Infrared radiation can be categorized by wavelength as follows:
Short-Wave IR (SWIR, 1–3 μm): Reflects sunlight; used for imaging in low-light conditions.
Mid-Wave IR (MWIR, 3–5 μm): Emitted by high-temperature objects (e.g., engines, explosions).
Long-Wave IR (LWIR, 8–14 μm): Emitted by objects at ambient temperatures (e.g., humans, buildings).
MWIR and LWIR dominate thermal imaging applications, but their distinct characteristics demand careful evaluation.
1. Spectral Sensitivity and Temperature Range
MWIR: mid-wave infrared excels in detecting high-temperature sources (100°C to >1000°C). It has strong atmospheric transmission in the 3–5 μm range, ideal for long-distance imaging and requires cryogenic cooling (e.g., Stirling coolers) for photon-based detectors (InSb, HgCdTe) to reduce noise.
LWIR: long-wave infrared is optimized for ambient-temperature objects (−50°C to 100°C). It operates in the "thermal infrared" band, where most terrestrial objects emit peak radiation. It also uses uncooled microbolometers, enabling lightweight, low-power devices.
2. Detector Technologies
MWIR Detector: it is a kind of photon detector with high sensitivity, fast response time, but costly and complex (e.g., HgCdTe, InSb). It can be used in security, gas detection (e.g., methane absorbs at 3.3 μm), industrial furnace monitoring etc.
LWIR Detector: it is a kind of thermal detector with microbolometers dominate; no cooling required, affordable, but slower response. It can be used in many applications, such as building inspections, medical thermography, firefighting, consumer thermal cameras etc..
3. Performance Metrics
Resolution: MWIR systems often achieve higher spatial resolution due to shorter wavelengths.
Sensitivity: MWIR detectors (photon-based) outperform LWIR in low-signal scenarios (e.g., distant targets).
Cost: LWIR systems are generally cheaper, driven by uncooled microbolometer arrays.
Environmental Robustness: LWIR performs better in humid or dusty conditions, as atmospheric absorption (e.g., water vapor) is lower in the 8–14 μm range.
