Short-Wave Infrared (SWIR)
The sensitivity in the short-wave infrared range (0.9–1.7 um) has become a reality only recently due to the development of InGaAs sensors. But why use short-wave infrared?
Firstly, there's a fundamental fact: light in the short-wave infrared spectrum is invisible to the human eye. The visible light spectrum extends from wavelengths of 0.4 micrometers (close to ultraviolet, appearing blue to the human eye) to 0.7 micrometers (deep red). Wavelengths longer than visible light can only be seen using specialized sensors like InGaAs. However, even though light in the short-wave infrared region is invisible to the human eye, it interacts with objects in a similar way to visible light. That is, short-wave infrared light is reflective; it reflects off objects just like visible light. Because of this reflective property, short-wave infrared light creates shadows and contrasts in its images, similar to visible light. Images from InGaAs cameras can match visible light images in resolution and detail; however, the colors in short-wave infrared images are not actual colors. This can make objects easy to identify, while also constituting one of the tactical advantages of short-wave infrared, namely object or individual identification.
This makes InGaAs interesting, but what makes it useful? InGaAs sensors can be extremely sensitive, capable of counting individual photons one by one. Thus, when made into arrays with thousands or millions of tiny sensor points or pixels, SWIR cameras can work in very dark conditions. Night vision goggles have been used for decades; they work by amplifying visible starlight or other ambient light that's sensitive and reflected, often called image intensifiers. This technology is suitable for direct observation with night vision goggles. However, there's still no practical method available that's reliable and sensitive enough for transmitting an image over a distance. As they convert light to electrical signals, they are inherently suitable for standard storage or transmission technology.
Using short-wave infrared at night has another major advantage. The brightness of the night sky, known as sky radiance, is 5 to 7 times stronger than starlight and is almost entirely in the short-wave infrared wavelength. So, with a short-wave infrared camera, combined with this often-called nightglow sky radiance, we can "see" targets very clearly on moonless nights and share these images over networks, something other imaging devices cannot do.
But are there other cameras that can operate in the short-wave infrared range? Yes, sensors made from materials such as mercury cadmium telluride (HgCdTe) or indium antimonide (InSb) are very sensitive in the short-wave infrared band. However, to raise their signal-to-noise ratio to a useful level, they must be mechanically cooled to extremely low temperatures. Cooling is not a problem in large military aircraft designed for monitoring and surveillance, as there is ample space and power on these platforms to run mechanical cooling systems. In contrast, InGaAs cameras can achieve the same sensitivity at room temperature.
Essentially, InGaAs cameras can be small, use minimal power, and yield significant results. InGaAs was originally used in the telecommunications industry because it is sensitive to light used in long-distance optical communication (typically around 1550 nanometers). By using short-wave infrared illuminators, such as 1.55-micrometer lasers or light-emitting diodes, it has become possible to illuminate a scene that can only be observed with a short-wave infrared camera. This illumination is safe for the human eye, and their use is almost unrestricted.
Thermal imagers are another type of camera with good detection capabilities. While thermal imaging can detect warm objects against a cold background, short-wave infrared cameras can actually identify what that object is. That's because thermal imagers cannot provide the resolution and dynamic range achievable with InGaAs short-wave infrared focal plane arrays.
Lastly, short-wave infrared imaging has a major advantage over other technologies: it can image through glass. For short-wave infrared cameras, specialized, expensive lenses or housings adapted for harsh environments are almost unnecessary. This allows them to be used in a variety of applications and industries. This capability also allows short-wave infrared cameras to be installed within a protective window, providing great flexibility when mounting camera systems on a potential platform.
So, why use short-wave infrared? Because short-wave infrared offers several advantages:
- High sensitivity
- High resolution
- Observation under night sky radiance
- Day and night imaging
- Covert illumination
- No need for cryogenic cooling
- Can use conventional low-cost visible light lenses
- Small size
- Low power