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Light sensing keeps soldiers safer

With SPIE DSS approaching, Tom Eddershaw investigates some of the sensing technologies used to protect troops, including lidar and spectroscopy

The latest sensing capabilities for defence will be presented at the upcoming SPIE DSS event, which takes place from 5 to 9 May in Baltimore, USA (for more on the show, see the preview on page 18). Lidar (light detection and ranging) and spectral sensing will be covered in depth, both of which can give advanced warning to soldiers of harmful gases or explosives in the vicinity. The technology is not necessarily developed originally for the military, but there are certain adaptations that have to be made for defence, typically by making it smaller, more rugged and less expensive.

The Swedish Defence Research Agency conducts research into lidar and other sensing technology. Ove Steinvall, director of laser research at the agency, commented that the systems hold promise as sensing solutions for automated vehicles: ‘Automated vehicles need to have a sensor that can keep track of lots of vehicles and other potential hazards on the road. Hopefully we will see lidar enter motorised or aerial vehicles, so the systems need to be small and less expensive. This is helping drive the technology. There are a lot of industrial and robotic applications.’

While remote sensing has been used for terrain mapping since the 1970s, Steinvall added that now lidar can be used to give advanced warning of airborne chemical weapons, giving infantry troops valuable time to put on gas masks.

Steinvall said: ‘Detecting a cloud at a few kilometres could give the troops minutes to put on gas masks, depending on wind speed and direction. To some extent the system can estimate particle sizes and detect what the cloud is, which helps the system differentiate between safe and dangerous gases.’

One technique used, named DIAL (differential absorption lidar), calculates the concentration of a gas. The method involves sending out a pulsed laser beam into the atmosphere, which is backscattered by particles in the beam path and picked up by a sensor. Two closely spaced wavelengths are used, one of which is strongly absorbed by the gas. When the resulting light is collected, the difference between the two beams is calculated, which indicates concentration. The higher the concentration, the more absorption occurs and the larger the difference in pulses at the two wavelengths.

For imaging of objects at long range, more than 10km, and hidden in vegetation or smoke and fog, a time-gated sensor can extract clear images for identification purposes. The time gating function allows images to be seen at different range intervals, thereby excluding surrounding clutter. However, Steinvall pointed out that in bad weather or for longer range applications radar is still, and always will be, better for surveillance – especially over wide areas.

For detecting explosives, advances in spectroscopy have meant that spectrometers are finding their way from commercial applications into defence and border control-type tasks. One such product is the TacticID-GP from B&W Tek sold by Pacer. Originally – as Stephen Newport, product specialist at Pacer, explained – the technology behind the general purpose handheld Raman spectrometer was developed with the pharmaceutical industry in mind. Now, though, a more robust and much smaller system has been produced that can be used in the field to check for explosives, either as a handheld unit or attached to an automated bomb disposal robot.

Newport said that it is still early days for the TacticID-GP used to be used as a defence tool: ‘People talking about it would be early adopters,’ he said. For the pharmaceutical industry, B&W Tek had offered portable instruments for around three years. However, he said that until the last few years there hasn’t been anything robust enough to be applied to defence applications. ‘They had good systems, but it’s not suitable to give it to a soldier. You needed a laptop, there were fibres, and other components; they [the soldier] needed to be an expert at what they were doing to make them work.’ He continued: ‘It’s like everything; the technology has moved on, everything is getting smaller, which makes it easier to get this kind of capability into these types of devices. But that’s only recently been possible and it still takes a long time to get it into military applications, because of the testing and standards it has to meet.’

Optics-based military solutions need to be resistant to dust, mud, and water as well as working at night, in high temperatures, and still functioning following shocks and vibration. For dismounted troops, the product needs to be portable enough for a soldier to carry long distances and to work for long periods of time. It also needs to be simple enough for non specialists to use and to understand the results, while still providing the highest accuracy possible because lives could potentially depend on it.

Newport said: ‘The portable device contains a laser and a spectrometer, a PDA of some kind to run the software, and a battery pack. It is small enough to be carried.’ The device holds a library of spectral signatures. The amount of signatures is dependent on the model, but for the general purpose device, the initial library can compare the results with around 4,500 matches. If a match is found the device then flags the compound as a controlled substance or as safe.

