As with anything that infantry is expected to carry, night vision device design must incorporate compromises balancing capability against the load they impose in terms of weight, bulk and power consumption.
Although core sensor technology is getting smaller, lighter and more frugal in its use of power while covering more bands of the electromagnetic spectrum than ever before, those spectral bands are made visible by different technologies. This forces the soldier to choose which spectral bands they will need to see, to carry several different devices, or to carry bulky apparatus that combine two or more sensor technologies. Those spectral bands include visible light, of which there is usually enough from the moon, stars or artificial sources. Sensors that amplify visible light often have an extended response into the longer wavelengths of the near Infrared (IR) spectrum. These include the analogue image intensifiers that have dominated infantry night vision for decades, and digital Complementary Metal Oxide Silicon (CMOS) chips that are finally challenging these legacy devices in some of these applications.
Israel’s Meprolight, for example, makes infantry observation devices and weapon sights based on image intensifiers. As analogue devices in an increasingly digital world, image intensifiers couldbe considered anomalous, but their performance continues to improve: “Slowly and surely, I have seen the technology advancing, mainly in the quality of the picture produced by the tubes themselves,” said Yonatan Pinkas, director of sales and marketing at Meprolight: “Once you could have got a certain detection, recognition, or identification range on one tube with a certain magnification, but now with the same magnification on a sight, for example, you can get to longer ranges because of the sharpness of the image”, he told AMR: “That is what has been happening over the past five years, and I think the next five years will just see a really great leap in the technology.”
Among Meprolight’s latest products is the Mini Hunter, a magnified sight for small calibre weapons such as assault rifles created to fill a gap in the market: “Most night vision sights are made for the longer range applications; for DMRs (Designated Marksman Rifle), for sharpshooters and for snipers. This sight was specifically made for assault rifles, and offers two times’ magnification,” he said: “With the hunter family, it was important to implement a very precise click system so the sight could be very accurate at the longer ranges and the user would have an easy way to zero the sight and compensate for ballistics.” In the Minimon-L, Meprolight created a device that can be mounted on a face mask, a helmet or a weapon, Mr. Pinkas told AMR: “Not all monoculars on the market have that ability. It was very important for us to have all three abilities in the same device.” Mr. Pinkas regards thermal clip-ons based on uncooled microbolometer detectors that attach to image intensified sights as the most practical way of providing dual sensor capabilities, meeting one of the market’s most pressing demands.
Another barrier is connectivity to get the image from the observation device or the weapon sight to a remote display on a helmet, for example, and out to the network: “There is a certain technology boundary with that, but it’s a race,” Mr. Pinkas added: “Every manufacturer has the same demand from end users and it seems like everybody’s racing towards a solution.” That boundary lies in digitising the intensifier’s image, which involves coupling a high resolution digital imaging device such as a CMOS chip closely to the phosphor screen, which is difficult without degrading the image unacceptably. Pressed to look ahead ten years or so, Yonatan Pinkas said that he expects the performance differences between analogue and digital night vision technologies to slowly but surely evaporate: “I think the quality will be pretty much alike, but the price factor will have a bigger impact on which equipment the user will buy in the end.”
Night vision devices using digital imaging technologies are, of course, already available. One of the latest is the CMOS Night Observation Device (CNOD) from Rochester Precision Optics. Designed to serve as a handheld monocular optic and as either a clip-on or stand-alone weapon sight, it is offered with a base for attachment to an M-1913 Picatinny Rail and is hardened to withstand the shock from firing most weapons of up to 7.62mm in calibre. Its CMOS sensor responds to wavelengths between 500 and 1080 nanometres (nm) and provides high-resolution images in conditions ranging from bright sunlight to dark nights, operating through the dusk and dawn periods during which soldiers must usually switch between day and night optics. The CNOD’s spectral coverage also enables users to see spots from most tactical lasers by day and night. It can also record still images internally and export video, allowing the user to minimise exposure to a potentially dangerous target, while RS-232 and USB (Universal Serial Bus) interfaces provide connectivity with tactical communications systems so its output can be networked. While CMOS imagers can produce sharp colour images by day, as light levels drop the colour gives way to grey scale. However, small amounts of full spectrum visible light from the moon and stars are available on most nights.
