NETD, pixel pitch, lens and frame rate: thermal specs explained

Short answer: for most buyers, look for a 384×288 sensor, 12 µm pixels, a 19 to 25 mm lens and NETD under 25 mK. Those four numbers decide the detail, how far you see and how well the picture holds on damp nights; then check the battery hours, which tell you whether you swap at midnight. Everything else on the spec sheet is second order.

The spec sheet, line by line

NETD: the number for damp nights

NETD measures noise. A sensor rated at 20 mK can separate two surfaces that differ by 0.02 °C from its own background noise; at 50 mK it needs a difference 2.5 times larger. On a cold, dry night, almost every sensor looks good because animals are much warmer than the ground. On a warm, humid evening, or in rain, the differences shrink, and that is when a low NETD keeps the outline of a hog crisp instead of mushy.

Two caveats. First, brands measure NETD differently: some quote the sensor alone, some the whole system with the lens, at different temperatures. Pulsar lists both for some models. Our methodology keeps the higher, less flattering figure. Second, Vortex and ATN publish no NETD for the Veil 400 and OTS LT 320, so we leave that criterion out of their scores instead of guessing.

Pixel pitch and focal length: the reach pair

Pixel pitch is the distance between the centers of two sensor pixels. Smaller pixels mean each one covers a smaller slice of the scene, so a given lens resolves an animal farther away. That is why a 35 mm lens on 12 µm pixels (Pulsar Axion 2 XG35) reaches about as far as a 50 mm lens on 17 µm pixels (Pulsar Telos): 35 ÷ 0.012 = 2,917 and 50 ÷ 0.017 = 2,941 pixels per radian.

That ratio is the single best way to compare reach across brands, and it is what our range calculator and our "computed reach" use. Brand range figures use different targets and odds, so they are listed on each review but not scored.

The flip side of a long focal length is a narrow field. The Burris BTH35 v3 sees far through a 7.8° window; the Pulsar Axion XQ19 shows 19.5° across but reaches about 40% less far than the same lens on 12 µm pixels. Woods want the second, open fields the first.

AGM ReachIR LRF 25-3848.8/10Read the review →

F-number, germanium and the lens itself

Thermal lenses are made of germanium or similar materials because ordinary glass blocks long-wave infrared. The F-number is focal length divided by lens diameter: F1.0 means the lens is as wide as it is long, which gathers plenty of energy. The Nocpix Vista H35R uses F0.9, slightly faster. Lower F-numbers help sensitivity in practice, which is why brands pair their best sensors with fast lenses.

Some brands leave the focal length off the spec sheet. ATN does on the OTS LT 320; we compute about 25 mm from its 8.8° field and 320-pixel, 12 µm sensor, and label it "computed" everywhere.

Frame rate, display and the rest

• Frame rate: 50 Hz (Pulsar, AGM, RIX, TOPDON, Vortex, Burris) or 60 Hz (Nocpix, ATN). Both pan smoothly.
• Display: more screen pixels make the picture comfortable, not more detailed. A 1280×960 screen on a 256 sensor is still a 256 image.
• Battery: brand hours are measured warm, often with Wi-Fi off (AGM says so). Swappable 18650 cells or packs (AGM, RIX, Pulsar, Nocpix, Vortex, Burris) beat built-in batteries (TOPDON, ATN) on long, cold nights.
• Water protection: IP67 and IPX7 both cover immersion. "Waterproof" without a code (Vortex, Burris) or "weather resistant" (ATN) tells you less.
• Warranty: from 1 year (TOPDON) to lifetime (Vortex VIP). We score it at 5% of the overall weight.

Figures from official manufacturer pages and US prices checked on November 9, 2026. Images: manufacturers. How we score: see our methodology.