Focal-plane-array camera eliminates mechanical parts and boosts resolution
Bruce Carocci
Infrared imaging cameras have been used in a growing number of industrial applications over the past 20 years. Originally developed for military use, infrared imaging is increasingly applied to such "mission critical" industrial tasks as predictive maintenance, research and development, process monitoring and control, and nondestructive evaluation for quality assurance in manufacturing industries.
An infrared camera is generally used as a field data-acquisition device to capture images and temperature data for subsequent analysis on a personal computer. Portability and small size are key attributes, along with the ability to digitally store infrared images. Image quality is extremely important. In the world of infrared imaging, it is only a slight exaggeration to say that "image is everything." If you can`t see a problem, it is difficult to recommend a solution. Finally, because not all users are technically skilled, IR cameras need to be simple to operate.
Solid-state solution
A number of technological advances have enabled the latest infrared cameras to meet these market requirements. A prime example is the Prism DS from FLIR Systems Inc. (Portland, OR). The camera design is made possible by the fusion of focal-plane-array (FPA) detector technology, the 486 microprocessor, PC card memory technology, and miniature Stirling coolers. The availability of Windows-compatible software for image analysis and report generation completes the package.
A comparison of this imaging system with the prior generation of infrared cameras highlights dramatic differences (see Fig. 1). Older cameras are bulky (weighing between 15 and 25 lb). A user may require one to two days of training to become a skilled operator, and image resolution is often insufficient to locate and identify problem conditions. In contrast, the Prism DS weighs only 7 lb, while supplying four times the image resolution of older systems. In addition, the power of an internal 486 microprocessor allows automatic "point and shoot" operation to replace of the numerous manual adjustments required of older cameras.
The most important improvement involves replacement of the mechanically scanned systems with solid-state FPA detectors. Older systems use a single detector that is mechanically scanned over the scene using a complex mechanism of nodding mirrors and a rotating polygon. This approach adds weight, consumes space, and offers relatively low-resolution imaging. In contrast, the Prism DS uses a solid-state, two-dimensional array of more than 78,000 detector elements that continually "stare" at the scene or object of interest. The 320 ¥ 244 array increases infrared energy-collection efficiency, as well as improving image resolution to near video quality.
The internal 486 microprocessor manages overall camera function. When the operator selects the automatic mode of operation, the camera continuously adjusts to optimize image quality, regardless of the range of temperatures represented. In addition, the MS-DOS-based 486 processor is an open system that can be easily upgraded with new software. This is important, considering that an infrared camera remains in service for eight to ten years.
Removable, solid-state PC cards provide several advantages over floppy disks. PC cards are about the size of two credit cards sandwiched together and are capable of storing large amounts of data (a popular size is 5 Mbits). Dust, smoke, or other contaminants often found in industrial or experimental environments cannot corrupt stored data. This digital storage technology is smaller, lighter, and more rugged and consumes less power than disk storage.
To eliminate the effect of incidental infrared energy, the detector in high-performance infrared cameras must be cooled to 77 K (-196°C). This energy would otherwise compromise accurate temperature measurement, in an effect analogous to that of stray light in optical systems. Cooling is typically provided by a Stirling cooler. However, the first generation of Stirling coolers had a rotary design that suffered a short mean time to failure. The wear of rotary parts and mechanical linkages combined with contamination from lubricants leaking from broken seals often resulted in frequent replacement or servicing of these units.
The recent generation of Stirling coolers uses twin linear-opposed pistons in micromachined cylinders. This design eliminates the need for lubrication and vulnerable moving parts. The result is a service life five times as great as that of earlier designs.
A complete infrared imaging system includes software for postprocessing images and temperature data; operating under Windows makes the software simple to use. The Prism DS is available with AnalyzIR, a Windows-compatible package that provides a range of capabilities for users from novice to skilled. Users can change color palettes or perform operations such as line profiles, DT, or histogram calculations with a single keystroke. The package includes a full-function report generator to design custom forms and reports or provide report templates.
Applications of thermal imaging
The practical applications of this technology are extremely diverse, even within a single company. Semiconductor manufacturers use infrared cameras for predictive maintenance of processor-fabrication facilities. Circuit breakers, motor control centers, and the connections of batteries used for backup power are surveyed on a regular basis to help prevent the unplanned stoppage of fabrication lines. The cost of these outages in lost revenue can exceed several hundred thousand dollars per day.
These same companies also use infrared imaging as a thermal management tool during the development and failure analysis of new integrated circuit designs (see Fig. 2). The use of magnification optics and emissivity correction software allows systems to accurately detect small temperature differences in areas as small as 5 µm. In this role, infrared imaging provides a noncontact analysis method that complements traditional modeling and parametric testing tools.
With accessory lenses and software, infrared cameras can be adapted to meet a lengthy list of requirements for nondestructive-testing applications. The inspection of aircraft fuselages, storage tanks, or other metal structures is a prime example. Flashlamps can be used to introduce a significant temperature differential to a surface that would otherwise exhibit a uniform ambient temperature. An infrared camera fitted with a wide field-of-view lens can then be used to capture the differential cool-down rates that often indicate corroded metal.
The industrial market for infrared cameras is changing due to the advent of new technology and the contraction of product design cycles. Enhanced camera performance accompanied by substantial improvements in ease of use has opened the use of infrared imaging to large groups of professionals for whom the technology was previously impractical. Although the use of infrared imaging has traditionally produced a return on investment in about six months, this next generation of infrared camera extends these benefit to a greatly expanded market. Future generations of infrared detector technologies and low-power/high-performance logic will continue to rapidly push infrared imaging technology into broader industrial use.
FIGURE 1. Focal-plane-array detector (top) eliminates the need for complex scanning system of rotating polygon and nodding mirrors (bottom).
FIGURE 2. Semiconductor manufacturers check circuits for defects using thermal images.
BRUCE CAROCCI is director of marketing at FLIR Systems Inc., 16505 S.W. 72nd Ave., Portland, OR 97224.