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Chapter 3. Advanced CCD Theory 33
9
1
2
4
3
The CCD is exposed
to light and a charge
pattern accumulates
in the subarray.
Subarray Readout
A subarray is speci-
fied.
The charge in the
subarray is fast-
shifted toward the
serial register.
The first row is
shifted into the serial
register.
Steps 4 through 8
are repeated until
the entire subarray
is read out.
The charge at the output
node is collected for
signal processing.
The charge is fast-
shifted toward the
output node.
The first pixel of the
subarray is shifted into
the output node.
5
6
7
Steps 6 and 7 are re-
peated until the entire
serial register is read out.
8
Subarrays
It is not necessary to read out all of the pixels on the parallel register. One or
more subarrays on the parallel register may be specified and read out, with a
resultant reduction in pixel count and increase in readout speed. Subarray
Readout illustrates the subarray readout concept.
Binning
Binning is a technique of combining charge from adjacent pixels during the
readout process. Binning improves the signal-to-noise ratio and extends the
dynamic range of the CCD imager, but at the expense of spatial resolution.
Binning is thus most useful in applications where resolution in one or both axes
is not of primary concern. Because binning reduces the number of pixels to be
processed and digitized, readout speed is also increased.
As shown in Binned Readout, on page 34, charge is collected in the normal
fashion, but the readout is programmed differently. With parallel binning, when
charge is shifted from the parallel register into the serial register, charge is
accumulated from two or more rows before serial shifting begins. With serial
binning, two or more charge packets are similarly accumulated in the output
node before the charge is digitized and read out.
Binning is specified by a binning factor: the number of pixels to be combined on
the CCD. The example shows 2 x 2 binning. Any combination of parallel and
serial binning factors is possible. The default values of 1 provide no binning.