The
shutter:
a crucial feature of ICCD high speed cameras
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The
shutter of an ICCD high speed camera is not a separate, operational
system
component but an
operation mode of the image
intensifiers photocathode,
called gating.
There are three
voltages applied to the image intensifier as shown in
the drawings.
This operation is called gating, the voltage UCM is called gating
voltage and the ICCD camera itself is therefore also called gateable
CCD camera.
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If
the voltage UCM between photocathode and multichannel
plate is negative, the photoelectrons are accelarated towards the multi
channel plate. This means that the shutter is open.
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If the voltage UCM
is positive, the photoelectrons are kept at the photoathode, thus the
shutter is closed.
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An unique
advantage
of the gateable ICCD camera over all other kinds of
cameras is the ability of ultrafast gating. Shutter speeds, i.e.
exposure times of 5 to 10 nanoseonds (ns) are mostly standard with
gateable
ICCD cameras nowadys. Some cameras on the market yet offer sub-ns
gating down to 200
picosecond (ps) shutter speed, though measured as full width at half
maximum
(FWHM) of the gaussian-like transmission curve, as shown by the blue
curve in the picture
below. However, this gaussian like shape exposes the CCD chip to the
light over typically 500 to 600ps, thus severeley
degrading time resolution far beyond 200ps.
However, STANFORD COMPUTER OPTICS gateable ICCD high speed cameras are
unrivaled
concerning ultrafast gating. Our unique Coaxially Coupling Technology
(CCT) achieves outstanding rectangular shutter transmission
characteristics down to the physical limits. The
1%-to-90% rise time as well as the 90%-to-1% fall time amount to
30ps each. As shown in the diagram (green curve), our
shutter transmission yields 200ps at 90% opening hence only
260ps at 1% opening.
This is realized by our unique integrated system
design. The
photocathodes are coated with an in-house developed anti-iris grid and
connected to the gating electronics by a dedicated coaxial waveguide.
The light travels 6cm in 200ps which approximately is the
distance between the gating electronics and the photocathode. During
the rise and fall times of 30ps the light travels only 9mm, which is
exactly the half diameter of the photocathode. Thus, our
rectangular curve shape represents the physical limit.
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Among a lot of ultrafast phenomenon
applications' firsts this unique rectangular shutter transmission
enabled
range gating with sub-millimeter precise spatial resolution.
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In
addition, STANFORD COMPUTER OPTICS gateable ICCD high speed cameras are
able to
stationary maintain outstanding 10MHz repetition rate in multiple
exposure mode. This unique feature makes them extremely valuable in a
large
variety of fluorescence microscopy applications like FLIM, FRET, FISH, TIRF
and others, where the ICCD camera can be synchronously triggered to
ultrafast laser excitation. Unlike to EMCCD cameras the readout noise
of the ICCD camera is negligible compared to the already intensified
light signal. There is no need for slow scan modi and the maximum frame
rate
is also applicable in difficult low light conditions and even in single
photon counting mode. |
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