Teledyne PIXIS-XF, System Manual

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Chapter 5 Operation 45
When using an X-ray source, you might run in Preview mode while positioning the
camera or the subject of the experiment. Then either:
• Stop Preview mode;
• Continue running in Preview mode;
• Begin Acquire mode;
• Wait for the CCD to reach the operating temperature before beginning Acquire
mode.
5.4 Exposure and Signal
The section discusses factors that can affect the signal acquired on the CCD array.
These factors include array architecture, exposure time, CCD temperature, dark charge,
and saturation.
5.4.1 CCD Array Architecture
Charge coupled devices (CCDs) can be roughly thought of as a two-dimensional grid of
individual photodiodes (called pixels), each connected to its own charge storage well.
Each pixel senses the intensity of light falling on its collection area, and stores a
proportional amount of charge in its associated well. Once charge accumulates for the
specified exposure time (as programmed in the software,) the pixels are read out
serially.
CCDs are rugged and compact: unintensified CCDs can withstand direct exposure to
relatively high light levels, magnetic fields and RF radiation. They are easily cooled and
can be precisely thermally controlled to within ±50 millidegrees.
Because CCD arrays are always sensitive to light, light must not be allowed to fall on the
array during readout (with a few exceptions). Unintensified full-frame CCD cameras like
the PIXIS-XF cameras may require a customer-provided X-ray shutter to prevent light
from reaching the CCD during readout.
5.4.2 Exposure Time
Exposure Time, which is set on the Experiment Setup
►
Main tab {Common Acquisition
Settings expander}, is the time between start acquisition and stop acquisition
commands sent by the application software to the camera. In combination with
triggers, these commands control when continuous cleaning of the CCD stops and
when the accumulated signal will be readout. The continuous cleaning prevents
buildup of dark current and unwanted signal before the start of the exposure time. At
the end of the exposure time, the CCD is readout and cleaning starts again.
Because PIXIS-XF cameras do not incorporate an internal shutter, some signal may
accumulate on the array while it is being readout. This continuous exposure of the array
during readout may result in some smearing. However, exposures that are significantly
longer than the readout time can be performed without a shutter, as the amount of
smearing will be low.
If smearing or other factors require a shutter, the NOT SCAN {Not Reading Out} or the
SHUTTER {Shutter Open} signal on the LOGIC OUT connector on the rear of the
PIXIS-XF can be used to control a customer-supplied external X-ray shutter. By using
one of the signals to synchronize the shutter operation with exposure, the CCD can be
read out in darkness. Alternatively, the X-ray source can be interrupted elsewhere in the
system while readout is taking place.