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.

A FOM number gives an indication of a tubes quality and usefulness but 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.