SWIR and the Value of Shortwave Infrared

11 Aug.,2022


infrared lenses

That makes InGaAs interesting, but what makes it useful? Well, InGaAs sensors can be made extremely sensitive, literally counting individual photons. Thus, when built as focal plane arrays with thousands or millions of tiny point sensors, or sensor pixels, SWIR cameras will work in very dark conditions. Night vision goggles have been around for several decades and operate by sensing and amplifying reflected visible starlight, or other ambient light, in what are called image intensification (I-Squared) tubes. This technology has worked well for direct view night vision goggles. But when an image needs to be sent to a remote location (an intelligence center, for example), there is no practical method which does not introduce reliability and sensitivity limitations (e.g. I2CCD). Because all of SUI’s SWIR sensors convert light to electrical signals, they are inherently suitable for standard storage or transmission techniques.

Using SWIR at night has another major advantage. An atmospheric phenomenon called night sky radiance emits five to seven times more illumination than starlight, nearly all of it in the SWIR wavelengths. So, with a SWIR camera and this night radiance - often called nightglow - we can "see" objects with great clarity on moonless nights and share these images across networks as no other imaging device can do. For more information on night vision, go to our Night Vision application page.

But aren't there other cameras that operate in the shortwave infrared range? Yes, sensors constructed from materials like mercury cadmium telluride (HgCdTe) or indium antimonide (InSb) can be very sensitive in the SWIR band. However, in order to increase their signal-to-noise ratio to usable levels, these cameras must be mechanically cooled, often to extremely low temperatures. In large military aircraft designed for surveillance and reconnaissance, cooling is not a problem since these platforms are designed with plenty of space and power to run mechanical cooling systems. In stark contrast, similar sensitivity can be achieved at room temperatures with a Sensors Unlimited InGaAs camera.

Essentially, InGaAs cameras can be small and use very little power, but give big results. InGaAs found early applications in the telecom industry because it is sensitive to the light used in long distance fiber optics communications, usually around 1550 nm. Sensors Unlimited's InGaAs diodes and arrays continue to be used in telecom, often in Optical Power Monitors that employ spectrometers operating in the SWIR band. Surprisingly, interesting capabilities have resulted from this combination. By using SWIR illumination like 1.55 micron lasers or LEDs, it is possible to covertly illuminate a scene that can be viewed only with SWIR cameras. So, imagine a military truck able to temporarily illuminate its path when nightglow is not available (for instance, a road under thick tree branches or in a tunnel) in order to safely navigate a route in hostile territory. SUI InGaAs cameras are of great value in such applications. And because such illuminators are eye safe, there are few restrictions on how they are used.