Figure of merit (FOM)
A single number for night vision tube quality: resolution multiplied by signal-to-noise ratio. Higher is better, but check whether it is a guaranteed minimum.
What it means
Figure of merit (FOM) combines two tube measurements into one number: resolution in line pairs per millimeter, multiplied by signal-to-noise ratio. A tube with 64 lp/mm and a signal-to-noise ratio of 25 has a FOM of 1,600. Tubes sold to hunters commonly fall somewhere from about 1,200 to over 2,000.
How it is stated
The same number can mean different things depending on how a seller states it:
- Guaranteed minimum: the tube will measure at least this.
- Typical: an average; your tube may be lower.
- Range: somewhere in this band.
- Floor, such as “1800+”: at least this, with no upper figure.
- Export minimum: the lowest figure the tube can be sold abroad at.
- Unstated.
The arithmetic is not always tidy either. At least one tube maker’s published minimums do not multiply out from its own resolution and signal-to-noise minimums. Treat FOM as a summary and look at the two parts.
What it changes in the field
A higher FOM generally means a cleaner, sharper picture in low light. But the two parts matter differently: signal-to-noise ratio does more on the darkest nights, while resolution matters more when there is plenty of light. Two tubes with the same FOM can behave differently.
Export rules can cap the FOM of tubes sold outside their home country, which is one reason the same tube line may carry different figures in different markets.
What to look for
- A guaranteed minimum or a per-tube data sheet, over a typical figure.
- The signal-to-noise ratio and resolution behind the FOM.
- Halo, phosphor and autogating, which FOM does not cover.
How we use it on this site
We record FOM as the maker states it, with its basis, and never derive it from other figures.