Colour at night
Cameras able to exploit this technology to produce colour images on dark nights have obvious attractions. One company that offers them is SPI Infrared whose X27 Osprey camera core is based on Broad Spectrum Thin Film Array (BSTFA) CMOS technology. With the equivalent of an ISO (International Standars Organisation) light sensitivity rating of five million and a spectral coverage of 390nm to 1200 nm, the X27 Osprey achieves a claimed luminance gain of x 85000. The sensor itself is a ten megapixel device that produces 4K High Definition (HD) colour imagery down to light levels as low as 1 millilux, achieving video frame rates of 60 Hertz and signal-to-noise ratios that meet or exceed those of the latest image intensifiers. Video footage on the company’s website shows impressive full-colour imagery taken under overcast starlight conditions.
Other low-light CMOS colour cameras, says the company, use filters to ensure natural looking colour, but block some light. The X27 Osprey avoids this with on-chip processing for colour correction. The module is ready to be integrated into infantry weapon sights and both monocular and binocular observation and targeting devices. One of these is a handheld binocular with an eye-safe laser rangefinder. Also under development is a version of this camera known as the X28, which extends its spectral response to 1600nm to take in 1550nm IR lasers.
SWIR for infantry
Further into the IR spectrum is the SWIR (Short Wave IR) band between approximately 900nm and 3000nm. SWIR radiation is plentiful during the day from the sun and at night from the sky glow phenomenon. SWIR cameras produce natural looking grey scale images from reflected light rather than the emitted light associated with the longer thermal wavelengths. SWIR has another advantage over thermal IR wavelengths in that it will pass through glass and can, unlike thermal imagers, see through windows. SWIR cameras are now offered in sizes suitable for infantry equipment. Sensors Unlimited has pioneered the technology and now offers a number of devices including the Warrior Handheld/Weapon/Helmet (HWH), the SWIR Pocket Scope (SPS) and the Warrior Illuminator for covert laser illumination. The Warrior HWH enables users to see through haze, smoke and dust and to see targets marked by battlefield lasers in daylight and at night. Its SWIR detector uses a 640 x 512 element Indium Gallium Arsenide (INGSAS) focal plane array sensitive to wavelengths between 700nm and 1,700nm. Like all SWIR sensors, it needs no cooling, helping achieve more than four hours of continuous operation on its battery. Attaching to a Picatinny rail, the weapon mount allows the sight to be flipped aside if necessary.
The Warrior PRF Decoder Module attaches to the hot shoe on the left side of the Warrior HWH and draws power from its battery. Using its own SWIR focal plane array detector and nine degree field-of-view lens, it will track and decode up to three laser markers or designators simultaneously and continuously by day or night out to a range of four kilometres (2.5 miles), says the company, overlaying colour symbology and PRF (Pulse Repetition Frequency) codes on the image. The SPS, meanwhile, offers very similar advantages to the Warrior HWH but in a smaller, lighter package. It’s 640 x 500 element chip provides images in the same 700nm to 1700nm band and uses a 25mm lens and provides more than two hours of continuous operation on a pair of CR123A batteries. Like its larger sibling it can be hand held or mounted on a helmet or weapon rail. It features a digital zoom capability and can save still images and video to an internal memory card. The US Government subjects products with SWIR sensors to its International Traffic in Arms Regulations (ITAR).
