Heat as an Image, Not Light
Thermal imaging is fundamentally different from regular cameras and even from night vision. A standard color camera records red, green, and blue light reflecting off a scene. A near-infrared sensor (the kind used inside most night vision tubes and digital NV cameras) is still recording reflected light — just light at wavelengths the human eye cannot see. Thermal is something else entirely. It is not really behaving like light at all. There are no shadows on a face viewed through thermal, just hot and cold areas. The sensor is reading emitted long-wave infrared radiation — the heat signature of every object in the scene — and translating relative temperatures into a visible image.
This distinction matters in practice. Transparent water that is invisible to a color camera and to a near-IR camera shows up clearly on thermal because the water is colder than its surroundings. Conversely, things that are visually obvious — a printed pattern, a colored shirt, a logo — can vanish on thermal if their surface temperatures match the background. Thermal does not see color, ink, or texture. It sees temperature differences.
The Sensor and the Pixel
The core of a modern thermal device is a microbolometer array. Common specs across current handheld and weapon-mounted units make this concrete:
- Resolution. 640 × 480 is the current high-end baseline for units like the iRayUSA RICO MICRO RH25 V2, the N-Vision NOX18, the Trijicon SkeetIRx, and the Trijicon REAP-IR. More pixels means more detail and more usable digital zoom before the image becomes blocky.
- Pixel pitch. 12 µm is the current small-pixel standard (RH25 V2, NOX18, REAP-IR). Older or larger-pixel sensors run 17 µm (SkeetIRx). Smaller pixels allow a smaller, lighter sensor and shorter optics for the same field of view.
- Frame rate. 60 Hz is standard on serious units. 30 Hz appears on some configurations of the SkeetIRx and REAP-IR. Higher refresh rates produce noticeably smoother imagery when scanning or moving.
- Thermal sensitivity (NETD). The RH25 V2 lists <15 mK, meaning the sensor can resolve temperature differences smaller than fifteen thousandths of a degree. Lower numbers translate to more contrast between objects of similar temperature.
Because the sensor reads heat, the lens cannot be ordinary glass — glass blocks long-wave IR. Thermal optics use germanium objectives (the RH25 V2 uses a 25 mm f/1.0 germanium lens; the NOX18 uses an 18 mm f/1.0). Germanium is opaque to visible light and transparent to long-wave infrared. This is also why thermal lenses are expensive and why thermal optics generally have fixed, simple objective designs rather than complex zoom assemblies.
Calibration, Display, and Palettes
Microbolometers drift. To keep the image clean, thermal devices perform a non-uniformity correction (NUC) — a momentary recalibration where an internal shutter closes, gives the sensor a uniform thermal reference, and the unit recomputes the offset for every pixel. This is the brief blink or pause many thermals do periodically. Devices typically offer manual, automatic, and semi-automatic NUC modes (the RH25 V2 supports all three; the NOX18 is manual).
The processed sensor data is shown on a small internal display — AMOLED on the RH25 V2 at 1440 × 1080, OLED 640 × 480 monochrome on the NOX18 — viewed through an eyepiece. Color palette options determine how temperature gets mapped to brightness:
- White hot / black hot. The most common modes. Hot objects appear bright (or dark) against a cooler background.
- Red hot / color. False-color overlays that highlight only the hottest portions of the scene in red or another saturated color, useful for drawing attention to warm targets in cluttered imagery.
- Edge detect. Available on the NOX18 and similar units. Outlines hot/cold transitions rather than rendering full grayscale.
None of these palettes change what the sensor sees — they only change how the data is presented to the eye.
Range, Field of View, and Magnification
Thermal “range” is reported as detection, recognition, and identification distances. The N-Vision NOX18 publishes 753 yards detection, 267 yards recognition, and 136 yards identification. Detection means seeing that something warm is there; identification means knowing what it is. The gap between those numbers is large because thermal resolution is low compared to a daytime optic, and a hot blob at distance can be a deer, a person, or a running engine.
Field of view is a function of focal length and sensor size. The RH25 V2’s 25 mm lens gives 17.5° horizontal; the NOX18’s 18 mm lens gives 24.3° horizontal; the SkeetIRx covers 28.1°. Wider FOV is better for scanning and helmet-mounted use. Narrower FOV with a longer focal length (the REAP-IR’s 35 mm lens, 12° horizontal, 1.75× native magnification) is better for shooting at distance.
Most thermals offer digital zoom on top of native magnification — 2×, 4×, 8× is typical — but this is just cropping and upscaling pixels from the same sensor. It does not produce new detail.
Where Thermal Wins, and Where It Doesn’t
Thermal sees through smoke, dust, light foliage, and total darkness. It sees body heat through visual camouflage that perfectly matches the surrounding color and texture. It is genuinely transformative against an opponent who has no comparable capability.
It is not, however, a magic bullet. On a hot day, when ambient temperature approaches body temperature, a person blends into the background more than expected. Sun-baked rocks and dirt at 95–100°F radiate heat similar to a person, and small warm targets become genuinely difficult to pick out of that clutter. Walls, vehicles, and dense foliage block thermal entirely — it does not see through solid objects. Glass blocks long-wave IR, so a person behind a window is invisible to thermal even though they are visible in the daytime. Wet surfaces, recently disturbed ground, and warm machinery all create false signatures that compete for attention.
This is why fusion — combining thermal with image-intensified or digital night vision — is increasingly common. Edge-detected thermal overlaid on top of a near-IR image lets the user see fine detail (terrain, gear, faces) from the NV channel while still catching heat signatures that would otherwise hide. The same idea applies to color overlays that highlight only hot spots while preserving the underlying scene.
Form Factors
Thermal hardware shows up in a few standard configurations:
- Helmet-mounted monoculars like the SkeetIRx and NOX18, which can also be hand-held or moved to a weapon.
- Multi-role units like the RH25 V2 that are designed to swap between helmet, clip-on (in front of a day optic), and standalone weapon-sight roles.
- Dedicated thermal weapon sights like the REAP-IR, with selectable reticles, BDCs for common calibers, and onboard DVR for recording.
Power is almost always CR123 or 18650 lithium cells, with USB-C external power as an extended-runtime option. Runtimes on internal batteries land in the 1.5–7 hour range depending on cell choice and refresh rate.
Civilian thermal is now broadly accessible. The infrared and thermal worlds are no longer the exclusive domain of any single military, which means thermal light discipline — heat signature management, exhaust, glass, and body heat through doorways — has become as relevant to consider as visible-light and near-IR discipline.