Airborne SWIR Imaging for Surveillance and Situational Awareness

Modern security and surveillance technology is relied upon to deliver increasingly accurate situational awareness while remaining dynamic and autonomously adaptable in the field. As a result, airborne surveillance platforms, including both crewed aircraft and unmanned aerial vehicles (UAVs or drones), have become indispensable in meeting many security challenges. This article outlines the advantages provided to airborne surveillance by Short-Wave Infrared (SWIR) imaging systems and describes the attributes that must be considered when selecting a camera for this application.

The Advantages of SWIR

Imaging at SWIR wavelengths (0.8-2.5 µm) offers significant advantages over imaging at visible wavelengths in surveillance applications. Because SWIR photons penetrate much further through air than visible photons before being absorbed or scattered, SWIR imaging systems can clearly image targets at increased range (Wiley et al. 2022).

Figure 1 - The electromagnetic spectrum showing the Ultraviolet (UV), Visible, Short Wave Infrared (SWIR), and Mid/Long Wave Infrared MWIR/LWIR wavelength regions.

For airborne surveillance this means that accurate SWIR imaging can be achieved over a larger range of distances from (or altitudes above) a target on the ground. The increased range afforded by SWIR imaging is shown in Figure 2.

Figure 2 - Demonstration of the increased range achieved by imaging at SWIR wavelengths instead of visible wavelengths. In this image SWIR light also cuts through the fog to image the countryside at much greater distances. An uncooled C-RED 3 camera combined with a 16 mm SWIR lens was used to capture the SWIR image.

While atmospheric conditions such as fog, haze, dust, or smoke reduce visibility at visible wavelengths they are effectively transparent in multiple bands of the SWIR spectrum. SWIR imaging systems are therefore readily adaptable to use in a wide range of real-world conditions without technical intervention or reconfiguration. Figure 3 demonstrates the ability of SWIR imaging to detect what otherwise might be obscured by the elements.

Figure 3 - Haze across the Potomac River in visible light (left) and in SWIR (right; Driggers et al. 2013). Images courtesy of Peter Judd and James Waterman, NRL.

Many materials reflect and absorb light more distinctly at SWIR wavelengths than they do in the visible range. The enhanced contrast between different materials in SWIR images often makes it much easier to discriminate objects with different construction or composition. For example, figures 2-4 all demonstrate how SWIR imaging starkly reveals the contrast between artificial structures and their surrounding environment. Different materials comprising these buildings, like glass, concrete, and metal are also readily identifiable. Likewise, it’s even possible to both distinguish different types or species of vegetation and to separate the trunks of trees from the canopy. This ability to firmly discriminate and characterise objects aids rapid accumulation of detailed situational awareness that supports well-informed decision making, strategic planning, and accurate identification of threats.

Figure 4 - An uncooled C-RED 3 camera with a 70-200 mm tele-objective is used for a surveillance application.

Airborne SWIR Imaging

Accurate SWIR surveillance imaging from a drone requires a sensitive, reliable, high-speed, low latency SWIR camera with minimal size, weight, and power consumption (SWaP). Some degree of autonomous operation is often also desirable.

Andor recommends the C-RED 3 for drone-mounted SWIR imaging. C-RED 3 is a compact, uncooled, high-speed SWIR VGA camera that incorporates the 640 x 512 InGaAs Snake SW sensor from Lynred. Let’s break down this camera’s attributes (also see Table 1):

• High sensitivity and dynamic range. C-RED 3’s sensor delivers quantum efficiency > 65% at 1.0 - 1.65 µm (peaking at 73% ) and read noise below 40 e- rms. It has 14-bit native quantization, but this can be improved to 16-bit (91 dB dynamic range) when the camera is operated in High Dynamic Range (HDR) mode. Combined, these properties simultaneously deliver low light sensitivity and excellent quantitative discrimination of bright and dark regions in a scene without pixel saturation.
• High frame rate. C-RED 3 is capable of 600 FPS full frame imaging. These high frame rates effectively ensure that recorded images remain sharp and unblurred when acquired from a drone in flight.
• Low latency. C-RED 3 can be provided with either a USB 3.1 or a full Camera Link data interface. Camera Link reduces delay between the end of a full frame integration and the arrival of the first valid data on a UAV’s onboard computer to only 22.2 µs. In this configuration C-RED 3’s readout introduces negligible lag to a UAV’s data downlink and minimises imaging feedback delay to a ground-based pilot.
• Low SWaP. A standard C-RED 3 is designed to have compact dimenstions (56 x 56 x 59.5 mm), low mass (0.23 kg), and low power consumption (6.5 W). The OEM format, delivered without an external case for better integration into larger systems, weighs only 0.1 kg.
• Electronic shuttering. C-RED 3 embeds an electronic shutter with integration pulses shorter than 5 µs in full frame mode, effectively eliminating image distortion caused by the rolling shutter effect. Electronic shuttering also negates the need for a mechanical shutter, eliminating any downtime and cost associated with servicing it if it fails.
• Adaptive autonomous image corrections. C-RED 3 incorporates patented technology that compensates for the effects of temperature and exposure time variation on the buildup of dark current in images. Adaptive on-head corrections are performed with dark frames that are automatically computed by the camera firmware. Calibrated in factory, this process eliminates the need to perform multiple experimental dark acquisitions, simpifying camera setup and automatically optimizing image quality on the fly.

Table 1 - Relevant measurements of C-RED 3’s performance and properties.

Camera-System Integration

C-RED 3’s compact low SWaP design simplifies its integration into a larger system. Multiple options are provided for operational, optical, software, and data integration for C-RED 3. (see Table 2). Mechanical integration flexibility can be further enhanced in the camera’s OEM (without housing) format, and we can work with customers to deliver custom formats that precisely match their needs. The electronic boards of the sensor can be located several centimeters from the stack.

Figure 5 - C-RED 3 camera in standard housed (left) and OEM (right) formats.

Table 2 - Interface Options for C-RED 3.

Figure 6 - Example of a C-RED 3 camera in a custom format.

References

1. Driggers et al. 2013, Proc. SPIE 8706, Infrared Imaging Systems: Design, Analysis, Modeling and Testing XXIV, 87060L
2. Wiley et al. 2022, Proc. SPIE 12106, Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XXXIII, 1210606