD image sensors
The term binning comes from scientific CCD image sensors. The prominent
feature of charge-coupled-devices (CCD) is the lossless transport or shifting
of charge packages until an amplifier circuit converts them into a voltage at
the output, where the main readout noise contribution occurs.
If charge packages from two or more pixels are added before they are read
out (past the output amplifier) because of very low light signals, this process
is called binning. Since the signal is increased before it is read out, and the
image sensor’s readout characteristics remain unchanged, binning
improves the SNR, and the image sensor’s resolution is reduced. The
lossless transport feature of CCD image sensors makes binning possible.
Binning in CMOS image sensors
In general, there is no binning possible in CMOS image sensors because
there are voltages processed instead of charges transported. In each pixel,
the light generated charges are converted into voltages with the readout
noise contribution of these amplifiers. Therefore, as opposed to CCD
image sensors, if these voltages were combined, the readout noise would
also be combined, which would not have the same positive effect on the
SNR.
Nevertheless, such a summation or even an averaging would be beneficial
for the SNR, but with a smaller impact compared to CCD image sensors.
Since such “CMOS binning” cannot be done within the image sensor, it
either has to be done in the camera or it has to be done in the computer.
There are two options for “CMOS binning”:
“CMOS binning” – accumulation
Pixel values can accumulate, causing an effective dynamic reduction or
larger number formats, because the result might exceed the original format
-- two times maximum 8 bit values will result in a 9 bit value. This will not
be a problem if 12 bit values are accumulated and transported as 16 bit
images. However, in the case of scientific CMOS, if 16 bit values are
transmitted in 16 bit images, only two times 15 bit maximum values are
allowed. The signal of the binned pixels will be accumulated, due to the
properties of readout noise, the increase of the noise itself will only be as
big as the square root of the number of binned pixels. The SNR will improve
and in addition, a reduction of the stored image data is achieved.
“CMOS binning” - averaging
Pixel values can be averaged, which has the same effect on the SNR as
accumulation due to the properties of noise. This would keep the image
output format the same and would reduce the amount of image data that
can be stored. When this type of “CMOS binning” is processed in the
camera it is called “hardware binning”. This should not be confused with
real binning in CCD image sensors, because the “hardware” that processes
this binning is not much different from the “hardware” in computer
processing. Therefore, the term “hardware binning” may be misleading.
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