Cleaning; Figure 32. Clean Cycles Timing Diagram - Princeton Instruments ProEM+ EMCCD User Manual

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time limit for on-chip accumulation, 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.
Note: Do not be concerned about the DC level of this background. What you see 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 will significantly reduce
dark-charge effects.
Caution
If you observe a sudden change in the baseline signal, there may be excessive humidity in
the camera vacuum enclosure. Turn off the camera and contact Princeton Instruments
Customer Support. See page 146 for contact information.

Cleaning

The basic cleaning function is implemented by clean cycles. These cycles start when you
turn the camera on and a clean pattern is programmed into the camera. Their purpose is to
remove charge that accumulates on the array while the camera not acquiring data (i.e.,
exposing and reading out the array). Figure 32 is a timing diagram for an experiment set
up to acquire four (4) images in Freerun {No Response} timing mode. In this diagram
clean cycles occur before the first exposure and after the last readout period.
Note: The start of the exposure is signaled by SCAN going high but will not occur until
the current clean cycle has finished.
The configuration of clean cycles is performed on the Hardware Setup|Cleans/Skips
tab {via the Sensor Cleaning pane accessed on the Sensor expander}. When you set
up the camera for the first time, default values are automatically inserted into these fields.
These will give the best results for most applications. Even so it is a good idea to know
what these entries mean with regard to cleaning.
Cleaning Parameters:


ProEM

Figure 32. Clean Cycles Timing Diagram

Number of Cleans {Number of Clean Cycles} value is usually set to one (1).
These are additional clean cycles that can be required after a start exposure signal
is received and the current clean cycle has finished. The maximum value for this
entry depends on the camera.
Number of Strips per Clean {Clean Cycle Height} sets the number of rows that
will be shifted and discarded per clean cycle. While a large number such as the
number of rows in the array may result in the best cleaning of the array, the tradeoff
is that there may be a significant delay between the receipt of a start exposure signal
and the beginning of the actual exposure. This delay occurs because the current clean
cycle must be completed before a start exposure signal received during the cycle will
be implemented. Typically, the default setting is much smaller and in time critical
experiments, the setting should be 1 or 2.
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+ System Manual Version 1.B