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Uncooled IR is the new cool.

In the world of night vision, infrared (IR) detectors are the core of the thermal imaging (TI) systems and come in two basic types: cooled (based on photon detection) or uncooled (thermal detection). While the former (either a photoconductor or phatovoltaic device) is considered the best technology far true IR heat detection, they are expensive to own and operate. In recent years, however, the development of uncooled IR detectors has moved forward.

Without a cooling circuit in the system, uncooled IR thermal detectors can be less bulky, lightweight and use less power than their cooled brethren. Additionally, they can offer a wide spectral response and longer periods of operation than the cooled photon detector. Therefore, uncooled technology provides military users with high sensitivity sensors in a lower cost, more reliable package. Consequently, they are cheaper to procure and operate and therein lies their attraction.

The downside is that uncooled IR sensitivity is not yet considered sufficient for all military applications, as its range is still somewhat limited. No doubt this will change as more investment is ploughed into the technology. Where long range is not the overriding requirement, uncooled IR is increasingly being applied. A prime example is the plethora of national projects to develop integrated 'future soldier systems' where night vision capability is a requirement. Here cost, weight and power consumption are as much elements of the equation as performance.

What's Inside

To set the scene, it may assist the reader to identify the components and explain some thermal terminology, as the search for night vision can be something of a 'black art'(pun intended) to the uninitiated.

An infrared detector produces an output signal, which depends on the amount of radiation hitting the active region of the detector. The thermal (uncooled IR) detector converts incident radiation into heat, thereby raising the temperature of the detector element. This change in temperature is then converted into an electrical signal that can be amplified and displayed. It responds to a wide range of wavelengths without appreciable variation in responsiveness, while displaying sufficiently high sensitivity at room temperature to permit imaging.

The infrared spectrum ranges from 0.7 to 14 [micro]m and is broken down into short wave IR (Swir), or near range, covering the 0.7 to 3 [micro]m band; medium wave IR (Mwir), or mid range, covering three to five [micro]m; and long wave IR (Lwir), or far range, covering the 5 to 14 [micro]m spread although most Lwir detectors cover the eight to twelve [micro]m spread.

Uncooled IR detectors come in one of three flavours:

* the bolometer, which measures electric resistance changes with temperature,

* the pyro-electric (or ferroelectric) detector, which measures spontaneous electric polarisation changes with temperature, and

* the thermopile, which measures junction potential changes with temperature (which is known as the Seebeck or thermoelectric effect).

Of these, the bolometer is the most widely used of uncooled IR detectors, due to its compatibility with Cmos (Complementary Metal Oxide Silicon) technology. The bolometer can be monolithically integrated with standard Cmos circuitry, which results in a low cost of production. It also permits the use of VLSI (Very Large Scale Integration) allowing 'active pixel' architectures, which incorporate all camera functions on a single chip.

To become part of a TI camera, the individual IR detectors (cooled or uncooled) are usually arranged in groupings known as a focal plane array (FPA), although for specific applications, linear arrays can be constructed. Each detector provides one pixel (picture element) of detection and the more pixels that are available to an array, the clearer (and more accurate) the resulting image (think digital camera).

In addition to the IR detectors or FPA, a TI camera requires a power source, a signal processor, various optical subassemblies and a video monitor system. Having produced the TI camera, it can be applied to a wide variety of applications, such as forward-looking infrared (Flir) systems for airborne use (either in pods or as part of a sensor turret payload), fire control systems for armoured fighting vehicles, missile systems for naval weapons, as sighting scopes for individual or crew served weapons or in missile seeker heads. Specific applications for uncooled IR detectors presently cover simple surveillance, lightweight helmet sights, smart munitions, weapon sights, unattended ground sensors and missile/smart bomb seekers.

One may summarise the requirements for further development in the uncooled arena as being an imager with increased sensitivity, smaller pixels and larger arrays (aiming for 640 x 480 elements and larger). Additionally, the imager needs improved temperature stabilisation, less expensive optics and lower power consumption. Bringing these requirements together in varying combinations will result in light, more compact designs that should prove cheaper to produce.

In the United States, the Night Vision and Electronic Sensors Directorate of the US Army's Communications-Electronics Command (Cecom) works closely with the Defense Advanced Research Projects Agency (Darpa) on uncooled technology. Three major companies are conducting work on uncooled microbolometers for military applications--BAE Systems North America, DRS Technologies (which has acquired the IR businesses of other companies including Texas Instruments, Hughes and Boeing) and Raytheon.

