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It must be remembered that the fiber plates replace a glass window and a ( mica ) membrane, in addition to an optical output lens system.

The efficiency Af of an Af lens at the magnification Af is: Af.

Neglecting absorption, the end losses of the coupling membrane and the output window Af would be 6 percent and 8 percent.

Thus, the combined efficiency of the elements replaced by the two fiber plates ( with a combined efficiency of 0.25 ) is 0.043 or about six times less than that of the two fiber plates.

Gain of fiber coupled image intensifiers

Including the brightness gain Af due to the Af area demagnification, the overall gain of a fiber coupled double stage image intensifier is: Af.

It is obvious that the careful choice of photocathode which maximizes Af for a given input E ( in the case of the second stage, for the first phosphor screen emission ) is very important.

The same consideration should govern the choice of the second-stage phosphor screen for matching with the spectral sensitivity of the ultimate sensor ( e.g., photographic emulsion ).