Technology shapes war, and war shapes technology. Gaining a tactical advantage in warfare has been a primary impetus for invention and engineering. Of course, many of these technologies have beneficial peacetime advantages, as well. Examples include Alfred Nobel’s dynamite and the technology behind the Internet as developed by DARPA.
This two-horn acoustic listening device was developed in order to hear approaching aircraft engines from a distance. Needless to say, detection technology has come a long way since then. This photo was taken at Bolling Field, USA, 1921.
The field of imaging and detection is no exception. For the warfighter on land, in the air, or at sea, vision and reconnaissance are of supreme importance. Infrared imaging capability provides superior vision in the dark, through smoke and obscurants, and even through obstacles such as trees or structures.
At longwave and midwave (LWIR or MWIR) wavelengths, imaging devices can detect the emitted thermal energy of objects such as skin. LWIR devices are often used to see through smoke and battlefield obscurants. At the higher-energy MWIR band, devices can see as far as several kilometers. MWIR devices are often used for intelligence, surveillance, and reconnaissance (ISR) systems.
Infrared imaging systems are used in buildings, vehicles, and aircraft, but one of the most valuable applications are human-portable systems.
When longwave and midwave (LWIR or MWIR) thermal infrared cameras were first developed and used for military applications, the sensors needed to be cryogenically cooled to liquid-nitrogen temperatures (77K or -200C) to increase the thermal sensitivity and reduce noise. Initially, the systems had either single or multi-element sensors or linear arrays and there were mechanical scanning devices used to “paint” a complete thermal image. This resulted in a complex device that was quite large and bulky, had frequent maintenance issues, and was very costly. The technology at the time made it impossible to have a truly portable soldier system.
The FLIR Boson is an IR camera module with LWIR, MWIR and SWIR variants. Note quarter for scale. (Image courtesy of FLIR.)
In the early 2000s, microbolometer technology solved this problem, enabling much higher pixel densities and smaller sensors for the midwave and longwave (MWIR and LWIR) wavelengths. This gave hope for true soldier-portable LWIR systems. More recently, FLIR has refined microbolometer technology to the point where a LWIR module can be about the size of a sugar cube, and size, weight, power, and cost (SWAP-C) requirements continue to decline.
High Performance Infrared Imaging Systems
However, some military applications still need the performance of a cooled MWIR or LWIR system. Typically, these are applications where the higher sensitivity of a cooled system will allow longer standoff distances. Applications that benefit from such performance benefits are airborne ISR missions, long range surveillance applications, high performance weapon sights, ground vehicle fire control systems, and missile seekers, to name only a few.
Advances in cooled infrared imaging technology has also provided a significant benefit to military applications. Historically, MWIR sensors have been Indium Antimonide (InSb) and LWIR sensors have been Mercury Cadmium Telluride (MCT). Both sensor materials are extremely high performance, but both have constraints.
In the MWIR, InSb is a well proven material that has amazing thermal sensitivity and uniformity, which translates into consistent performance. However, InSb also must be cooled to 77K (-200° C) which requires significant mechanical cooling. The typical coolers involved utilize the Stirling cycle, a miniature compressor that cools the sensor from ambient temperature to 77K in less than 10 minutes. Historically, these coolers have been relatively large and bulky, consumed a significant amount of power, and had limited mean time between failures (MTBF). Recent innovations in both material and cooler technologies have demonstrated significant progress in reducing SWAP while maintaining performance, increasing MTBF, and offering a path toward cost reduction.
At SPIE DCS 2018, FLIR Systems introduced the Neutrino LC, a miniature MWIR camera module. The FLIR Neutrino LC offered significant technical advances. The first advance was the incorporation of a sensor material that operates significantly “warmer” than traditional InSb, while maintaining the same performance. This material, Strained Layer Superlattice (SLS), is the result of many years of work both by the US Government and industrial organizations such as FLIR. A consortium called VISTA, Vital Infrared Sensor Technology Advancement, was established in 2011 to bring industry participants together to collaboratively make strides in advancements in new detector technology.
The potential for high quantum efficiency and low production costs make SLS an attractive alternative to InSb and MCT detectors. The consortium investigated using commercial foundry molecular beam epitaxy (MBE) machines to create the substrates which allows standard processing equipment to build detector arrays.
Neutrino LC MWIR cooled camera core. (Image courtesy of FLIR.)
The FLIR Neutrino LC incorporates a 640x512 pixel density MWIR SLS focal plane array utilizing VISTA materials. This SLS MWIR sensor is considered “High Operating Temperature” (HOT) as it operates significantly warmer than the traditional 77K InSb sensors. The sensor in the Neutrino LC operates >120K which is nearly 40% warmer. Such a reduction in sensor operational temperature allowed FLIR to develop a SWAP optimized microcooler specifically optimized for HOT technology.
The Neutrino LC utilizes the new FLIR linear cooler specifically designed for sensors operating at 120K and higher. This miniature cooler and entire integrated detector dewar cooler assembly (IDDCA) can fit in the palm of a hand. This miniature cooler has faster cool down times, consumes a fraction of power compared to traditional cooling methods, occupies a smaller volume, has a lower weight, and offers the promise of longer MTBF.
