The
micro channel plate multiplies the
incoming electrons from the photocathode.
It can be considered as a matrix of tiny linear
channeltrons in which the single channels are arranged by an angle of a
few degrees towards the axis of the image intensifier. The
multiplication mechanism is shown in the enlarged section of the
picture.
The inner diameter of the micro channels is typically 4µm and the
pitch amounts to 6µm. The diameter of the active surface of the micro
channel plate, i.e. the image intensifier, is 18mm as a
standard.
We use
the 18mm image intensifier in combination with a 1/2" CCD chip,
that measures 8mm in the diagonal. Therefore the image from the
intensifier is scaled down by a factor of 2.25 by a coupling lens. The
effective pitch of the micro channels as seen on the CCD chip in this
case is 1.5 x 6µm/ 2.25 = 4µm. For comparison, the pixel
size of a high resolution CCD chip is 4.7µm (1360 x 1024) and 8.6µm for
standard resolution (780 x 580).
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Micro
channel
plates are available as single stage MCP, double stage
MCP or triple stage MCP. The typical electron multiplication factor of
one MCP stage is 1000 secondary electrons per incoming electron.
Thus,
a
two stage MCP will give a multiplication factor of 1000 x 1000 = 106.
In case of a triple stage MCP the maximum multiplication factor is
limited to less than 108 due to saturation effects.
 
When
choosing a microchannel plate with more than one stage it must be
considered
carefully, that additional stages significantely degrade the over-all
optical quality of the image intensifier as principally described in
the table below.
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