Figure of Merit (FOM)

Figure of Merit (FOM) is a dimensionless value that has been introduced as an arbitrary reference and shorthand overall performance grade of an image intensifier.

It is calculated by the formula $$\mathrm{F}\mathrm{O}\mathrm{M}=\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{.}\cdot \mathrm{S}\mathrm{N}\mathrm{R}$$ where $\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{.}$ is the resolution (in lp/mm) and $\mathrm{S}\mathrm{N}\mathrm{R}$ is the signal-to-noise-ratio.

Considerations

Due to the resolution being usually around 2-3x higher than the SNR it has a larger impact on the value, although some consider SNR the more important factor of the two.

The FOM is not to be taken as a tell-all-end-all as a full spec sheet gives far more detailed insight as to a tubes performance albeit it might seem slightly more esoteric to people who are unfamiliar with interpreting a spec-sheet.

In order to fairly judge a tubes FOM rating one should have some knowledge of the category to which a tube belongs as well as an average or minimum number for at least one of the factors involved in producing a FOM rating. If one knows that a particular category of tubes has a minimum SNR of 30 with average at 36 then going by FOM alone when considering an individual tube of that category is less arbitrary.

Downsides

• The same FOM can be produced by a low SNR and high resolution and vice versa
• Other important specifications like gain, sensitivity, halo, MTF, ... are not taken into account