64 MMIC receivers built for mm-wave imaging have been systematically characterized. Each receiver comprises three InP MMIC low noise amplifiers and a biased Schottky diode detector in a compact package with a horn antenna. The characterization includes spectral response, responsivity and noise. The noise has both white and 1/f components. The average (±standard deviation) receiver bandwidth is 23.8±3GHz, and the overall noise equivalent temperature difference (NETD) in an integration time of 0.2ms, as used in the imager, is 0.66±0.1K.
The cost of thermal imaging technology has, up until now, precluded its widespread use in sensor systems which require sensors to be deployed in very large numbers. This paper describes a method of achieving this goal of bringing low-cost 'disposable' thermal imaging into the dismounted military environment. Infrared detectors based on the manufacturing processes used in the production of conventional silicon chips offer a breakthrough in cost compared to other technologies. Despite having modest performance, this technology offers a route towards a very cost-effective thermal imaging sensor for dismounted applications. A flexible detector format which permits the detector to operate as a conventional close-packed 2-d array or as a faster update linear array gives the opportunity for performance optimisation and data reduction at the sensor, important attributes for a remotely deployed sensor with limited power resources. This paper describes a sensor architecture which is well matched to the cost, power consumption, and performance levels suited to short-range dismounted and networked operations, and demonstrates some of the imaging capability achievable with such a simple (and hence potentially extremely low cost) sensor.
Acoustic sensors have been the primary sensor of choice for many UGS network concepts. This is primarily due to their low cost, non line of sight performance and the fact that most targets of interest are noisy. This paper explores the benefits to be gained by attaching additional sensors to an acoustic sensor network to provide extra information. A methodology is described to assess the cost of acquiring a certain level of information and this is used to explore the context in which the sensor network is operated. It is demonstrated that the optimum choice of sensors is dependent on the target set and the information required from the network. The potential benefits of a 'plug and play' sensor suite are examined in the context of using this concept for targeting.
Some years ago QinetiQ introduced a short-range reconnaissance unmanned air vehicle (UAV), known as OBSERVER, which carried a visible three-camera sensor. To increase its versatility, a compatible infrared (IR) thermal imaging (TI) sensor was developed for the vehicle for operation in the 8-12mm waveband with a dual field of view function. The sensor incorporates a specially designed camera board, employing two IR lead scandium tantalate (PST) detectors based on UK un-cooled TI technology. Since no cooling engine is required for the detectors, the sensor module is very lightweight and hence well suited to its UAV application. So as to achieve the minimum possible payload for the vehicle, in addition to the lightweight detectors and electronics board, compact low mass optical solutions were devised for the camera objectives. These functioned at a relative aperture of f/1.0 and were designed to provide stable focus and imaging performance over a comparatively large temperature span (-10°C to + 50°C) to enable all weather operation. In order to achieve an athermalisation scheme devoid of elaborate electro-mechanical drives, thermally passive solutions were developed for the objectives in which the differing thermal characteristics of the components were designed to self-cancel optically. In this paper, the design and performance limitations of the optics are discussed and the procedure employed for establishing a thin lens pre-design for one of the objectives is described.
The cost of thermal imaging technology has, up until now, precluded its use in networked sensor systems which require sensors to be deployed in very large numbers. Detectors based on the manufacturing processes used in the production of conventional silicon chips offer a breakthrough in cost compared to other technologies. Despite having modest performance, this technology offers a route toward a very cost-effective thermal imaging sensor for networked applications, where the limited performance of each individual sensor is less significant due to the advantage given by large numbers of sensors covering the target area. By carefully optimising the detector format, this low-cost technology is able to achieve useful performance at short ranges which are suited to a networked sensor system.
The advent of uncooled thermal imaging has produced an order-of-magnitude reduction in the cost of thermal imaging compared to first-generation cooled systems. To reach a truly mass market, this process needs to be continued. One of the key cost constraints is the specialist nature of the sensitive material used in infrared detectors. This paper describes thermal imaging technology which can be entirely manufactured in a silicon IC foundry on a standard CMOS process. As a result the detector cost in volume production is extremely low. Careful optimisation of the other system components such as packaging, optics, and signal processing maintains this low-cost approach, giving a predicted production cost well below $100.
