FOM, SNR, gating — understand an image tube in 10 minutes
Three acronyms set the price of an image intensifier tube — and intimidate buyers. FOM, SNR, gating. None are mysterious once you have the map. We draw it here.
FOM — Figure of Merit
The FOM is a synthetic performance indicator, defined as resolution (lp/mm) × SNR. An FOM of 1800 means a tube with 64 lp/mm resolution has an SNR of 28.1. The higher the FOM, the better the tube — but the number alone is just an average.
Civilian market brackets for 2024-2026: FOM 1200 (entry), FOM 1600 (mid-tier), FOM 1800+ (high-end), FOM 2200+ (apex). Beyond 2000, you're paying mostly for performance in very low-light conditions (deep dark, no moon, no pollution).
SNR — Signal-to-Noise Ratio
The SNR measures image cleanliness — how much useful signal reaches the eye relative to the photonic and electronic noise inherent to the tube. An SNR of 25 is decent, 30 is good, 35+ is excellent.
It's the SNR that makes you feel the difference between two tubes with similar FOM. A tube with high resolution but average SNR produces a grainy image that tires the eye over long use. A high-SNR tube produces a smooth, almost cinematographic image.
Gating — electronic shuttering
An autogated tube includes a circuit that cuts power hundreds of times per second, protecting the tube from strong light sources (headlights, flashes, streetlights).
In urban areas, gating changes the game. Without gating, a passing car overloads the tube and causes a visible blackout. With gating, the transition is imperceptible. It's also a generation marker: most tubes > FOM 1600 are gated by default.
How to combine the three
For rural observation, prioritise SNR (35+) over high FOM. For mixed or urban patrol, aim for FOM 1600+ gated. For apex operational use, FOM 2000+ with systematic gating and individually measured SNR at delivery.
None of these numbers are arbitrary. All are measured on a bench, certified at delivery, and reported on the tube's data sheet. Ask for them before buying — a vendor who refuses to provide the measured SNR of the tube sold should raise suspicion.