Teledyne Princeton Instruments PI-MAX 3, Системное руководство

Страница: 97 / 250

97
Chapter 8: WinX/32 and Gated Operation
This chapter discusses gated operation with the SuperSYNCHRO™ timing generator
and aspects of operation of the PI-MAX3 not covered in
Chapter 6, First Light , on
page 49. We additionally suggest that you review Chapter 15, Tips and Tricks , on
page 183, which contains information that should prove helpful in getting good results
in more complex measurements.
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 fall in a valid gate width during a valid exposure. Many
gate pulses can be placed into one exposure at repetitive or sequential 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
≤
4 ns Full Width at Half Maximum (FWHM) 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-MAX3's MCP bracketing
feature can be used to extend the On:Off ratio to 10
6
:1.