White Paper: Short Wave Infrared (SWIR) for Surveillance Applications in Defense

SWIR technology has long been used across a wide range of applications due to its distinct features.

Included in these are civil applications (solar cell inspection, bottling plant inspection, fruit inspection, firefighting, etc.) and military applications (laser guiding systems, camouflage detection, surveillance, night vision, border monitoring, etc.).

The principal advantages of using SWIR technology in military applications are that SWIR cameras generally “see through” and produce higher-contrast images than visible-light cameras do through haze, mist, rain, fog, and challenging atmospheric conditions. This paper demonstrates visibility of 35 km through the thick haze of North Eastern Taiwan and explains the potential to see even further.

This is due to reduced Rayleigh scattering in the atmosphere at relatively longer wavelengths:

A simple way to think about this is to imagine a sunset. As the sun lowers towards the horizon, less and less blue, green, and yellow light permeates the atmosphere, as they have wavelengths shorter than red. Red light, with a longer wavelength, is not scattered so easily (and infrared light even less so), so the sun appears red. Therefore, as the concentration of particles of water vapor in the atmosphere increases, shorter wavelengths are scattered more than longer wavelengths. This makes imaging in visible, UV, and X-ray very difficult or impossible in bad weather.

Secondly, SWIR can be used for night-vision applications; however, care must be taken to understand how to integrate these cameras with the right accessories to achieve the best results. This is discussed further on in this article. Lastly, and most importantly from a military standpoint, SWIR is invisible to the human eye, enabling covert operation. It is therefore a good choice for targeting with laser designators between 1060 nm and 1550 nm. These laser designators also have the advantage of being eye-safe at very high powers.

Good surveillance operations rely on robust equipment that can handle any environmental conditions. This is why governments and private-sector organizations worldwide are turning to SWIR technology for naval, air, and ground security.

Modern fighter jets are equipped with the latest stealth technology, making their presence virtually undetectable with even cutting-edge RADAR surveillance systems. When they are at their fastest before an attack, as they are travelling well beyond the speed of sound, the noise of a fighter’s engine will arrive only after the attack is completed.

For these reasons, optical technologies are being incorporated into an increasing number of anti-air systems. As an aircraft accelerates, the exhaust air temperature can exceed the minimum “visible” temperature detectable with SWIR imaging.

The exhaust jets appear as large white spots contrasted against the highly absorbing aircraft exterior. The visibility of this heat is only enhanced at night, when the jets contrast against the black night sky. One further advantage is that the high image contrast provides an easy target for tracking software to lock onto.

SWIR for Very Long Range Land & Sea Surveillance

Identifying targets the size of fishing boats or cars at ranges over 35 km is critical in an era when threats can be found in isolated mountain ranges or far out at sea. These threats are further compounded by poor visibility in bad weather conditions. Even in good weather, coastal visibility with traditional optics is limited to around 13–16 km. In these conditions, however, long-range SWIR is only limited by the magnification power of the optics being used.

In a recent field test carried out in Northern Taiwan by Allied Scientific Pro, boats as far away as 35 km were visible, as demonstrated in figures 3.i) and 3.ii).

Another example of the tactical advantage of SWIR in long-range, bad-weather scenarios is shown in Figures 4.i) and 4.ii).

The quality of information from targets within 20 km is also improved when SWIR technology is used. Figures 5.i), 5.ii), and 5.iii) show how fine details on a minaret less than 5 KM away can be exposed with just a 0.4 megapixel SWIR camera versus a 5.0 megapixel CCD visible light camera.

These pictures were taken using a modified 6” aperture telescope optimized for SWIR applications.

Water is a good absorber of SWIR light. This means that water appears black when viewed with a SWIR camera. An immediate use for this physical property is detecting maritime or other sea-borne objects. Figures 6.i) and 6.ii) demonstrate how many boats can be spotted and tracked quickly and easily over very large areas.

Large and small navy vessels painted to blend into the natural environment, as well as stealth boats with anti-RADAR surfaces, can be detected over 20 km away as large white spots on a black background with just simple SWIR optics. Shallow submersible objects will also be detectable. By measuring the position of a boat relative to the horizon, it is also possible to give a reasonable estimate for the range and therefore the GPS position of that boat.

When choosing a SWIR camera, consider your environment, application, and budget. Is the application long-range surveillance or large-area surveillance? Will the camera be used mainly during the day or also at night? Will many cameras be used for an extensive area network or just one for a special scenario?

Legal Restrictions:

SWIR technology from the USA is considered highly sensitive, and in most cases, ITAR restricts export to any other country. If you are outside the USA, we recommend ensuring your camera is manufactured outside the USA to avoid lengthy export license issues and other legal challenges. All ASP SWIR cameras are manufactured outside of the USA.

The resolution of a camera defines the level of detail obtained in an image. VGA resolution is equivalent to 1.3 megapixels. Most SWIR technology is available in only one of these two formats, and the price and performance can differ substantially. It is also important to note, however, that the level of detail over a particular field of view can be altered much more cost-effectively by careful selection of optics. Figures 7.i) and 7.ii) are two images of the same landscape taken in both ¼ VGA and full VGA formats.

Wavelength of light versus pixel size in SWIR and LWIR cameras

For an experienced purchaser of thermal imaging cameras (LWIR), it is not uncommon to confuse the pixel size of a camera’s imaging chip with the wavelength of light the chip is to image. Typically, an LWIR camera images a wavelength range of 8–12 µm but has a pixel size of 1–2 µm. A SWIR camera, on the other hand, images a wavelength of approximately 0.9 – 1.6 µm and has a pixel size of around 20 – 30 µm. For reference, the wavelength of visible light is often written as 0.4 – 0.7 µm.

Night Vision

Many sources on SWIR technology offer conflicting opinions on the use of these cameras for low-light and night-vision applications. Below (figure 8) is a typical image from an uncooled SWIR camera at night over a nearby streetscape:

There are three main ways of improving the signal-to-noise ratio for night use:

1) Use a thermo-electrically cooled SWIR camera instead of an air-cooled version

Each sensor brand has its own ability to handle low-light levels, and care must be taken to ensure you choose the right one. The selected cooling level must also be considered: the higher it is, the better the signal-to-noise ratio (i.e., image quality). Today, almost all SWIR camera manufacturers offer air-cooled or electronically cooled models, allowing them to be placed in remote locations without maintenance.

2) Collect more light: increase exposure or use a larger aperture lens

This is a simple method for improving image quality from any camera, SWIR or otherwise – the more light you have, the better your SNR. For example, figures 10.i) and 10.ii) show the same outdoor night scene with different integration times. The following images are taken using a Photonic Science Ltd. VGA SWIR Camera.

Another critical point here is that viewing a larger area (50 m) will give you a much better signal than viewing a smaller area (5 m).

3) Use an accessory, e.g., laser illuminator

Using a lighting accessory is by far the most effective way of making your SWIR system operate at night. There is a wide selection of light sources available that are invisible to the human eye and to standard night vision technology but are bright white to SWIR (as shown in figures 11.i through 11.iv). The bottom two images were taken using a Photonic Science Ltd. VGA SWIR Camera.

Allied Scientific Pro distributes cameras, telescopes, and lenses for SWIR, LWIR, and Visible camera solutions.