Technical Reference
Choose the wrong band, and every lens, algorithm and mechanical design after it is forced to compensate. This page puts the physics, detector options and applications of each band side by side, so that selection can be done on paper first.
The horizontal axis is wavelength (log scale); the definitions of NIR and SWIR overlap at 0.9–1.0 µm, and the chart uses 1.0 µm as the boundary. Between 5 and 8 µm, water vapor absorbs strongly and the range is rarely used for imaging in practice, which is why there is a gap between MWIR and LWIR.
By Wien's displacement law, the peak emission wavelength of an object is λpeak ≈ 2898 / T (µm·K). This formula gives the first answer to "which band should I use".
NETD values are typical; actual specifications vary with the model and integration time, and are meant only for order-of-magnitude judgment at the selection stage.
| Band | Wavelength | Common detector materials | Cooling | Typical NETD | Applications |
|---|---|---|---|---|---|
| NIR near infrared |
0.75 – 1.0 µm | Extended sensitivity of silicon (Si) CCD/CMOS | Not needed | — | Night-vision illumination, machine vision, biological fluorescence imaging |
| SWIR shortwave infrared |
0.9 – 1.7 µm (up to 2.5 µm) |
InGaAs (indium gallium arsenide) | Not needed, or TEC-stabilized | — | Defect inspection inside silicon wafers, moisture and foreign matter detection, viewing 1064/1550 nm laser spots, haze penetration, sorting of agricultural products and recyclables |
| MWIR midwave infrared |
3 – 5 µm | InSb (indium antimonide), HgCdTe, T2SL type-II superlattice | Cooling required (InSb about 77 K; HOT types about 110–150 K) |
20 mK | High-temperature process monitoring, gas detection by characteristic absorption (CO₂ 4.3 µm, CH₄ 3.3 µm), long-range observation payloads, glass and metal melting |
| LWIR longwave infrared |
8 – 14 µm | Uncooled microbolometers (VOx/a-Si), HgCdTe | Mostly not needed | 40 – 60 mK (uncooled) |
Thermal imaging of objects at room temperature, power equipment and building inspection, body temperature screening, PCB hot spot analysis, thermographic non-destructive testing |
| VLWIR very long-wave infrared |
Above 14 µm | HgCdTe, Si:As | Deep cooling required | — | Astronomy, space remote sensing, very cold targets |
Answer these five questions in order, and usually only one or two bands remain.
Use λpeak ≈ 2898 / T. Room temperature at 300 K falls at 9.7 µm (LWIR), 500 K at 5.8 µm (between the two atmospheric windows), 800 K at 3.6 µm (MWIR), and 1500 K is already at 1.9 µm (the edge of SWIR). The higher the temperature, the shorter the best band.
MWIR and LWIR see the thermal radiation the object emits itself and need no extra illumination; SWIR mostly sees reflected light and needs sunlight, night-sky glow or an active light source. This question directly decides whether the system needs an illumination module.
Silicon becomes increasingly transparent above about 1.1 µm (above 1.15 µm it hardly absorbs at all), which is why SWIR can see inside wafers; ordinary optical glass gradually stops transmitting above about 2.5 µm, so MWIR lenses mostly use silicon or germanium, windows often use sapphire or calcium fluoride, and LWIR mostly uses germanium. Long-range atmospheric transmission in the thermal infrared relies mainly on the 3–5 µm and 8–12 µm windows; within SWIR there are also windows at about 1.0–1.35, 1.5–1.8 and 2.0–2.4 µm.
To reliably resolve temperature differences of the 0.1 K order (for example thermographic inspection of delamination in composites), the signal usually needs to be 3 to 5 times the NETD, which is very tight for uncooled cameras (about 40–60 mK); if you only need to find overheating spots, uncooled LWIR is more than enough, and the cost can differ by an order of magnitude.
The Stirling cooler in a cooled camera is a consumable with a defined service life in operating hours, and the camera needs time to cool down at start-up. For production lines with 24-hour continuous monitoring, uncooled solutions are usually evaluated first.
The difficulty in selection is usually not the table, but that your object meets several conditions at once. Describe the scenario to us, and we can do a feasibility assessment first.
Before requesting a quotation, use these three formulas to check whether a lens focal length is feasible.
p) behind a lens of focal length f subtends a fixed angle IFOV = p / f; projected onto the object plane, this angle becomes the size the pixel actually covers, GSD = IFOV × distance. (Schematic, not to scale.) To see smaller defects there are only three ways: shorten the distance, lengthen the focal length, or use a smaller pixel pitch.| To calculate | Formula | Example |
|---|---|---|
| Field of view FOV | FOV = 2 · arctan( D / 2f ) | Sensor width 10.88 mm, focal length 25 mm → about 24.6° |
| Single-pixel angle IFOV | IFOV = p / f | Pixel pitch 17 µm, focal length 25 mm → 0.68 mrad |
| Ground/workpiece resolution | GSD = IFOV × distance | 0.68 mrad × 10 m → about 6.8 mm per pixel |
A thermal camera measures radiation, not temperature. Set the emissivity wrong and the temperature is wrong: this is the most common source of measurement error on site.
| Material surface | Emissivity ε (approximate, LWIR) | Measurement notes |
|---|---|---|
| Polished aluminum, stainless steel | 0.05 – 0.10 | Almost a mirror; what you measure is mostly reflected surroundings. Apply high-emissivity tape or black paint as a reference point |
| Oxidized or rough metal | 0.60 – 0.90 | The degree of oxidation varies widely, and may differ even within the same batch |
| Painted surfaces (non-metallic paint, any color) | 0.90 – 0.95 | Colors differ greatly in visible light but hardly at all in the LWIR; metallic paints with aluminum or other metal pigments are the exception, with emissivity as low as 0.3–0.6 |
| Plastics, rubber | 0.90 – 0.95 | Thin films may be partly transparent; check the thickness |
| Glass | 0.85 – 0.95 | Opaque in the LWIR; you measure the surface temperature, not the interior |
| Human skin | ≈ 0.98 | Close to a blackbody, but strongly affected by ambient temperature and airflow |
| Abbreviation | Full name | Description |
|---|---|---|
| NETD | Noise Equivalent Temperature Difference | Noise equivalent temperature difference. The temperature difference equivalent to the noise (signal-to-noise ratio = 1); the smaller the number, the more sensitive. To resolve reliably, a temperature difference usually needs to be 3–5 times the NETD |
| FPA | Focal Plane Array | Focal plane array, the sensor itself in an infrared camera |
| NUC | Non-Uniformity Correction | Non-uniformity correction. Compensates for differences in pixel response; key to thermal image stability |
| MTF | Modulation Transfer Function | Modulation transfer function; quantifies how well an optical system preserves contrast at each spatial frequency |
| EOIR | Electro-Optical / Infrared | Combined visible and infrared payload, often also integrating a laser rangefinder (LRF) |
| LRF | Laser Range Finder | Laser rangefinder; boresight alignment is needed when it shares a line of sight with EO/IR |
| T2SL | Type-II Superlattice | Type-II superlattice, a new generation of detector material for MWIR/LWIR |
Items related to this page that we also supply, each with specifications and selection notes.
Original illustration, not a product photo. Actual appearance varies by model.
Infrared imaging (SWIR, MWIR, LWIR) principles consulting and advisory services: we help you confirm your choice of waveband and lens focal length, and can extend into system integration, early validation and custom test validation of camera modules.
