Chapter 9 Exposure and Readout 63
the IIC-200 (IIC-300, IIC-100, or MCP-100), the intensifier can be turned ON or OFF in
much the same way as it is in Gated operation, but at slower speeds, allowing exposures
from 50 µs to 23 hours to be set from software.
"Smearing" of signal features can occur if the CCD is exposed to light during readout.
The additional light will result in continued charge accumulation even as charge is being
moved across the CCD's surface to the shift register. The result is blurring of the image
along one direction only.
The fraction of total signal due to smearing is the ratio of the amount of time spent
shifting divided by the exposure time between frames. Faster shifting and/or longer
exposure times will minimize this effect. Note that while 1% smear is insignificant in an
8-bit camera (256 gray levels), in a 12-bit camera (over 4,000 gray levels) 1% smearing
is over 40 counts, enough to obscure faint features in a high dynamic range image.
With a full-frame CCD, smearing in shutter mode operation can be avoided by designing
the experiment so no light falls on the intensifier during readout or so the intensifier is
biased OFF during readout. The latter can be accomplished by connecting the
SHUTTER signal (available at the LOGIC OUT connector when selected via software)
to the SHUTTER IN connector of the IIC-200 (IIC-200 or MCP-100).
With a frame transfer CCD, smearing is minimized. This is due to the speed (a few
milliseconds) at which the image is shifted under the masked portion of the array. Once
the image is under the mask, it can be read out without being affected by light incident
on the array.
Saturation
When signal levels in some part of the image are very high, charge generated in one pixel
may exceed the “well capacity” of the pixel, spilling over into adjacent pixels in a
process called “blooming.” In this case a more frequent readout is advisable, with signal
averaging to enhance S/N accomplished through the software.
For signal levels low enough to be readout-noise limited, longer exposure times, and
therefore longer signal accumulation in the CCD, improves the S/N ratio approximately
linearly with the length of exposure time. There is, however, a maximum time limit for
on-chip averaging, 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 (see below).
Dark Charge
Dark charge (or dark current) is the thermally induced buildup of charge in the CCD over
time. The statistical noise associated with this charge is known as dark noise.
Dark charge values vary widely from one CCD array to another and are exponentially
temperature dependent. At the typical operating temperature of a standard I-PentaMAX
camera, dark charge is reduced by a factor of ~2 for every 6º reduction in temperature.
When acquiring data using long exposure times, taking a dark charge “background
image” under identical conditions is essential. This image should be subtracted from the
raw image in software.
