Teledyne PIXIS-XF, System Manual

Page: 51 / 116

Chapter 5 Operation 51
The readout rate for n x n binning is approximated using a more general version of the
full resolution equation. The modified equation is:
5.5.2.2 Software Binning
One limitation of hardware binning is that the shift register pixels and the output node
are typically only 2-3 times the size of imaging pixels. Consequently, if the total charge
binned together exceeds the capacity of the shift register or output node, the data will
be lost.
This restriction strongly limits the number of pixels that may be binned in cases where
there is a small signal superimposed on a large background, such as signals with a large
fluorescence. Ideally, one would like to bin many pixels to increase the S/N ratio of the
weak peaks but this cannot be done because the fluorescence would quickly saturate
the CCD.
The solution is to perform the binning in software. Limited hardware binning may be
used when reading out the CCD. Additional binning is accomplished in software,
producing a result that represents many more photons than was possible using
hardware binning.
Software averaging can improve the S/N ratio by as much as the square root of the
number of scans. Unfortunately, with a high number of scans (i.e., more than 100,)
camera 1/f noise may reduce the actual S/N ratio to slightly below this theoretical value.
Also, if the light source used is photon-flicker (1/f noise) limited rather than photon
shot-noise limited, this theoretical signal improvement cannot be fully realized. Again,
background subtraction from the raw data is necessary.
This technique is also useful in high light level experiments, where the camera is again
photon shot-noise limited. Summing multiple pixels in software corresponds to
collecting more photons, and results in a better S/N ratio in the measurement.
t
r
N
x
N
y
t
sr
n
-----
t
v
n
2
-----
+
 
 
 
 
–
N
x
t
i
  
+