New uncooled thermal imaging systems
The last five years have seen rapid progress in thermal imaging technology. In fact, these years have seen a transition from scanning devices with one sensitive element and a linear block to large-format non-scanning two-dimensional flat focal plane arrays. This process has allowed for a sharp increase in the performance characteristics of thermal imaging cameras. However, some problems remain unresolved. These include, first of all, the large dimensions and relatively high cost of cameras, as well as the need for cryogenic cooling of sensitive elements. Because of this, such a promising sales market as environmental monitoring devices remained virtually closed to thermal imagers.
It seems that an acceptable solution to the problems has been found today by specialists from Amber, a Raytheon company, who have developed an uncooled IR focal plane array based on microbolometers. The Sentinel camera with such an array has a higher sensitivity compared to many existing cooled cameras and improved image quality. Weighing just over 1800 g, the Sentinel camera, which looks like a regular video camera, is equipped with a 320×240 pixel array of microbolometers in the focal plane, sensitive to radiation with a wavelength of 8-12 microns.
A significant boost to the development of thermal imaging technology was provided by the development by Amber of a custom CMOS integrated circuit for reading with elements of internal signal processing, including on-chip pixel offset correction and voltage amplification. The read signal is fed to a conventional 12-bit analog-to-digital converter. The sensitivity of the camera exceeds 100 mK.
The developers believe that the new camera will find wide application in the civilian and military sectors: devices for monitoring production conditions and environmental pollution, combat thermal imaging sights, reconnaissance devices, inexpensive missile guidance systems, image enhancement systems for vehicles, etc.
Back in the late 1970s and early 1980s, scientists and developers at Texas Instruments and Honeywell realized that the new integrated circuit technology could be the basis for creating uncooled thermal imaging devices. As a result, a large number of different associations and groups emerged, mostly financed by the U.S. Department of Defense, trying to create production of uncooled thermal imagers. The first mass-produced uncooled thermal imaging system was based on a pyroelectric or ferroelectric focal plane array developed by Texas Instruments.
Ferroelectric cameras usually use a mechanical chopper to stabilize the image. This unit ultimately determines the thermal sensitivity of the camera. However, the sensitivity, and therefore the image quality, of uncooled microbolometer cameras are usually lower than that of ferroelectric uncooled cameras. As a result, the equivalent noise temperature difference of the Sentinel camera is 0.07°C. In the next year or year and a half, the company's specialists plan to increase the sensitivity of the bolometer array by up to two times.
It is important to note that Sentinel senses the slightest temperature fluctuations in the observed scene, unlike changes in illumination in conventional television. This is in stark contrast to night vision devices with image intensifiers, which require at least the weakest light source to operate. Image intensifiers are easily blinded by light sources. Light sources do not pose any danger to a thermal imaging camera. Therefore, it produces a clear image both on a clear sunny day and in complete darkness.
The «eye» of the Sentinel camera is a focal plane bolometer array measuring less than 6.45 cm. The sensitive microelement of such an array is a thermistor element of a microbridge. The microbridge is a suspended structure that holds the heat-sensitive element above the substrate. This design provides good thermal insulation and mechanical strength of the suspended microbridge.
Each pixel has dimensions of 0.05×0.05 mm, and the fill factor of the sensitive area is 48%. The thermistor element is a 50 nm thick vanadium oxide semiconductor film, the temperature resistance coefficient of which is 2% per 1 °C. When the pixel absorbs heat, the microbridge heats up, and the temperature increase is recorded by the thermistor element. According to measurements of the manufactured devices, the thermal conductivity of the microbridge is 2×10 W per 1 °C.
Conventional photolithography technology is used to form the thin-film elements of individual pixels. The pixel sensitivity is 70,000 W/W for a blackbody radiation source at 300 K. On an optical system with an aperture of f/0.1, this corresponds to an equivalent noise temperature difference of less than 0.1 ° C. The bolometers are formed directly on the silicon substrate of the signal readout, eliminating the need for
soldering the electronics to the microbridge. It is expected that as the technology becomes more mature, the yield will increase and the cost of manufacturing the devices will decrease.
It is known that most high-end thermal imaging systems cannot operate without cryogenic cooling of the sensor. It takes about 10 minutes after switching on the device to reach the required sensor temperature (about 200°C). The Sentinel device is ready for operation just 1 minute after switching on the power.
Less than 5 minutes are required to «warm up» the uncooled thermal imaging sight LION (Lightweight Infrared Observation Night) produced by the Dutch companies Signaal USFA and Delft Sensor Systems. The first sample of the device was supposed to appear in mid-1996. The Royal Netherlands Army settled on this model in November 1995 after a thorough study of its prospects. Having made a decision on the device, the Ministry of Defense paid for the development and plans to purchase 300 serial samples starting in 1997. In addition, both companies are counting on exporting the device abroad.
Thermal imaging sight LION AN on a lead-scandium tantalate (PST) grid with 256×128 pixels manufactured by GEC-Marconi Sensors. Operating wavelength is 8-13 μm, optical system magnification is x3, field of view is 10×5 degrees. Vehicle detection range is 2 km, recognition is 700 m, identification is 350 m. The image is formed on a cathode-ray tube in the sight's eyepiece. The sight weighs 2 kg, its dimensions are 10x20x24 cm, and its power consumption is 7 W. Power supply is from six lithium batteries for 10 hours or from standard alkaline batteries for 2 hours. The noise of the operating device is not audible at a distance of 2 m from it.
The development of uncooled focal plane array thermal imaging cameras will undoubtedly expand market opportunities. However, it should be noted that cooled thermal imagers continue to be the most effective in a number of military and civilian applications, since uncooled devices cannot provide the necessary sensitivity, resolution, and response time for a number of tasks.
For example, surveillance reconnaissance and forward-looking aircraft systems typically use long-focus optics, which are not compatible with uncooled thermal imaging systems today. Cooled InSb and HgCdTe cameras are capable of operating with f/4 aperture optics. They use smaller, less expensive optics than those required for uncooled cameras.
Work is currently underway to improve the performance of uncooled systems, and the differences in operating parameters between the two classes will narrow in the coming years. The advent of uncooled technology represents a major step forward in reducing the cost and complexity of thermal imaging systems, but the current price of optical assemblies, especially those made of germanium, will continue to be a major obstacle to reducing the cost of both cooled and uncooled thermal imaging surveillance devices. Thus, any significant reduction in the cost of thermal imaging equipment will be associated primarily with the development of new optical systems.