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Shortwave Infrared Sensors: A New Way to See

Compact, lightweight, and uncooled imaging sensors are fueling a revolution in intelligence, surveillance, and reconnaissance (ISR). They are the key enablers for a powerful new breed of sophisticated, small, unmanned aerial systems (UAS) and man-portable sensor systems. Very capable small UAS can work from lower-altitude vantage points and still remain undetectable. With their smaller sensor payloads, they are a fraction of the cost, and still produce uncompromised results. Uncooled indium gallium arsenide (InGaAs) shortwave infrared (SWIR) and longwave infrared (LWIR) thermal microbolometers have been primary enablers in this imaging revolution.

SWIR light bridges the spectral gap between the visible and thermal bands. SWIR imagers detect reflected light, offering more intuitive, visible-like images. SWIR light propagates longer distances undistorted and scatter-free than does shorter-wavelength visible light, so it is better suited for imaging in adverse environments and weather conditions including fog, dust, and smoke. SWIR imagers can also see in low light conditions, and use eyesafe 1550-nm illumination, totally undetectable by regular night vision equipment. Most importantly, InGaAs SWIR imagers can see all the lasers on the battlefield, a capability lacking in other night vision technologies. SWIR imagers generate digital video outputs and offer more dynamic range than traditional image intensifier night vision equipment.

LWIR imagers, on the other hand, see thermal emissions, making them excellent for detecting hot targets such as people or vehicles. LWIR imagers can see in very dark conditions when there is not sufficient light available for other night vision technologies to see anything. Thermal imagers operate at even longer wavelengths than SWIR, sometimes providing better obscuration penetration, but not during dawn and dusk transitions. Capturing shadows and contrast in reflected light is characteristic of SWIR images, and makes interpretation intuitively easier than with thermal images. SWIR imagery offers an essential complement to thermal imagery for positive target identification.

altCompact, lightweight, and uncooled SWIR sensors based on high-quantum-efficiency InGaAs technology — like the 90-gram SU640KTSX imager from Sensors Unlimited-Goodrich ISR Systems (Princeton, NJ) shown in Figure 1 — have been developed. Compact imaging sensors are capable of full-motion video at a 640 × 512 pixel resolution from daylight to quarter-moon, while operating uncooled at room temperature. SWIR imagers feature spectral response from 0.9 to 1.7 microns, and extend down to 0.7 microns — a broad spectral range encompassing all the key battlefield laser wavelengths. Requiring as little as 2.5 watts of electrical power, these imagers are suitable for small UAS payload and man-portable sensor applications.

Hinted SWIR Imagery

No single sensor modality works best in all scene variations, but combining complementary features of multiple sensing modalities through image fusion comes closest. Image fusion is a process of combining video streams from multiple sensors into a single composite video stream in real time without losing contrast or resolution. The complementary combination of LWIR for detection and SWIR for positive identification enables potential threats to be triaged at greater ranges and in more weather conditions and environments than each sensor is capable of doing alone.

SWIR imagers work primarily with reflected light, in contrast to thermal imagers. In very-low-light-level conditions at night under no moon or even on heavily overcast days, there can be times when contrast with the background is insufficient for good detection of people. Hinted SWIR™ from Goodrich is a variant of image fusion in which a primary grayscale SWIR scene is complemented with colorized thermal hints provided by an uncooled LWIR microbolometer. Figures 2 and 3 show examples of Hinted SWIR imagery, where, for example, important low-light-level illuminated scene detail from the SWIR sensor forms a contextual background for thermally active target highlights detected with the LWIR sensor.