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Chapter 5
Readout
Introduction
After the exposure time has
elapsed, the charge
accumulated in the array
pixels needs to be read out of
the array, converted from
electrons to digital format, and
transmitted to the application
software where it can be
displayed and/or stored.
Readout begins by moving
charge from the CCD image
area to the shift register. The
charge in the shift register
pixels, which typically have
twice the capacity of the
image pixels, is then shifted into the output node and then to the output amplifier where
the electrons are grouped as electrons/count. This result leaves the CCD and goes to the
preamplifier where gain is applied.
WinX and LightField allow you to specify the type of readout (full frame or binned), the
output amplifier, and the gain (the number of electrons required to generate an ADU).
Full Frame Readout
The upper left drawing in Figure 19
represents a CCD after exposure but
before the beginning of readout. The
capital letters represent different
amounts of charge, including both
signal and dark charge. This section
explains readout at full resolution,
where every pixel is digitized
separately.
Note: With PIXIS-XB cameras you
have a choice of amplifier (low noise
or high capacity). Depending on the
selected amplifier, the shift register
may be read out to the right or to the
left. For simplicity this drawing
shows the readout to the left.
Readout of the CCD begins with the
simultaneous shifting of all pixels
one row toward the "shift register,"
in this case the row on the top. The
shift register is a single line of pixels
along the edge of the CCD, not
sensitive to light and used for
Operation
Figure 18. Array Terms for a CCD with a Dual Output
Figure 19. Full Frame at Full Resolution
Amplifier
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