The image intensifier
The three functional units of a image intensifier: the photocathode, the micro channel plate and the phosphor screen.
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.
Fast gating capability of image intensifier tubes
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. Even mechanical shutter systems are not able to provide short gating times like the image intensifier. Therefore image intensified CCD cameras are a unique tool for ultra high speed imaging.
Image intensifiers consist of three functional units:
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.