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122 PI-MAX
®
4 System Manual Issue 4
Refer to Table 7-1 for additional Internal Sync information when using Gated mode.
NOTE:
The Accumulations and Images (or Spectra) parameters
(WinView/32 or WinSpec/32 Acquisition Setup
Main tab)
determine how data will be processed. Integrating multiple
events on the CCD really brings the power of the PI-MAX4 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.)
7.4 Micro-Channel Plate (MCP) Bracket Pulsing
The primary advantage gained from incorporating 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. Limits for this technique are determined by the
light leakage of the intensifier, which, although very good, is not without bounds.
• Within the visible spectrum, the on/off ratio of a typical Gen II Intensifier with just
the photocathode gated is excellent, typically between 10
6
and 10
7
. For most
measurements, this ratio is more than adequate to guarantee that any signal reaching
the CCD during intensifier Off times is too small to affect the data.
• However, when working with wavelengths below 350 nm, optical leakage through
the photocathode to the UV-sensitive MCP occurs. At 200 nm, the on/off ratio is
reduced to approximately 2 × 10
4
. 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.
MCP bracket pulsing
1
keeps the MCP biased OFF except for an interval that brackets the
timing of the photocathode gate as shown in Figure 7-1.
Table 7-1: Internal Sync Information for Gated Operation
Timing Mode Shutter Control Ext. Trigger Input
Controlled by Internal Timing
Generator
Controlled by Internal Timing
Generator
Trigger In BNC
1. Bracket pulsing is not available for cameras having a filmless Gen III Intensifier. Gen III Intensifiers do not
respond in the UV.