Image intensifier: Multi-channel plate - MCP
The Micro Channel Plate (MCP) 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.
Schematic drawing of a single MCP image intensifier (left) and double MCP image intensifier (right).
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.
As standard the ICCD cameras from Stanford Computer Optics are equipped with a 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).
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.
|Single stage MCP||double stage MCP||triple stage MCP|
|Electron multiplication||good (103)||excellent (106)||extremely (108)
|secondary e- per incoming e-|
|Optical resolution||excellent||very good||adequate||line pairs per mm|
|Linear mode repetition rate (photon current, saturation effects)||excellent||good||adequate||images per second|
|Signal to noise ratio (SNR)||good||excellent||adequate|
|best image quality standard||high sensitivity optional||not recommended|