Teledyne PIXIS-XF, System Manual

Page: 47 / 116

Chapter 5 Operation 47
5.4.4 Dark Charge
Dark charge (or dark current) is the thermally induced buildup of charge in the CCD
over time. Even with the light into the camera completely blocked, the CCD will collect
a dark charge pattern, dependent on the exposure time and camera temperature. The
longer the exposure time and the warmer the camera, the larger and less uniform this
background will appear. Thus, to minimize dark-charge effects, you should set the
camera temperature at the lowest CCD temperature within the recommended range
for your camera. Refer to
Table A-2, Typical Deepest Operating Temperature , on
page 91 for complete information.
The statistical noise associated with dark 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 thermoelectrically-cooled
camera, dark charge is reduced by a factor of ~2 for every 6-7 degree reduction in
temperature. In the case of cameras such as the PIXIS-XF cameras, which have MPP
type arrays, the average dark charge is extremely small. However, the dark-charge
distribution is such that a significant number of pixels may exhibit a much higher dark
charge, limiting the maximum practical exposure.
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 system. Contact
Teledyne Princeton Instruments Customer Support for
information about how to refresh the vacuum. Refer to
Contact Information on page 114 for complete information.
NOTE:
Do not be concerned about either the DC level of this
background or its shape unless it is very high, i.e., > 1000
counts with 16-bit ADC. What you see is not noise. It is a
fully subtractable readout pattern. Each CCD has its own
dark charge pattern, unique to that particular device. Simply
acquire and save a dark charge background image under
conditions identical to those used to acquire the actual
image. Subtracting the background image from the actual
image will significantly reduce dark-charge effects.
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 S/N (Signal-to-Noise 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.