Chapter 6 Gated Operation 55
Timing Mode
When you are in Gate Mode, the internal timing generator will only operate using
Internal Sync timing. This timing mode initiates a readout cycle after each internal
timing generated pulse ensemble.
*
The handshakes that prevent a readout from occurring
while the timing generator is busy and that prevent the timing generator from pulsing the
photocathode ON while a readout cycle is in progress are performed within the camera.
For this mode, the following parameters are automatically set by the application software
and cannot be changed: Exposure Time is set to 0, Continuous Cleans is disabled,
and Disabled Opened is selected for Shutter Control.
Controlled by Internal
Timing Generator
Controlled by Internal
Timing Generator
Table 3. Timing Mode, Shutter Control, and Ext. Trigger Input
when using the Internal Timing Generator
Notes:
1. These automatically set parameters may not be visible in some versions of the
software.
2. The Accumulations and Images (or Spectra) parameters (WinView/32 or
WinSpec/32 Acquisition Setup Main tab page) govern how the data will be
processed. Integrating multiple events on the CCD really brings the power of the
PI-MAX3 to bear on low-light gate-mode experiments. If the experiment allows,
many pulses can be summed on the CCD with no pulse artifacts in the readout. The
signal increases nearly linearly with the number of pulses (within limits imposed by
the CCD).
MCP Bracket Pulsing
Introduction
The principal utility of gating is that it allows temporal discrimination against background
light. By allowing the photocathode to “see” only during the event of interest, very high
background illumination levels can be tolerated without materially degrading experiment
results. The limit on this technique is set by the light leakage of the intensifier, which,
although it is very good, is not without limit. In the visible, the on/off ratio of a typical Gen
II Intensifier with just the photocathode gated is excellent, typically between 10
6
and 10
7
. In
most measurements, this ratio is sufficient to assure that the signal reaching the CCD
during the intensifier Off times will be too small to affect the data.
Below 350 nm, however, a second leakage mechanism occurs, optical leakage through
the photocathode to the UV-sensitive MCP, which reduces the On:Off ratio to about 2 ×
10
4
at 200 nm. This is the dominant response of a Gen II image intensifier to UV photons
when the photocathode is electrically off. At an On:Off ratio of 20,000, the ability of a
camera with conventional photocathode (only) gating to perform certain kinds of
measurements in the UV is adversely affected.
*
An internal timing generator pulse ensemble consists of a Gate Start pulse, a Gate Stop pulse,
and one or more Auxiliary pulses.
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