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What is FOM in picture tubes?
FOM (Figure of Merit) is a value to better compare image intensifier tubes. Especially if you compare Gen.2 to Gen.3.
Top Gen.2 tubes have a cathode sensitivity of 650-800µA/lm. Gen.3 from 1.800 µA/lm.
So the Gen.3 tube would have to be 3 times as "bright" as a Gen.2 tube. But she isn't. This is due to the special structure of the Gen.3 tubes. These have a thin aluminum film built into the tube. This protects the photocathode from ions and thus from rapid aging or destruction. On the other hand, this film also blocks a large part of the electrons created in the photocathode and these cannot get into the MCP (Miro Channel Plate) for electron multiplication, i.e. amplification. Therefore the µA/lm value of Gen.2 and Gen.3 tubes is not directly comparable. FOM can help here.
The FOM value is calculated by multiplying the resolution and the signal/noise ratio of a picture tube. Example: 64lp/mm times S/N 24 = FOM 1536