Dark Charge; Saturation - Teledyne PIXIS Manual

70
7.4

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. In the case of cameras with 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. Dark charge effect is more pronounced in cameras
having a non-MPP array such as deep-depletion devices.
With the light into the camera completely blocked, the CCD collects a dark charge
pattern that is 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, always operate with the
lowest CCD temperature possible.
7.5

Saturation

When signal levels in part of an image are very high, charge generated in one pixel may
exceed the well capacity of the pixel and spill over into adjacent pixels in a process
called blooming. In this case a shorter exposure 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 accumulation that is 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.
PIXIS System Manual
WARNING!
If a sudden change in the baseline signal is observed, there
may be excessive humidity in the camera vacuum enclosure.
Turn off the camera and contact Teledyne Princeton
Instruments Customer Support. Refer to
Information on page 154 for complete information.
NOTE:
Do not be concerned about the DC level of this background.
What is seen is not noise. It is a fully subtractable bias
pattern. 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 significantly reduces dark-charge effects.
Contact
Issue 5