Teledyne Princeton Instruments PI-MTE, Системное руководство

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50 PI-MTE System Manual Issue 6
Because dark charge is thermally-induced, reducing the array temperature significantly
reduces the rate at which of dark charge accumulates in the pixel wells. Even so,
enough dark charge could accumulate in the pixels between data acquisitions to affect
dynamic range at the beginning of an exposure. To prevent this from happening, Clean
Cycles repeatedly shift and discard any signal that has integrated on the array while
waiting for a Start Acquisition command. For additional information, refer to
Section 5.4.6, Clean Cycles , on page 51.
After the Start Acquisition command is received by the controller and exposure begins,
both the signal of interest and dark charge integrate on the array. The longer the
exposure time and the warmer the camera, the larger and less uniform the dark charge
will appear. To minimize the dark charge contribution to the acquired signal, you
should operate with the lowest temperature possible for your camera. Reducing the
exposure time may also be helpful.
To further reduce the dark charge contribution to an acquired signal, you can perform
background subtraction, which subtracts a dark charge background from raw data
acquired using the same experiment conditions. For additional information, refer to
Section 5.5.3, Background Subtraction , on page 55.
CAUTION!
!
If you observe a sudden change in the baseline signal you
may have excessive humidity in the camera's vacuum
enclosure. Immediately turn off the controller. Then,
contact Teledyne Princeton Instruments Customer Support
for further instructions. Refer to
Contact Information on
page 116 for contact information.
NOTES:
1. Do not be concerned about either the DC level of this
background noise or its shape unless it is very high, i.e.,
> 1000 counts with 16 bit A/D. What you see is not
noise. It is a fully subtractable readout pattern. Refer to
Section 5.5.3, Background Subtraction , on page 55 for
more information.
2. Offset and excess noise problems are more likely to
occur if the controller and camera were not calibrated
and tested as a system at the factory.
5.4.5 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 Signal-to-Noise (S/N) ratio 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, will improve 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 pixels
by the signal or the loss of dynamic range due to the buildup of dark charge in the
pixels.