Research by DERA aimed at unmanned air vehicle (UAV) size reduction and control automation has led to a unique solution for a short range reconnaissance UAV system. Known as OBSERVER, the UAV conventionally carries a lightweight visible band sensor payload producing imagery with a large 40°x90° field of regard (FOR) to maximize spatial awareness and target detection ranges. Images taken from three CCD camera units set at elevations from plan view and up to the near horizon and are 'stitched' together to produce the large contiguous sensor footprint. This paper describes the design of a thermal imaging (TI) sensor which has been developed to be compatible with the OBSERVER UAV system. The sensor is based on UK uncooled thermal imaging technology research and offers a compact and lightweight solution operating in the 8-12 μm waveband without the need for cryogenic cooling. Infra-red radiation is gathered using two lead scandium tantalate (PST) hybrid thermal detectors each with a 384 X 288 pixel resolution, known as the Very Large Array (VLA). The TI system is designed to maintain the imaging format with that of the visible band sensor. In order to practically achieve this with adequate resolution performance, a dual field of view (FOV) optical system is used within a pitchable gimbal. This combines the advantages of a wide angle 40°x30° FOV for target detection and a narrow angle 13°x10° FOV 'foveal patch' to improve target recognition ranges. The gimbal system can be steered in elevation to give the full 90° coverage as with the visible band sensor footprint. The concept of operation is that targets can be detected over the large FOV and then the air vehicle is maneuvered so as to bring the target into the foveal patch view for recognition at an acceptable stand-off range.
This paper reviews the use of thin film ferroelectric materials for application in microbolometer infrared detector arrays. A key issue is the need for high temperature processing to achieve the required ferroelectric crystal phase. Results of thermal trials on silicon readout circuits are presented which indicate failure due to disruption of the AlCuSi metallisation. Higher temperatures can be used if oxygen is excluded. A low temperature lead zirconate titanate (PZT) sol-gel deposition is reported which has been used to fabricate fully integrated detector arrays directly on readout silicon wafers. Higher performance is obtained from dielectric bolometer materials, and materials merit figures nearly 4 times that of PZT are reported for sputtered lead scandium tantalate (PST) films. These require post-deposition annealing to temperatures above that allowable for silicon readout survival. Results on excimer laser annealing are presented which demonstrate crystallisation of a ferroelectric film without heating the underlying substrate. A new composite thermal detector array design is introduced, based on an indirect fabrication method. This uses a high density interconnect wafer as a high temperature substrate for ferroelectric film growth. After fabrication of the detector pixels, individual arrays are flip-chip bonded to readout silicon die.
The staring array is the basis of many modern thermal imaging systems, cooled and uncooled. A major drawback in all staring array thermal imaging systems is the need to provide thermal referencing in order that the non-uniformity inherent in all IR detector technologies can be corrected. A common approach is to use a mechanical shutter operated intermittently, typical of uncooled resistance bolometers and cooled photon detectors, or a rotating chopper, typical of ferroelectric uncooled bolometers. Although these methods are inexpensive and consume little power, they are inappropriate to environments where high g-forces or shock loads are encountered. This paper describes a solid state modulator operating on the 8-12micrometers band. The modulation mechanism is induced absorption in high-purity intrinsic germanium. Electron-hole pairs are created in the germanium modulator; the electrons are weak absorbers but the holes absorb strongly by means of the light-hole/heavy-hole inter-sub-band transition. The transmission of the modulator can thus be varied by varying the hole concentration, for example by illuminating the modulator with near IR light. Very good modulation depth (5% to 90% transmission) has been measured, at optical power densities of approximately 10 W/cm2. Switching speeds are controlled by the carrier lifetime, and are a few milliseconds in our prototype device. The high power requirement rules out this approach for hand-held applications. However for intermittent use, or where the environmental constraints are dominant, this technology offers a potentially robust 8-12micrometers modulator.
There is a widespread requirement for low cost lightweight thermal imaging sensors for both military and civilian applications. In Europe, these requires are now being met by systems using large uncooled ferroelectric detector arrays offering performance levels which, until recently, could only be achieved by expensive cryogenically cooled systems. The uncooled technologies a result of collaboration between the UK Defence Evaluation and Research Agency (DERA) and Marconi Electronic Systems (MES) under a 'Dual Use Technology Program (DUTP). The successes from this program have resulted in developments for civil applications, including both hand held and helmet mounted fire-fighter's thermal imaging cameras. Military applications include personal surveillance sensors, vehicle driving aids, airborne flying aids and thermal weapon sighting systems. The products available to date have been based on hybrid ferroelectric detector technology in which the IR sensing material is manufactured separately from the silicon readout circuit to which it is subsequently bonded. Meanwhile, the ongoing DUTP program is developing a high performance 'integrated' detector technology in which the ferroelectric ceramic material is deposited as a microbridge structure directly onto the silicon readout circuit. The improved performance available from this approach will realize major enhancement and cost reductions to be achieved in future thermal imaging sensor developments.
