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Chapter 5 Gate Mode Operation 69
• Fast Gate Tubes
When using Fast Gate Tube and the programmed gate width is < 10 ns, the width of
the
MONITOR pulse is approximately 2 ns wider than the intensifier ON time.
• Slow Gate Tubes
When using Slow Gate Tube the width of the MONITOR pulse is approximately the
same as the timing of the electrical gate drive to the intensifier. However, because
of the slow gate nature of the intensifier, the optical gate will be delayed and
distorted relative to the
MONITOR pulse. The error is related to the slowness of the
individual slow-gate intensifier tube. Typical deviations are 50 -100 ns.
5.6.2 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.” If this is the case, you could reduce the intensifier gain or read out more
frequently, with signal averaging to enhance S/N (Signal-to-Noise ratio) accomplished
through the software.
CAUTION!
!
Signals that are large enough to bloom may damage the
intensifier.
For signal levels low enough to be readout-noise limited, longer exposures, 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.
5.7 Background Subtraction
Each CCD has its own dark charge pattern or background that can be subtracted from the
total acquired signal. By subtracting this background, you can eliminate the dark charge,
which might otherwise hide low-intensity signal.
When setting up for background subtraction, set up the experiment conditions for acquiring
the actual image (i.e., camera temperature, gating sequences, region of interest, timing
mode, etc.,) and then, while blocking the incoming signal from the array, acquire a dark
charge background image under those conditions. Once the background image is acquired,
save it to disk.
After storing the background data to disk, you have two choices for background subtraction:
automatic or post-processing.
•Automatic
In WinX, this approach requires that you activate “Background” and specify the
background filename on the
Acquisition —> Experiment Setup… —> Data
Corrections
tab before acquiring an image. In LightField, you must check the Apply
Background Subtraction
box on the Online Corrections expander and select the
appropriate background file or acquire a new background. When you acquire an image,
the specified background will be subtracted automatically from the raw image data
before the corrected data is displayed and is available for storage to disk.