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Thermal imaging goes digital
May 22, 1995
12 Min Read
Heat can be deadly to products and processes. It can shorten the life of electronics, indicate the impending failure of electrical or mechanical systems in buildings or equipment, and predict impending problems or design flaws.
Infrared thermal imaging can help analyze, monitor, and solve such heat problems. With this track record, why isn't industry teeming with IR cameras? Mainly because, in the past, IR cameras were bulky, limited by low-resolution imagery, and had only rudimentary analysis capabilities.
On the other hand, mechanical scanning systems could measure temperatures everywhere, but not very accurately. Even more advanced solid-state cameras measure temperature only at a single point in the center of the screen.
Last year, all those drawbacks went out the window. Attribute the advance to engineers at FLIR Systems Inc., Portland, OR, and the Prism DS-a handheld infrared thermal-imaging camera packed with advanced features.
"An emerging need for bringing very-high-resolution digital data from an IR imaging camera to a PC prompted the design," says Gary Causley, director of industrial engineering and manager of the Prism DS project. "This would allow the user to deal with the data in a more orderly and sophisticated format."
"In our previous camera, we digitized inside, did all sorts of DSP, and then regurgitated the output in analog video," Causley chuckles. "Our customers had no access to the digital data. The biggest thing about the DS is the link to the customer of "true' digital data."
Other project goals: temperature measurement across the entire field of view, instead of at just one point; 12-bit resolution; real-time operation; and light weight. There were also challenges: calibrating 78,080 IR detectors, connecting the detectors to the digitizing electronics, and processing a huge amount of data in real time.
Design of the Prism DS started in March 1994. However, the DS is the third product in the Prism line, so the engineering team didn't start from scratch. Also, FLIR believes in modular designs. Engineers reused or redesigned what components they could from the previous models, designed parts that could be retrofitted into those cameras, and developed others that they could easily integrate into future designs.
The result: the 7-lb Prism DS camera that uses solid-state technology to detect infrared radiation and measure temperature at more than 78,000 points simultaneously. PMCCIA flash-memory cards store the digital images, a 486 microprocessor runs the show, and real-time digital signal processing (DSP) enhances the images. Then, Windows(R)-based AnalyzIR image-analysis and report-generation software let users post-process images and analyze trends on their PCs.
Front to back. Walking through the camera from the lens to the digital output shows how the DS works-and the challenges the designers faced. First, the lens. FLIR developed a family of lenses-25, 50, and 100 mm-for the DS. The IR-transmissive material consists mainly of germanium and silicon.
The company buys the material as blanks, then uses its own diamond-turning optical fabrication facility to build lenses in house. "That gives us tremendous leverage," says Causley. "We can make common parts to keep costs down."
In the process, the engineers developed a new color-correction lens technology for the DS using diffractive optics and a sophisticated grinding technique. Because of IR light's long wavelength, it's critical that the energy doesn't shift as the lenses move the light. If there is a shift, the new technology passes the energy in the desired wavelengths.
Heat at 78,080 points of heat. The infrared radiation travels through the optics and then to the focal plane array (FPA). The most sensitive part of the camera, the FPA comprises a 320x244 array of 78,080 IR detectors.
Traditional systems have just one to four detectors, and you could calibrate each one independently. With 78,080 detectors, FLIR engineers developed new calibration techniques and algorithms that would let them deal with each. The results would be compatible with real-time imaging and temperature measurement.
"One of the biggest issues involved the vast quantity of data required to calibrate each pixel. Each needs several data points to provide a uniform output from an input signal," says Dwight Dumpert, application engineering manager. This is done at the factory. FLIR shows the camera a series of "black body thermal references"-objects at a known temperature uniform across the surface. Each detector must agree on the temperature not only at that point in time, but also at different ambient temperatures.
To cool the calibrated IR detector chip to its working temperature of 77K, FLIR chose a system developed by the military for a missile program. Called a dual-opposed linear Stirling cooler-or the "mini-liny"-it uses helium for cooling. The compressor's twin-opposed pistons move in opposite directions to minimize vibration. It's also electrically isolated from the detector to prevent noise and interference.
IR energy hitting the detector surface creates a charge, which transfers to a CCD (charge-coupled device). The CCD sends the microvolt-range signal to the digitizing electronics, which rest on a flexible circuit board on the cooler assembly. The board includes a speedy A/D chip from Analog Devices that digitizes the signal in real time.
Complex tradeoffs. The board mounts on the back of the cooler to get critical timing signals as close to the detector as possible. But, the components generating the signals can't be too close. If they were, they could generate noise that the detector could pick up.
