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Chapter 5 Operation 59
59
For signal levels low enough to be readout-noise limited, longer exposure times, and
therefore longer signal accumulation in the CCD, will improve the S/N ratio
approximately linearly with the length of exposure time. There is, however, a maximum
time limit for on-chip accumulation, determined by either the saturation of the CCD by
the signal or the loss of dynamic range due to the buildup of dark charge in the pixels.
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
Figure 24. Array Terms for a CCD with a
Dual Output Amplifier
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).