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Shrinking The Sensors – recent trends in night-vision technology By Peter Grant

 


PIC.J  Fused dual-channel digital image L3EOSThe ability to see in the dark has come a long way since night fighter pilots and bomber aircrew were urged to eat carrots to improve their night vision during the Second World War. Some 70-odd years later, for soldier, sailors and airmen, the use of night-vision devices peculiar to their own roles has become second nature.  Products available today owe as much to the commercial market as they do to industry and military research and development establishments. Just check how many mega-pixels resolution the camera on your cell-phone has, and you’ll get the message.

As has been noted on these pages before, the parallel technologies of image (or light) intensification and infrared (or thermal) detection have led the evolution, each one bringing its own specific solutions to the challenge. The choice of which to use has depended on the application’s suitability to the mission.

Early model infrared detection devices, generically called thermal imagers, were big, clunky devices due to the size of the detector elements and the need for cryogenic cooling of the detector. They were also noisy and required much power to perform their function. So initially much too bulky for goggles and similar man-portable applications, for a soldier’s hand-held monocular device, a head-mounted goggle the image intensifier proved more suitable. They were also applied to surveillance devices or fire-control systems used on armoured fighting vehicles and on board warships.

Image intensification

Image intensification (II or I2) involves the collection and conversion of ambient light photons into electrons that are then multiplied by a cascading process before being reconverted back into visible light. They cover the visual and near infrared (V/NIR) spectrum from 0.4 to ~0.9 microns but for practical purposes is usually limited to the red end of this range.

A ‘classic’ example of the I2-powered night-vision goggle (NVG) is the almost ubiquitous AN/PVS-7 model. Conceived by what is now the US Army’s Night Vision and Electronic Sensors Directorate (NVESD); originally refined and produced to MIL-SPEC by ITT Night Vision (later Exelis, now absorbed into the Harris Corporation) and Litton EOS (later Northrop Grumman Electro-Optic Systems, and now part of L-3 Warrior Systems). It has been licensed produced in several parts of the world and commercially in the US (to non-Mil-Spec standards). Well over 215,000 units have been produced since it entered US Army service in 1988 (and remains in use), as well as being in service around the world.

As with all technology, I2 has evolved with performance and product life improving over the years. Some 15 year ago, most I2 tubes were of Gen 2+/early Gen 3 standards, as defined by the materials and technology used. At the turn of the Century, a so-called Gen 4 configuration, without a barrier film on the microchannel plate (MCP) in an attempt to extend tube life, but then just as quickly died in favour of a thin-film tube. Since then, thin-film Gen 3 tubes, instanced by the Exelis (now part of the Harris Corporation) Pinnacle range or the L-3 Warrior Systems Ultra range, have been further refined to deliver better performance and longer life. Indeed, today’s I2 tube has a lifetime so long that it will likely be replaced as obsolete long before it fails.

Being the world leader in investigating I2 technologies and initiating applications, the NVESD, not surprisingly, guards the US technology fiercely. Even before ITAR regulations emerged, the export of US-manufactured I2 tubes to countries beyond close US allies was restricted. The measure used is known as the “Figure of Merit” (FoM), which is obtained by multiplying the tube’s resolution (in line-pairs per millimetre) by its signal-to-noise ratio (SNR). It still applies, which is why non-US organisations – notably, but not exclusively, instanced by the Franco-Dutch company Photonis – have developed equivalent products that in some criteria exceed and in others fall just short of US specifications but remain ITAR-free.

As the I2 tube evolved to provide better performance, lighter weight and lower cost, users found they could upgrade many of their existing devices, mostly the genre described by the catch-all moniker of NVG, by simply swapping over from a first-generation (Gen 1) I2 tube, to a Gen 2 tube and then, again to a Gen 3 tube. Other minor adjustments may be required to perhaps the optics or power system, depending on the actual device.

It is impossible to put an exact figure on the number of I2-powered systems in use worldwide but it is a fairly safe bet to say that, even with the introduction of the AN/PSQ-20 Enhanced NVG (which fuses or, more accurately, overlays a thermal image into the I2 image, of which more anon) that at least 90% of the NVGs (monocular, bi-ocular, binocular) in US military service today are pure I2: for the rest of the world, that figure has to be 99-100%. It certainly remains less expensive to upgrade existing stock than buy a whole new unit.

