Tube Specs Explained

Intensifier tube specs read like alphabet soup until you know what each number actually means. This page walks through the four numbers that matter most — FOM, SNR, Halo, and EBI — plus the brand and tier vocabulary you'll see on every tube product page.

FOM — Figure of Merit

FOM is the single best summary number for a tube's overall performance. Calculated as resolution (line pairs per millimeter) × signal-to-noise ratio. Higher is better. Bands: 1600+ entry commercial, 1800+ solid commercial, 2000+ strong commercial / entry Ground Spot, 2200+ Ground Spot territory, 2400+ Aviation Spot territory.

SNR — Signal-to-Noise Ratio

How much real image versus noise floor. Below 25 looks gritty under starlight; above 28 is dramatically smoother. Aviation Spot tubes regularly clear 30+.

Halo

The bright ring around point light sources. Lower halo = tighter ring = less obscuration of detail near a bright light. 0.7 mm is excellent; 1.2+ is noticeable. Matters most near lit environments.

EBI — Equivalent Background Illumination

How much self-light the tube produces in total darkness. Lower EBI = less glow = see fainter signal. Below 1.5 is excellent.

Tube tiers

Commercial (FOM 1600–1900) meets manufacturer's commercial spec. Ground Spot (FOM 2000–2200) exceeds commercial on key parameters. Aviation Spot (FOM 2200–2400+) is the top tier — lowest halo, highest SNR.

Brand families

L3Harris Unfilmed: most-stocked premium tube, Ground Spot and Aviation Spot tiers, mostly ITAR-controlled. Elbit Thin-Film: strong commercial-tier. NNVT: solid commercial White Phosphor at lower cost, not ITAR-controlled. Photonis: European, intermittently available.

White vs Green Phosphor

Performance specs are independent of phosphor choice. White Phosphor renders in white and gray, often easier on the eye over long sessions.