Tube Spec Guide

FOM is calculated as Resolution (lp/mm) × Signal-to-Noise Ratio (SNR). It's a single-number summary often used to compare tubes at a glance. A higher FOM generally indicates a higher-performing tube — but two tubes with the same FOM can perform very differently in practice depending on which underlying spec is dominant.

Measured in line pairs per millimeter (lp/mm), resolution describes the tube's ability to resolve fine detail. Higher numbers mean finer detail at the same distance — but bright-condition resolution and low-light resolution are different measurements.

SNR describes the contrast between useful image signal and the inherent noise of the tube. Higher SNR generally means a cleaner-looking image, especially in low-light conditions where the noise floor matters most.

EBI measures how much light the tube produces in total darkness — it's effectively the noise floor. Lower EBI is better, particularly for very dark environments. EBI is sensitive to temperature and is often spec'd at room temperature.

Halo is the visible glow around bright point sources (streetlights, vehicle lights). Lower halo is preferred. Different tube generations and manufacturers handle halo differently.

Gain describes how much the tube amplifies incoming light. Higher gain isn't automatically better — too much gain can wash out detail. Most modern tubes have automatic gain control built into the system.

Photocathode sensitivity (μA/lm) describes the responsiveness of the input photocathode. Higher sensitivity contributes to brighter, cleaner imagery — but interacts with other specs and is not meaningful in isolation.

Blemishes — cosmetic spots in the image — are common and usually not performance-affecting. Tubes are spec'd by zone (Zone 1 / 2 / 3), with Zone 1 being the center. Most builders prioritize a clean Zone 1 over a flawless overall image.

Green phosphor (P22 / P43) and white phosphor (P45) describe the color of the image you see through the tube. White phosphor is widely preferred for the more natural visual appearance, though some users still prefer green for specific applications. Performance differences between the phosphor screens themselves are minor — tube performance is driven by the components in front of the screen.

Gen2+ and Gen3 describe broad generations of image intensifier technology with different photocathode chemistry and performance envelopes. Gen3 tubes typically have higher photocathode sensitivity and longer lifecycle, at higher cost. Gen2+ tubes can be a strong value option, particularly with quality white-phosphor screens.