Examples

One of the major successes in getting uncooled technology onto the battlefield is the US Army's Thermal Weapon Sight II (TWS II) project, a follow-on to the TWS I programme, which resulted in the AN/Pas-13 TWS family, produced by Raytheon. Of the three variants of TWS, Light, Medium and Heavy, the Medium and Heavy versions incorporate 40 x 16-element scanning arrays of cadmium mercury telluride functioning in the medium wave IR band and employing a thermoelectric cooler. The Light TWS, however, is based on an uncooled staring 320 x 240 FPA of barium strontium titanate detectors (as used in the AN/VAS-5 Driver's Viewer Enhancer), operating in the long wave infrared band. Raytheon began full-rate production of the LTWS variant in August 2003 and completed deliveries of the 2500 units late last year.

The TWS II programme, developed by the Program Executive Office Soldier, is looking at new uncooled thermal imaging module (Tim) technology. The two sensors have been developed; a 320 x 240 array for lightweight (individual) weapons and a 640 x 480 array for medium/heavy weapons. Larger array sizes of 1000 x 1000 elements or greater are now seen as achievable. Other potential applications would be on micro-UAVs (unmanned air vehicles), head tracked viewers, AFV self-defence detectors and missile/smart bomb seekers.

As uncooled technology moved forward, vanadium oxide (VOx) was seen as the detector material of choice. It can be produced, although more expensively than is presently desired. An alternative material--amorphous (alpha) silicate (aSi)--is becoming as popular. A VOx bolometer membrane is thicker than one of aSi, thus its thermal insulation resistance is not as effective. Being inherently stiff, aSi permits the construction of smaller bridging structures in the array. The smaller the bridge structure, the smaller the pixel and, therefore more elements can be built into a given array size. This bridging structure is also shorter and, thus, more resistant to vibration.

BAE Systems' Information & Electronic Systems Integration IR Imaging Systems (Iris) sector (Lexington, Massachusetts) is already replacing cooled IR detectors in many applications, including the new Precision Guidance Set sensor for the Joint Direct Attack Munition. According to Dr Matt Dovidio, Iris programme manager: "Uncooled technology, with its recently demonstrated high sensitivity and resolution capabilities, has now achieved the performance level needed for tens of thousands of military sensors that will be built in the next several years."

The company's Iris sector uses VOx in its MicroIR series of uncooled microbolometers. The company has recently been awarded a three-year $ 4.5 million contract by the US Department of the Interior to improve the produce-ability and lower the cost of advanced MicroIR FPAs. Supplementing this with its own development funding, the company expects to evolve the capability to produce high-resolution, affordable IR cameras while reducing the manufacturing cost of FPAs by a factor of eight.

Based on the MicroIR FPS, the Tim 1500 dual FoV module was ordered in July 2004 to replace the existing cooled TI sensor used by the Kongsberg Defence & Aerospace Protector remote weapons station on later batches of the Stryker 8 x 8 wheeled APCS used by the US Army's Brigade Combat Teams. It is claimed to be able to identify targets up to 1.5 km away in all weather and obscurant conditions. (ten Protector units have been bought by Australia for its Aslav APCs and by Finland for its Patria AMVs, although without this TI module.) The Stryker TIM shares the same core with the Heavy TWS. Among other BAE Systems products emerging in this arena is its SCC500 family of uncooled thermal camera cores, based on micro-bolometer sensors, with resolutions of 160 x 126 or 320 x 240 pixels and employing common electronics. The units employ custom video processing hardware using general purpose control processors running a commercial real-time operating system rather than the usual approach based on digital signal processors.

BAE Systems has also introduced its HHC100 series of hand-held thermal imagers, also based on micro-bolometers and available in 160 x 120- or 320 x 240-element formats. Its most recent product is the Pole-Mounted Camera 300, which uses the company's uncooled 640 x 480 MicroIR FPA to produce a high-resolution thermal image for long-range day/night surveillance and vision enhancement.

One of the major successes in getting uncooled technology onto the battlefield is the TWS II. Although Raytheon brought the Light TWS to the field in 2004 BAE Systems and DRS Technologies outbid Raytheon and split the first TWS II production contracts, picking up the second tranche of a five-year programme earlier this year. The first sights for light, medium and heavy weapons were delivered in the third quarter of 2004.

As well as working on TWS II, DRS Technologies' Electro-Optical Systems Group is developing the Cost-Effective Targeting System (Cets) for the US Army, part of the Networked Sensors for the Objective Force advanced technology development project. It uses the company's U6000 Low Power Uncooled Infrared detector, based on a 640 x 480 array of VOx detectors. In addition to Lupair with dual FoVs for search, Cets brings together a colour TV camera (for improved daylight performance), a monobloc laser for ranging and illumination, and a Swir active gated camera for target identification at long range on a 13-inch diameter, low-cost, two-axis gimbal. It is envisaged that Cets' application will be principally for UAVs, unmanned ground systems (UGS) and mid capability troop carrying vehicles.

The company is also working under a US Army Cecom contract awarded in 2004 (worth $252 million over four years) to produce Driver Vision Enhancers for installation on a wide range of combat and tactical wheeled vehicles. The Lupair is the sensor of choice and its output will be shown on an 800 x 600-pixel active matrix liquid crystal display.