Uncooled Thermal Imaging (TI) in the UK involves large arrays of ferroelectric bolometer elements, at a pitch of from 100 micrometer down to 40 micrometer. A Hybrid Array Technology, exploiting the pyroelectric property of ferroelectric ceramic materials for the bolometer elements, has produced a range of successful solder bump bonded 2-D arrays. However, in innovative technologies under research, direct deposition of the ferroelectric material as a thin film onto suitable thermal microstructures on the silicon readout IC will provide substantial reductions in costs as well as improved performance. A route has been defined for this Integrated Array Technology, leading to performance enhancements by a factor of three over the Hybrids. In achieving the performance, the optimized ferroelectric signal readout, signal conditioning and processing architectures perfected for the Hybrid Arrays will be retained. Microscan mechanisms, readily incorporated in the IR chopped format used with ferroelectric imaging, have been demonstrated for the Hybrids, and will be even more closely matched to the improved thermal diffusion MTF of the Integrated devices. The ferroelectric capacitative detector filters the high frequencies, limiting noise bandwidths for very large arrays, and with microscan technology added, the ferroelectric arrays retain their potential to provide high quality IR imaging at very large equivalent array sizes.
The diffraction-based performance limitations of dual waveband infrared systems which incorporate hybrid refractive-diffractive lenses are examined. These limitations must be understood in order to identify the key trade-offs and optimize the design of the diffractive element. The correction of chromatic aberration is considered and the range of conditions under which hybrid solutions offer an advantage is established. A dual waveband hybrid objective lens for an uncooled staring array camera has been designed, manufactured and evaluated.
The development of a Petzval objective lens which is passively athermalized over the temperature range minus 20 degrees Celsius to plus 50 degrees Celsius is described. The lens is compatible with a latest generation uncooled staring array imager operating in the far infrared band, currently under development at the UK Defence Research Agency (DRA). In order to minimize the number of lens components which are required, a diamond turned hybrid refractive-diffractive element is employed. Design options are presented and the manufacturing issues relating to the diffractive surface are described. The goal is to produce a cost effective solution rather than placing the emphasis on achieving the ultimate in performance. Results from optical performance tests are given, including interferometry at a range of temperatures and broadband MTF.
Uncooled, compact and low power IR imaging is offered by large arrays of bolometer elements, at a pitch of 40 micrometers to 100 micrometers. A hybrid array technology, exploiting the pyroelectric property of ferroelectric ceramic materials in the bolometer elements, has produced a range of successful linear and 2-D arrays. High merit figures for the ferroelectric ceramic have been coupled to fabrication technologies including reticulation and solder bump bonding of the elements to the readout multiplexer IC. However, other designs will compete for cost-effectiveness in the large arrays now required for security and other civil applications such as night driving aids. In particular, direct deposition of thin film ferroelectric material onto suitable thermal microstructures on the silicon readout IC could provide substantial reductions in costs and improved performance. This integration will require processes compatible with the silicon IC. Already, lead-based perovskite films are showing considerable potential but other bolometer types are also candidates, such as thin film resistance bolometers. Thermal imaging systems research has resulted in signal conditioning and processing architectures which are optimized for the bolometer arrays. The ac coupling of the ferroelectric response to the IR radiation has been exploited through the use of radiation chopping and image difference processors (IDP), which remove fixed pattern noise and limit low frequency noise. The image detail observable with the pixellated aery has been enhanced by microscan modes. The successful technology will form the basis for a rapid growth of commercial IR imaging and monitoring into the next century.
Large arrays of bolometer elements have considerable potential for thermal imaging applications, offering uncooled operation, and a performance which challenges the cooled semiconductor detectors. A hybrid array technology, exploiting ferroelectric materials, is the basis of a successful range of linear and 2-D arrays. The success is based on hot-pressed ceramics (operating in both pyroelectric and dielectric modes), and the technologies for solder bump bonding and element reticulation. Arrays are increasing in size, from 104 elements up to 105 elements while the pitch is reducing, thus providing high resolution in compact systems. Ferroelectric thin films have recently shown marked improvement in the merit figures and, if compatible with the silicon IC, will allow a more direct array fabrication. These integrated array technologies have potential for high yield and low cost in very large area devices.