Newport said: ‘Similar compounds have similar signatures, so a high resolution is required.’ However he pointed out that, for these applications, groups of compounds are typically analysed. If any one of a number of compounds falls under a chemical group marked as an explosive, then it is flagged as such. The spectrometer has WiFi capabilities for automatic updates and to store records off-site when communicating with a centralised system. The user can have a fixed higher resolution system situated away from the field that can process unidentified or unrecognisable samples and update the portable devices with these signatures.

There are difficulties in taking a technical, traditionally lab-based product, out into the field. One of the issues is that while a number of materials will work with the device, certain compounds will fluoresce and spoil the reading. In response to this, the TacticID is typically fitted with a 785nm laser – which, Newport said, is the norm for these portable systems. This reduces most of the fluorescence issues that can occur while still gaining a good Raman signal.

Typically, Raman spectroscopy can struggle with noise from the surrounding light. So, to combat this, Newport said: ‘The spectrometer is touching the material, which mitigates a large amount of the ambient light. As long as this is done, the surroundings shouldn’t have too much effect.’ He continued to explain that the system also takes a ‘dark scan’, a scan without the laser running, and subtracts the signal from the measured reading. This reduces noise from the array and when combined, these techniques remove a large part of the issues with regards to the conditions.
While these types of spectrometers will get smaller – Newport compared the current size to the dimensions of older, suitcase-sized versions – these devices are a long way from a mobile phone-type add-on, according to Newport.

Thermal imaging lenses

Another technology that can be applied to both commercial and defence markets is the lens technology used for thermal imaging. Carl Burns, Eastern regional sales manager of Ophir Optics, explained that thermal imaging operates on the principle that all objects emit heat in the infrared as a function of temperature. The sensor technology in the camera sensor is sensitive to the changes. Commercial applications for security or for testing for heat loss just need to be able to tell if there is a warm body or hot gas within an area. Defence applications include night vision for soldiers, armoured vehicles and UAVs (Unmanned Aerial Vehicles), which tend to require a higher resolution of image.

In order to operate in near- to far-infrared, the lens’ substrate is made of exotic materials, such as germanium, silicon, zinc selenide, and zinc sulphide, Burns explained. He said: ‘They are CNC machined, diamond-turned or polished similar to visible and they often have anti-reflection coatings to get high transmission and hard carbon coatings for military specifications.’ This is to protect the lenses from adverse weather. As well as coatings, Burns said it was quite common to have a protective window built into a system to offer further protection to the lens.

Different applications have different requirements. ‘It all depends on what they want to see,’ said Burns. ‘How big is the field of view and target, do they want to recognise it, or do they want to identify it. If they want to see a target at X amount of distance, we have to calculate the number of pixels that is required on the detector for the specific lens required to do that.’

To focus on a target there are three main options, Burns explained: ‘You can have a fixed field view with one or two elements which are set to different fixed focal lengths, each covering a narrow range. If the application is only for a hallway or room, the fixed lenses suffice. For surveillance purposes at a perimeter, or from a UAV, higher-end duel field lenses are an option. These have a narrow and a wide field lens.’ When there is a target within the narrow field, the system changes to the wide lens to gain a better view of the scene. Or conversely, the wide angle FOV is used for situational awareness while the narrow would be more suitable for targeting-type applications. The third method is to use zoom lenses, which can change focus manually or with motors.

Traditionally the military has used I2 technology, which amplifies low level light to create an image. This however, as Burns pointed out, has limitations: ‘It works well as long as the stars are out, there’s no smoke or dust or anything like that.’ These ‘dirty battle fields’ are best suited to long wave applications. Because of this, Burns said that the military is now using a combination of the two technologies, I2 and thermal imaging, as well as others. He said that the image intensifiers are inexpensive when compared to thermal imaging, however more recently thermal technologies have come down enough in price to be more frequently used.

With prices coming down and components being made smaller to meet size, weight and power (SWaP) specifications, light-based technologies are now a huge part of defence applications for keeping soldiers safe on the battlefield.

About the author

Tom Eddershaw is a technical writer for Electro Optics, Imaging & Machine Vision Europe and Laser Systems Europe.

You can contact him on tom.eddershaw@europascience.com or on +44 (0) 1223 275 478.

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