In Britain a DRS Technologies 320 x 240 VOx detector has been incorporated into an uncooled Thermal Imaging Module by Thales Optronics Canada and adopted as a drop in replacement on the Raven sight on the British Army's Warrior repair and recovery vehicles as part of the BGTI (Battle Group Thermal Imaging) Group 2 fit on 94 vehicles, supplied by Thales Optronics.

Perhaps not so well known as the major players, Indigo Systems, which was acquired by Flir Systems in 2004, has produced what was originally named the Omega and is now marketed by Flir Systems as the ThermoVision Micron TI camera. Using an uncooled 160 x 128 FPA VoX micro-bolometer, Micron is one of the smallest uncooled cameras on the market and has been mounted on the AN/PVS-21 low-profile NVG.

The US Air Force has adopted Flir Systems' uncooled ThermoVision Sentry camera for its Tactical Automated Security System (Tass) programme, run by Northrop Grumman as prime contractor. In March 2004 Northrop Grumman ordered additional cameras under a contract worth $ 7.2 million. Most of the sensors will be used for perimeter security around US air bases worldwide under the Tass. ThermoVision Sentry uses an uncooled 320 x 240 micro-bolometer, with dual fields of view, on a pan/tilt mount that is designed specifically for perimeter security and surveillance applications.

In Europe, agencies such as Britain's commercial QinetiQ and national DSTL (Defence Science and Technology Laboratory) and the French CEA's Leti (Laboratoire d'Electronique de Technologie d'Information), who are working on uncooled technology, are licensing the technology to industry for refinement into useable military products. With the constraints of space, the best way to illustrate the progress made is to highlight some examples.

Thales Optics, based in St Asaph, Wales, part of Thales Land and Joint Systems business unit, used the Idex 2005 exhibition in Abu Dhabi earlier this year to launch the Vipir short-range weapons sight and the Vipir-S hand-held derivative, which uses uncooled technology (see Thales picture in our main title). It was developed for, and being bid on, the UK's Future Integrated Soldier Technology modernisation programme.

The Vipir uses an uncooled VOx detector micro-bolometer in the eight to twelve-[micro]m range with a 160 x 120-pixel array (320 x 240 interpolated), giving it a man-detection range of 400 to 600 metres. The detector assembly electronics packaging is most compact with a low power demand. Four AA-size lithium batteries give six to seven hours of continuous operation; an equivalent thermal weapon sight would require ten batteries.

Weighing less than 700 grams, the Vipir is designed to be attached to a Nato or Picatinny rail mount. It is available with 8[degrees] or 16[degrees] fields-of-view and magnifications of x3 or x1.5.

Vipir has an I/O port for auxiliary power, data and video transfer, as does the hand-held Vipir-S version.

The irony of this is that Thales is a French-based multinational group which, in 2004, brought all its various subsidiaries with thermal imaging capability together into its Land and Joint Systems business group, these include Thales Optronique (France), Thales Angenieux (part of the French Thales High Tech Optics), Thales Optronics (UK), Thales Optronic Canada and Thales BV in the Netherlands.

The leading exponent of uncooled technology in France is, however, Ulis (85 per cent owned by Sofradir and 15 per cent by CEA). To offer lower product costs it has opted to use an aSi bolometer combined with micro-bridging techniques (which maintain effective thermal insulation between the detector and its Roic (Read-Out Integrated Circuit). The standard Ulis micro-bolometer uses a 320 x 240 Lwir FPA, with a 45-[micro]m element pitch, although a variant with a 35-[micro]m pitch is planned for the future.

An initial application is the Thales Angenieux Elvir (Equipement Leger de Vision Infra-Rouge) hand-held thermal camera, which entered production for France (an evaluation batch) and Poland (200 units) in January 2004. Weighing 1.6 kg, it has detection range of 1 to 1.2 km, and comes with interchangeable lenses offering narrow and wide fields of view. With a common pupiliary display as the Thales Angenieux Lucie image intensifier-based night vision goggle, the Elvir image can be monitored using Lucie headgear. A hands-free variant is expected this year.

A prime target market for Ulis is the French Army's Felin future soldier project and the company is to supply the same aSi micro-bolometer to Sagem for its Jim (Jumelle Infrarouge Multifonction) medium-range camera on order for the Felin squad leader. (The long range Jim LR for Felin uses a cooled Mwir detector.) Other applications for this microbolometer include AFV driver's night viewers and hand-held imagers.

By another twist of irony, Ulis supplies Cedip Infrared Systems with its standard aSi micro-bolometer detector for the latter's Ruby IR-based night vision goggles (which were bought by the British Ministry of Defence in 2004), a direct competitor to Elvir, and also its Jade UC (uncooled) camera.
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Title Annotation:Sensors: night vision
Author:Maxwell, David
Publication:Armada International
Date:Jun 1, 2005
Words:2627
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