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Chapter 7 WinX and Gated Operation 123
Figure 7-1: Timing Diagram: MCP Bracket Pulsing
For emitted photoelectrons to be accelerated in the MCP, the MCP must be biased ON. In
conventional intensified cameras, the MCP is biased ON continuously. In PI-MAX4,
however, when bracket pulsing is ON, the MCP is biased OFF until just before the
photocathode is gated ON and is biased OFF shortly after the photocathode is biased OFF.
By bracket pulsing the MCP off (in addition to photocathode gating,) the on/off ratio of the
Gen II PI-MAX4 in UV is improved by two to three orders of magnitude. The resulting UV
ratio exceeds even the high levels normally achieved in the visible. Applications that benefit
from this new approach include LIF and nanosecond pump-probe experiments.
NOTE:
Bracket pulsing does not help in the visible region. Under
extremely low duty-factor conditions, the only remedy is to
install an external shutter ahead of the camera.
7.4.1 Bracket Pulsing in LIF Measurements
Most experiments using laser-induced fluorescence to probe combustion flows are
performed with UV probe/lasers. Atomic emission from flames also has significant UV
content. If the flame is continuous, the UV background will also be continuous. Even where
a flame is transient (e.g. internal combustion engine,) its lifetime can be many milliseconds,
compared to the nanosecond time scale of the laser used. This background can be a million
times as long. If the background is bright, then a UV on/off ratio of 20,000:1 will be
overwhelmed by the duty cycle and will not be adequate for extracting a signal of 10
-5
. In
high dynamic range quantitative measurements, backgrounds must be kept to an absolute
minimum. MCP bracket pulse gating dramatically improves the rejection of CW and even
millisecond time-scale background.
An alternative to suppressing background for imaging has been the use of very narrow
spectral bandpass filters. In the UV, these filters are expensive and they can have low
transmission at their central wavelength. An additional filter is required for each wavelength
to be imaged. The use of electronic temporal rejection of CW or quasi-CW background may
make it unnecessary to use these filters, thus increasing the optical throughput, sensitivity,
and quantitative precision of the measurement.
Off
On
Off
On
OffMCP Gating
Photocathode Gating
~1 μs
Off
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