Thermal (infrared) imaging

Thermal imaging cameras are based on photon detectors, using exotic materials such as indium antimonide (InSb) and mercury cadmium telluride (MCT, CMT or HgCdTe) for their sensing detector arrays, and require cryogenic cooling to deliver the sensitivity required for imaging. Although more sensitive than the newer uncooled microbolometer arrays, their size, weight and power (SWAP) requirements are greater, as is their cost.

The uncooled detectors, which use a somewhat less sensitive detection principal, have taken longer to refine to the point of practicality than did the photon detector technology. However, without a cooling system to complicate matters, uncooled detectors (microbolometers operating in the long-wave infrared [LWIR] spectrum of 8-12/14 microns) offer lower cost and reduced SWAP. For the future, HOT (Higher Operating Temperature) arrays are emerging, which will require less cooling, reduce SWAP, and increase reliability.

While cooled systems moved down the SWAP curve (offering both the mid-wave infrared [MWIR] spectrum of 3-5 microns as well as LWIR) they retained their place for more specific roles and platforms. Reduced SWAP, however, allowed uncooled weapon sights to become commonplace.

To illustrate the evolution of the uncooled sights, one need only look at the AN/PAS-13 Thermal Weapon Sight (TWS), produced in three variants: (V)1 Light TWS (for rifles and light anti-armour weapons),  (V)2 Medium TWS (medium machine guns) and (V)3 Heavy TWS (for heavy machine guns, sniper rifles and automatic grenade launchers).

Initially produced by Raytheon (A, B and E variants), with Low Rate Initial Production (LRIP) from 1995, procurement competition and technological development have seen various improved generations have been produced by BAE Systems (C version), DRS Technologies (D version) and L-3 Warrior systems (G version). Perusal of manufacturer’s datasheets show how the size and weight of the AN/PAS-13 has reduced over time as has the detector’s pixel pitch allowing, an increase in array format. The weight of the LTWS reduces from 1.4 kg in the B-model of 1998 to 0.88 kg in the E-model of 2007; MTWS from 2.3 kg to 1.3 kg and HTWS 2.5 kg to 1.5 kg; while the LTWS  array format stays at 320×240 pixels, those of the MTWS and HTWS rise to 640×480 pixels.

The array format is the configuration of the pixels within the detector’s focal plane array (FPA), expressed, for example, as 320×240 – this being the most common FPA format some dozen years ago. The pixel-pitch is the size of the individual pixels – the 320×240 format would probably have a pixel pitch of 25 or 30 microns, depending on the detector material.

The more pixels there are on an FPA, the better the image resolution and, depending on the optics, the longer the range. As pixel pitches reduced, pixel counts rose: a dozen years ago, a 640×480 FPA with a 20 micron pixel pitch was considered HIGH definition. Production detectors with pixel pitches of 17 and 15 microns are now the norm, with 12 microns emerging, and 10 and 7 microns under development.

Shrinking pixel-pitch

At this year’s Paris air show, Selex ES launched an MWIR detector which offers a 1,280 x 1,024 pixel cooled FPA on a pixel-pitch of just 8 µm, known as SuperHawk, which the company claims  as having “the smallest production-ready infrared pixels”. The SuperHawk’s 1,280 x 1,024 pixel FPA offers four times as many pixels as a conventional 640 x 512 pixel FPA with a 16 µm pixel pitch. The company says that SuperHawk is able to capture better than HD-quality images in total darkness by detecting temperature differences as small as 1/50th of a degree.

However, shrinking pixel size can lead to signal leakage between neighbouring pixels (known as ‘cross-talk’), which leads to image blurring. To overcome this drawback, Selex ES uses a technique that physically isolates the individual pixels from each other. Known as the mesa pixel format, it creates an inter-pixel ‘trench’ eliminating signal cross-talk between pixels which can cause image blurring. This, says Selex ES, allows the sharpest possible image. The MOVPE (Metal Organic Vapour Phase Epitaxial) process is used to grow the MCT infrared-detecting crystals used in the SuperHawk, a technology Selex ES notes it has been developing over 30 years. Trial samples of the SuperHawk detector are expected to be available by late-2015, with production quantities available in 2016.