Packing multiple sensors into handheld devices is a goal furthered by the latest CMOS cameras, whose extended spectral response is reducing the need for separate devices to track laser spots. Two of the latest to benefit from this are Safran’s JIM Compact lightweight, long-range multifunction binoculars and the Moskito-TI multi-purpose target locator from the company’s Vectronix division: “Their most innovative function is their capability to see spots from laser pointers or even designators without adding weight”, a Safran spokesperson told AMR. The JIM-LR contains two CMOS cameras, one being a 15 megapixel daylight colour television sensor offering a wide field-of-view of 13.5 degrees and a narrow field-of-view of 4.5 degrees, the other sensor is a low light camera with respective fields of view of 6.2 degrees and 4.5 degrees. Both are designed to provide lower weight and broader capabilities than any device in their class, according the spokesperson: “They address a large spectrum of users including dismounted infantry (and) also special forces and joint fire units such as JTAC (Joint Terminal Attack Controllers or Forward observers.” Weighing less than two kilograms/kg (4.4 pounds/lb), the JIM Compact also features a cooled MWIR (Medium Wave IR) thermal imager from Sofradir, which it jointly owns with Thales, providing fields of view that match those of the colour TV camera and a 640 x 480 element detector. The JIM Compact is also fitted with an eye-safe laser rangefinder with an effective range of more than twelve kilometres (7.5 miles), a digital magnetic compass, inclinometers and an embedded GPS (Global Positioning System) receiver.
Weighing less than 1.3kg (2.9lb), the Moskito-TI has a sensor suite that includes a direct view optical channel with a field-of6view of 6.1 degrees and six times magnification, which provides a daylight observation capability that does not require electrical power: “Also, Safran’s products include advanced connectivity such as Ethernet interfaces and picture/video streaming that contribute to increase efficiency in the battlefield,” the spokesperson continued.
There are two main thrusts of development with the cooled detectors used in such systems. The first of these is producing focal plane arrays with individual elements placed closer together to provide greater resolution in the same physical envelope or to preserve resolution while miniaturising the system.
While the industry standard is around 15 microns, Sofradir now offers detectors with a pixel pitch of ten microns in the form of the Daphnis-HD MW, a MWIR detector. The company claims better performance than comparable ten micron (µm) detectors along with up to a 55 percent improvement in the Detection, Recognition and Identification (DRI) range over the previous generation of IR detectors. Those pixel numbers still fall short of those associated with visible spectrum detectors, in part because of limitations with the manufacturing technology for thermal IR sensitive materials but also because of the diffraction limit, which has long been thought to be fundamental, as Claire Valentin, vice-president of marketing for Sofradir explained to the author: “In the visible you have wavelengths approximately between 0.4 and 0.9 microns, here in IR we have three to five microns µm and eight to twelve microns,” she said: “If you have a wavelength in long wave between eight and twelve microns, let’s say, the minimum pitch to be sure you will not have too much interference between the waves you need to be in the range of ten microns to 15µm.”
The second challenge is developing detectors that operate at higher temperatures without affecting sensitivity, the advantage being that the Stirling cycle cooler has an easier life so it can either last longer or be made smaller and lighter: “Our current generation works at 110 degrees Kelvin (K) and we are working to move from 110K to 140K or 150 K,” she said: “The main advantage is to reduce the size of the cooler for more compact products, which is very interesting for portable systems.”
Breaking the rules
Looking further into the future, the diffraction limit might not be fundamental after all and going beyond it is the purpose of the US Defence Advanced Research Projects Agency’s Extreme programme. Here, DARPA is challenging industry to exploit the emerging technology of engineered materials (ENMATS) that derive their optical properties from their structure and can behave in ways that “break away from traditional rules and “laws” that artificially constrain modern optics. One such technology that will be readily appreciated by any infantry soldier could result in practical night vision glasses a little thicker than an ordinary pair of sunglasses. This technology is being developed by a team from the Australian National University (ANU) led by Professor Dragomir Neshev and exploits a phenomenon known as second harmonic generation to convert IR wavelengths to visible ones. Using tiny ‘nano-antennae’ in the form of discs of dielectric aluminium gallium arsenide crystals embedded in optically transparent material, incident IR light is mixed with a strong laser ‘pump’ beam to generate a new visible image. The conversion happens directly without an intermediate electronic stage, but does require batteries: “This is to power a laser diode that is needed for the conversion of the light from IR to visible. A laser diode of one Watt, similar to a bright battery-operated head torch, would be sufficient for several hours of operation,” Prof. Neshev said. He added that a practical device for military use could be built within five years. Night vision technology in general and infantry systems in particular could be on the cusp of a revolution.