SubscribeThe word “infrared” refers to a broad portion of the electromagnetic spectrum: everything between visible light and microwaves.
Much of the infrared range is not useful for ground- or sea-based imaging because it is blocked by the atmosphere. The remaining portions of spectrum are often called “atmospheric transmission windows," and define the infrared bands that are usable on Earth: Near Infrared (NIR), Short-Wave Infrared (SWIR), Medium-Wave Infrared (MWIR), and Long-Wave Infrared (LWIR).
-Spectral-Band.jpg "IR (Infrared) Spectral Band")
Atmospheric transmission of infrared bands
(Courtesy Raytheon)
Since NIR and SWIR are so near the visible bands, their behavior is similar to more familiar visible light. Energy in these bands must be reflected from the scene in order to produce good imagery, which means there must be some external illumination. Both NIR and SWIR systems can take advantage of sunlight, moonlight, starlight, and an atmospheric phenomenon called "nightglow," but typically requires some type of artificial illumination at night. Arrays of infrared Light Emitting Diodes (LEDs) often provide a very cost effective solution for short-range illumination, but achieving good performance at distances of over tens of meters requires more directed illumination.
Typical medium to long-range systems employ a focused beam from a laser or specialized spotlight, though special consideration of eye-safety issues is required.
NIR and SWIR Imaging Systems
While NIR and SWIR imaging systems often employ sensors that are more exotic than those found in consumer-grade camcorders and digital cameras, glass is transparent to wavelengths as long 3μm, so normal lens systems can be used and windows can be seen through. Because NIR has a wavelength longer than visible light, and SWIR a wavelength that is longer still, energy in these bands is scattered less by particles suspended in the atmosphere. This means that SWIR, and to a lesser extent NIR, systems are tolerant of low levels of obscurants like fog and smoke.
The MWIR and LWIR bands are often called "thermal" bands because a typical scene emits radiation in these ranges. An imaging system that operates in these ranges can be completely passive, requiring no external illumination because it is able to sense the energy that is radiated directly from objects in the scene. Two major factors determine how bright an object appears to a thermal imager: the object's temperature and its emissivity. As an object gets hotter, it radiates more energy and appear brighter to a thermal imaging system. Emissivity is a physical property of materials that describes how efficiently it radiates. Because cloth has a lower emissivity than skin, to a thermal imager cloth will appear darker than skin even when both are exactly the same temperature.
At these long wavelengths, infrared radiation behaves differently from visible light. Glass is opaque in the LWIR band, and blocks most energy in the MWIR band. Consequently, LWIR and MWIR systems cannot use inexpensive glass lenses, but are forced to use more exotic materials like silicon or germanium. Glass windows are also not transparent in these bands, so they appear brighter or darker according to their temperature. Since radiation in the MWIR and LWIR bands is not transmitted by water,
rain can coat a scene and wash out much of its thermal contrast resulting in a duller image.
LWIR Bands
Atmospheric obscurants cause much less scattering in the MWIR and LWIR bands than even the SWIR band, so cameras sensitive to these longer wavelengths are highly tolerant of smoke, dust and fog. Even small effects like atmospheric turbulence can add up over very long distances to impact range performance, allowing LWIR an edge over MWIR.
Hotter objects emit more of their energy at shorter wavelengths. The peak emissions of an object at room temperature falls in the LWIR band, so for objects at normal earthly temperatures, a MWIR system must be more sensitive than a LWIR system to achieve identical imaging performance. The emissive peak of hot engines and exhaust gasses occurs in the MWIR band so these cameras are especially sensitive to vehicles and aircraft, but since hotter objects emit more total radiation, they are still easily detected by LWIR imagers.