As the pixel-pitch shrinks, for an FPA of a given physical size, the more pixels that can be accommodated within that size, the higher the resolution available. Turning it around, a given array format using a lower pitch can reduce the physical size of the FPA, making the resulting imager smaller and lighter. Either solution makes higher-resolution arrays practical in both smaller applications (such as weapon sights and digitally-fused night-vision goggles) and larger multi-sensor systems. For the latter, full High Definition resolution (1,280×1,024, 1,280×720 and 1,024×766) are available now with much larger configurations coming soon.

Returning to the AN/PAS-13 family, while the pixel-pitch of the detectors used in the D- and E-variants is 25 microns, the latest version as produced by L-3 Warrior Systems is labelled “AN/PAS-13G(V)1  Light Weapon Thermal Sight (LWTS)”, and features a 17 micron pixel-pitch detector. DRS Technologies offers what its documentation calls the “17-Micron Light Weapon Thermal Sight (LWTS)” without any designation. Photographs of these versions also bear a family resemblance to Raytheon’s undesignated Clip-On TWS launched at IDEX 2013.

Meanwhile, the US Army’s basic TWS programme is now complete, with over 237,000 units produced. The follow-on programme, known as the Family of Weapon Sights (FWS) will be basically similar to TWS with three somewhat differently defined variants covering rifle, machine gun and sniper applications. Unlike TWS, these sights are being designed to interface with a helmet-mounted display which can physically disconnect the user’s head from proximity to the rifle while sighting. It also allows provision for a look-around-the-corner acquisition device.

The FWS programme also illustrates how other technologies are impacting impact the night vision community, in the form of digital communications, which allows electronic imagery to be transmitted over radio and other frequencies. (Think cell phone again.) In the 21st Century the word ‘wireless’, once used to describe the domestic radio receiver, has taken a completely new application. It is now possible for the individual soldier to communicate with all levels of the command chain, and vice versa.

Micro-displays and fusion

Thermal imagery was, by the nature of its creation, fully digital, meaning imagery could be displayed on any of a number of digital displays and could transmitted into a network via wireless data links if required. This has seen the development of small but rugged display systems that can deliver data and imagery in immediate proximity to the soldier’s eye, in much the same way as a Heads-Up Display (HUD) on a combat aircraft.

The availability of such micro-displays enabled the advent of the fused-sensor goggle, bringing together both I2 and IR images. For the US military, the AN/PSQ-20 Enhanced NVG (ENVG), as the fused goggle was known, entered service in April 2008. This ENVG optically overlaid an uncooled long-wave infrared (LWIR) image onto the visible image provided by an I2 tube. Initially it was ‘clunky’ but worked and, since then, Exelis has evolved the ENVG to reduce its SWaP, with the current configuration being the AN/PSQ-20A Spiral Enhanced NVG (SENVG), now also being produced by L-3 Warrior Systems as the AN/PSQ-20B model.

The combination of I2 visible imagery and thermal imagery not only merges two different views of the world combined into one TV-like picture, the two images have tactically significant complementary nature. The visible image is most like what we are used to seeing with our eyes and provides higher resolution than current uncooled thermal imagers. This makes the overall picture “readily understandable”. On the other hand, because the thermal imager is sensitive to differences in the temperature of objects in the scene, people (and other mammals) tend to stand out strongly in the picture due to their body warmth. This provides for very fast detection of individuals (and active objects such as military vehicles) that is not available in visible imagery. Although a powerful combination it is expensive and not likely to be released across the worldwide market in a hurry.

A much simpler solution has, however, been found and one that is, compared with ENVG less costly – the clip-on thermal imager or COTI. This injects a thermal image onto the I2 image as an overlay. Among several examples of the COTI now available are the AN/PAS-29 COTI from Optics 1 (the US subsidiary of Switzerland’s Vectronix, which offers its own derivative Thermal Acquisition Clip-on System – Miniature  or TACS-M) and the ClipIR small Thermal Imager Clip-On (TICO) from the UK’s Thermoteknix Systems. In France, Thales has developed own clip-on thermal module specifically for its MINIE-D NVG to bring it up to MINIE-DIR configuration.