FLIR is investing in the advancement of “cooled” technology beyond simply MWIR SLS. FLIR is pursuing pixel pitch reduction, which will allow much higher density focal plane arrays while maintaining SWAP optimization. Higher pixel density arrays will provide the warfighter more pixels on target, ultimately resulting in more confidence of mission success. FLIR currently has an ultra-high resolution MWIR device (the Neutrino QXGA) that boasts a 2048x1536 pixel density using 10µm pixels. The company has plans to develop pixels 8µm and smaller.
In addition, significant advancement is being made on improving the inherent read out integrated circuit (ROIC) technology. The basis of all imaging sensors is the ROIC, which defines pixel pitch and performance characteristics (such as frame rate). For the most part, ROICs for infrared sensors are analog or perhaps may have analog/digital converters (ADC) embedded in the columns of the ROIC. Development of all-digital ROICs (DROICs) is a strategically important effort by both the US Government and sensor suppliers.
FLIR Systems is also investing in development of a three-dimensional stacked digital ROIC. This advanced design hybridizes an analog layer to a digital layer, resulting in the ability to have extremely wide dynamic range imaging at high frame rates, and the promise of very deep wells. The advancement of DROIC capability can expand the capabilities of important military missions such as degraded visual environment, aircraft survivability, and seekers.
As previously stated, cooled LWIR sensors have traditionally utilized MCT material. MCT is a well proven sensor material used by the US Army and the entire DoD for decades. It is well suited for LWIR response and is the basis for the primary fire control systems, called SADA2, within the US Army Abrams and Bradley vehicles. However, MCT is a challenging material to produce and its volatility in production can result in less than optimal yields, producing high per unit cost and fluctuation in performance. A more reliable sensor material has been the objective of the US Army for over a decade.
SLS material is unique in that it can be produced to be sensitive in SWIR, MWIR, or LWIR. While the MWIR band was the first to take advantage of SLS developments coming out of the VISTA consortium, equal attention has been given to improving performance in LWIR and dual band (midwave/longwave).
Military Applications of Shortwave Infrared Imaging
The final waveband of interest to the warfighter is shortwave infrared (SWIR). In the 1990s, SWIR detectors, primarily made of Indium Gallium Arsenide, were popular in telecommunication applications that used optical fiber. Extensive research was made in expanding the sensors to two dimensional arrays for imaging applications with the hope that SWIR would be a technology that could ultimately replace the low light image intensified (I2) tube-based technology commonly used in night vision goggles.
However, SWIR (defined in this context as 1.0-1.7µm) proved to be ineffective for nighttime imaging as a primary mission. SWIR energy is a reflected energy on all targets except those of very high temperature (i.e. >200°C), as opposed to MWIR or LWIR sensors that can detect passively emitted energy from targets as cold as -40°C. As SWIR sensors require reflected light, the absence of significant SWIR energy in dark conditions renders the sensors useless for traditional imaging unless there is supplemental illumination—which may compromise the covert nature of the mission. SWIR sensors traditionally also suffered the need to be stabilized with a thermoelectric cooler (TEC), resulting in a huge power draw. Ultimately, SWIR sensors were deemed unsuitable to replace I2 tubes for nighttime imaging.
SWIR technology, however, is very effective for many other tactical military applications. The primary benefit of SWIR is the ability to “see” battlefield lasers that traditionally operate at 1.06um and/or 1.54um. Therefore, it is common to see SWIR sensors used in target designation, or “see spot” applications where confirmation of the designation laser is required. In daytime imaging, SWIR is also beneficial under certain atmospheric conditions and certain geographic locations (such as misty conditions over water).
But SWIR sensors were written off as being ‘niche’ sensors, as they consumed significant amounts of power (because of the TEC) and the limited volume of sensors produced kept the price of SWIR cameras high.
Then, FLIR Systems was awarded a contract supporting a US Department of Defense DMS&T (Defense-wide Manufacturing Science and Technology) initiative to reduce the size, weight, power, and cost of SWIR sensors with the intent to make the technology more practical for widespread usage both on the battlefield and off. The result of the DMS&T effort was the FLIR Boson SWIR, which was a miniature SWIR sensor in the same form factor as the FLIR Boson LWIR.
The Boson SWIR utilizes a 640x512 InGaAs SWIR focal plane array operating without a TEC. FLIR incorporates Boson electronics and developed calibration algorithms to allow the sensor to operate under wide ambient conditions without the TEC. This allows the module to be extremely tiny and consume a fraction of the power typically used by a SWIR sensor with a TEC.
Additional technology advances within the SWIR realm are in development. In addition to the Boson SWIR advancements, FLIR has invested in a new ROIC design for SWIR sensors that incorporates the capability of Asynchronous Laser Pulse Detection. The concept is that the sensor would be incorporated into a portable soldier system. If a “high flux” (i.e. laser) event occurs in the field of view, the sensor can switch into high frame rate mode, pinpointing the laser and performing Pulse Repetition Frequency decoding. This allows the warfighter to quickly identify if the laser is a friend or foe.
Significant technology advancements in sensors within the SWIR, MWIR, and LWIR spectral responses have been accomplished in recent years. These advances have resulted in the ability of new sensor systems for the warfighter that provides more mission specific capabilities under more conditions and for more applications. These advances have moved from being science projects in government labs to production sensors put into the hands of warfighters today, allowing true overmatch capabilities.
For more information, check out our story Overmatch Capability for the Warfighter: Thermal Vision and Infrared Imaging, or visit the FLIR Systems website.
FLIR Systems has sponsored this post.