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Chapter 7
Gated Operation with a PTG
Introduction
This chapter discusses gated operation with a PTG and aspects of operation of the
PI-MAX not covered in Chapter 4, First Light. We additionally suggest that you review
Chapter 11, Tips and Tricks, which contains information that should prove helpful in
getting good results in more complex measurements.
Gate Mode operation is more complex than Shutter Mode operation because of the
additional cabling and timing considerations that apply. However, it is in gated operation
that the PI-MAX achieves its maximum power and utility, able to recover extremely faint
fast signals in the presence of large ambient light levels.
Gating provides electronic shutter action by controlling the photocathode bias, allowing
the detection of low light level signals in the presence of interfering light sources of much
greater energy. For instance, in combustion research, a pulsed laser probe is used to
investigate the chemistry within a flame. Since the flame itself emits broadband light
continuously, the integrated flame emission is much greater than the integrated signal
resulting shortly after the laser probe (such as laser-induced fluorescence or Raman).
Fortunately, since the laser pulse is very short and the time at which it occurs is known, it
is possible to gate for a few nanoseconds during the laser pulse, thus reducing the flame
emission interference by approximately the measurement duty factor ratio.
Exposure time is the 'time space' from which charge integrating on the CCD will get
summed into the reported data. Gate width is the time during which light will be detected
by the intensifier, intensified, and applied to the CCD. Basically, the intensifier controls
what the chip 'sees' during the exposure time.
For signal to be detected, it must both fall in a valid gate width and in a valid exposure
time. Many gate pulses can be placed into one exposure time at random intervals because
there is no temporal measure inside a given exposure (all incident signals get summed
(integrated) into one value per pixel / superpixel inside a given exposure).
Using presently available image intensifiers and gate pulse generators, optical gate times
2 ns FWHM (full width at half maximum) are possible. Since the control is electronic,
the gate width can be made virtually as long as desired, allowing a wide range of
experiment requirements to be satisfied in one instrument setup. Further, in UV
measurements, where the On:Off ratio is only 10
4
:1, the PI-MAX’s MCP bracketing
feature can be used to extend the On:Off ratio to 10
6
:1.
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