Looking at the pure display element, Exelis (now Harris) produces what it calls the ‘i-Aware’ family of imagery interface modules for night-vision devices, launched at the AUSA exhibition in 2011. This allows transmission and reception of imagery, as seen through the device, plus the ability to display live video feeds and geospatial information. As part of the family, the company also introduced the Tactical Mobility Night Vision Goggle (TM-NVG) with built-in ‘i-Aware’ in both monocular and binocular formats.

For thermal imagers, using electronic processing to produce the infrared image, it was comparatively easy to adapt to allow the transmission of images over the emerging network-centric battlespace. For I2-powered devices, based on pure light, the solution to the requirement for digital transmission has been to change the sensor. One emerging technology emerging is that of the EBAPS (Electron Bombarded Active Pixel Sensor), pioneered by US manufacturer, Intevac. This takes the photons from the scene, focuses them onto a photocathode and the resulting photoelectrons are then accelerated across a vacuum gap and proximity-focused on the back-illuminated CMOS (Complementary Metal Oxide Semiconductor) anode to produce digital image intensified (DI2) video with very little noise.

The Intevac Night Port digital goggle looks much like a ‘conventional’ binocular NVG with spectacles-compatible eyepieces and dioptre adjustment. However, with a back-illuminated CMOS architecture and a high definition OLED colour display, it has the capacity to import and export imagery and display data by symbology overlay on the image. A handful of prototype systems have been supplied to what the company describes as “US government laboratories”.

This is the stage we are at today. With I2 tube technology at an estimated 90% maturity – 30,000 hr MTBF, very high gain and saturation prevention – its biggest feature is low cost. As manufacturers continue to produce I2 tubes, it is most likely that industry R&D efforts will focus on the alternatives, such as EBAPS and dual-band EBAPS/uncooled devices. The reality, however, is that although such digital I2 and fused I2/IR products will become available, and make sales, they are unlikely to replace all the legacy I2 products around the world on a one-for-one basis.

Exploring wavelengths

While thermal imagers can produce an image in total darkness, the distance they can see depends on the atmospheric conditions. Different parts of the spectrum offer advantages for differing conditions. The MWIR range is generally considered more suitable for hot and humid climates; while the LWIR range is more suitable in cooler drier climates. Which you opt for depends on where you would expect to fight!

However, as digital technology has been emerged as an alternative to I2 in the visible to near infrared (V/NIR) spectrum (0.4 to ~0.9 microns), it has also brought the short-wave infrared (SWIR) spectrum (~0.9 to 3 microns) into focus. These use reflected “light” and are better able to penetrate dust, haze and smoke, while bright light or flashes will not degrade performance. Such SWIR cameras are now becoming available with a resolution and weight than can be accommodated on a head-mounted NVG for the foot soldier.

Two recent examples of SWIR products are the Nocturn camera range from Photonis USA and the Warrior HWH (handheld/weapon/helmet) SWIR viewer from Sensors Unlimited (part of UTC Aerospace Systems). The former uses the company’s Lynx CMOS solid-state imaging sensor with 1,280 × 1,024 resolution across the V/NIR spectrum; while the latter uses an indium gallium arsenide (InGaAs) FPA of 640 × 480 resolution, covering the 0.7 to 1.1 micron range.

kiteIn addition to applications for soldier systems, it is worth noting that several manufacturers of larger multi-sensor systems are also offering SWIR sensors as part of their imaging package for other platforms: ground vehicles, warships and airborne platforms (fixed- and rotary-winged, manned or unmanned).

Conclusions

While many of the recent developments across night vision have been covered, some elements have been only briefly mentioned, and the focus has been principally on soldier systems. Hopefully, one may explore other aspects and applications – such as the increasing use of multi-sensor above-armour surveillance systems by armoured fighting vehicles – in future issues.

 

 

 

 

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