The
image
intensifier is
the most crucial system component of an intensified CCD high speed
camera, beside
the CCD sensor itself. It allows for an optimum adaption of the ICCD
camera to any specific application. The main function of the image
intensifier is the multiplication of the incoming photons, i.e. the
amplification of the incoming light signal. This enables the ICCD
camera to take images at extremely low light conditions and/or at
extremely short exposure times down to 200ps, when the
integral of the photon flux over the exposure time is very small.
It is
the image intensifier that provides the so called gating
capability of the ICCD camera, i.e. the shutter
function. If the
high speed camera is gated "on", the shutter is open. In this case the
incoming
light is intensified, i.e. amplified, transmitted to the CCD chip and
collected. If the camera is gated "off", the shutter is closed and no
light is transmitted to the CCD sensor.
The ICCDs can be gated down to the picosecond time scale. The gating of
CCD and EMCCD in principle is only possible on interline CCD sensors.
Due to the high capacitance of these area sensors gating starts only in
the microseconds range. Frame transfer CCDs cannot be gated faster than
a few milliseconds.
|
|
The
image
intensifier consists of three functional units:
- the photocathode
that converts the incoming photons
to photo
electrons,
- the micro
channel plate (MCP) that strongly
multiplies
these photo
electrons,
- the phosphor screen
that converts the multiplied
photo electrons back
to photons.
There are
three voltages applied to the image intensifier as shown in
the drawing. If the voltage UCM between photocathode and micro channel
plate is negative, the photoelectrons are accelarated towards the
microchannel plate. This means that the ICCD high speed camera is
gated, i.e. the
shutter is open. If the voltage UCM is positive, the photoelectrons are
kept at the photocathode which means that the ICCD camera is not gated,
i.e. the shutter is closed.
|
