High Speed Gating Gating
is probably the most important advantage of the PI·MAX.
Gating allows the detection of low light level signals
in the presence of interfering light sources of much
greater energy by temporal discrimination.
Principle of Operation The intensifier
consists of a photocathode, a microchannel plate, and
a phosphor screen. A fraction (called the quantum efficiency,
or QE) of the photons incident on the photocathode is
converted into electrons. Single photoelectrons are
converted into electrons by the microchannel plate (MCP),
which acts as a distributed electron multiplier. The
electrons released from the MCP then strike the fluorescent
screen (phosphor) and cause it to emit far more light
than was incident on the photocathode. In the traditional
configuration, the voltage between the photocathode
and the input of the MCP is used to switch the intensifier
on and off. If the photocathode is electrically biased
more positively than the MCP, electrons will not enter
the MCP and the intensifier is gated off. If the photocathode
is negatively biased, electrons will be accelerated
toward the MCP and the intensifier is turned on.
MCP
bracket pulsing Another feature that contributes
to the PI·MAX extraordinary flexibility is provision
for MCP bracket pulsing. Traditionally, intensified
detectors discriminated against background signal by
gating the photocathode. Although this technique yields
very high peak Off/On ratios in the visible, background
signal can still prove troublesome in low-duty factor
measurements, particularly in the UV region.The PI·MAX
allows bracket pulsing of the MCP in addition to the
photocathode gating, to gain higher rejection (106:1)
in UV measurements.
MCP Gating The ability to gate the MCP
on and off gives rise to an additional technique, known
as MCP gating, which addresses the need to have the
same QE as that of slow-gate tubes, but with shorter
gate widths. The lower resistance of the two sides of
the MCP allows the MCP to be gated more quickly than
a slow-gate photocathode. This technique gives gate
widths of <10 ns (better than slow-gate tubes), but
does not compromise the QE of the system.
Photocathode Spectral Range Princeton Instruments offers a selection
of Gen II and Gen III fimless intensifiers, covering
the entire visible and NIR spectral region. Gen II intensifiers
are available with red enhanced, blue enhanced, and
compromise red-blue enhanced photocathodes. Intensifiers
with MgF 2 windows are also available with response
in the 120-700 nm range. PI selects image intensifiers
for minimum noise, negligible corona, highest gain,
and longest operational and shelf life. In addition,
because of our close relationships with most of the
major intensifier manufacturers worldwide, we can usually
provide state of the art custom photocathodes. For special
requirements contact our office or your representative.
Effective Quantum Efficiency (EQE) To calculate
the effective quantum efficiency of the image intensifier
tube the noise factor of the intensifier must be taken
into account. GEN II and GEN III filmless intensifier
have a typical noise factor of 1.6. If you move the
mouse cursor over the QE plot you will see the typical
Quantum Efficiency of the selected intensifier.
Gen
III filmless intensifiers for high sensitivity Princeton
Instruments offers intensified CCD cameras with generation
III GaAsP intensifiers for very high sensitivity. The
intensifiers are fiber optically coupled to a variety
of high resolution CCDs with 512 x 512, 1024 x 1024
and 1024 x 256 formats. The Gen III filmless intensifiers
allow greater than 50% QE and sub-nano second gate times
(500 nsec). This improved performance is ideal for ultra-fast
gated applications such as plasma diagnostics, fluorescence
lifetime imaging microscopy and planar laser induced
fluorescence.