Adds Dumpert, "At the same time, you have to minimize lead lengths. If the leads are too long, they'll act like antennas. Tradeoffs proved to be a careful balancing act."
Another concern: The signal leads had to go into the cooler, where they essentially act as conduits for heat. Taking heat into the cooler makes the cooler work harder, and changes the temperature of the detector. Engineers addressed this problem by using advanced ground-signal management to cut down on the number of leads to reduce the thermal load. Normally, you'd have separate grounds for each signal going into the array. Again, this required careful balancing: FLIR also wanted quality signals coming out of the detectors.
Programmable chips. After the signal makes its way through the digitizing electronics, it travels through several digital correction circuits. These circuits implement such features as automatic leveling, gain, and ranging. Xilinx programmable gate arrays perform the digital signal processing.
Causley estimates that the DS has 13 different Xilinx designs. "When you're developing a product, you can use these off-the-shelf programmables and load instructions into them. Someday, you could take what we've learned and reduce it into a custom chip set. But in markets like ours, where the feature sets move fast and the technology advances quickly, the flexibility of gate arrays proves very valuable." One circuit example: a sophisticated auto gain and level system that uses a histogram equalization algorithm.
Some of circuits are new, some are redos. No matter what the circuit type, the 12 bits gave the designers added arithmetic to do. The challenge, they say, was to build firmware that would support all the features, but still go fast enough to deliver the picture in real time.
Data depository. After you have the image you want, you can store it in the camera by pressing a button. The storage media include PCMCIA cards and an internal HardCard 10M-byte solid-state drive from Sundisk. The PCMCIA cards hold 5 megabytes of solid-state memory configured as a floppy drive. They can store 30 to 35 images per card.
"We've got two ways to get digital data out. One is to store them on the removable PCMCIA card; the other is to hook up a monitor to the camera in real time," Causley explains. "You'd opt for real-time, 12-bit data for fast events that you want to capture with high accuracy, such as a plasma, explosions, or charcoal igniting."
A third option: Use the video output and image capture simultaneously. The camera's multipurpose, 44-pin DIN connector makes this possible. The connector implements three serial ports for calibration, interrogation, and remote control; the power connection; and the high-speed serial digital link. The digital link lets users take real-time digital video off the camera and convert it to real-time imagery on a PC.
To save space, FLIR decided to adopt a serial digital link rather than the more traditional parallel digital link. "If we did it in parallel, it would take 34 wires instead of four," claims Causley.
Analysis software. The AnalyzIR software, a Windows program that provides off-line data analysis and dynamic report generation, completes the Prism DS package. It features temperature versus time trending, using up to 24 user-selected temperature points or areas of interest. A simple interface lets users perform analysis functions with a single selection from the graphical toolbar.
The software team, under the guidance of Project Manager Bill Sondermeier, worked with Optimas, Bothell, WA, to create the image-analysis program. The team provided Optimas the necessary software and data to convert an image analysis, which typically shows pixel luminance, into temperature calibration data.
"There are other digital cameras out there, but their picture resolution is so poor that you see blocks of data, not a true VGA-quality image," claims Sondermeier. "Now our customers say there's no need to take a visual. The IR resolution is so good you can tell exactly what you're looking at."
For Sondermeier, the biggest challenge was dealing with the 12-bit TIFF data. Communicating the extra data and displaying it in full on all computer platforms proved difficult. But the designers felt it was especially important to get a good display for laptops, given the camera's portable design.
"We see people wanting to go out and capture their images, then do the image analysis on a laptop in the car," explains Sondermeier. "This assures them they got what they wanted before driving back to the office."
But it's customer reaction that Sondermeier and the other team members like best. "It's a great technology," says Brian Johnson, a manufacturing engineer at SDC Submarine Systems in Portland, OR. "By the year 2000, there won't be an advanced manufacturing facility in the modern world that doesn't utilize IR imaging."
Finding the hot spot
Three examples of thermal imaging applications-volcano research, injection-molding quality management, and studying CPU heat dissipation in PCs-reveal the power and versatility of Prism DS technology.
Heat in Hawaii. Half a million students around the world learned the power of IR imaging firsthand last March, thanks to the JASON Project. Oceanographer Dr. Robert Ballard, discoverer of the Titanic, headed the Island Earth project, based in Hawaii Volcanoes National Park. Live satellite broadcasts transported students to the expedition site. "Primary Interactive Network Sites" let them watch the expedition live, interact with the scientists, and even control remote-operated vehicles.
"We went up in a helicopter with the camera and flew over a lava field," says David Katzive, producer of the JASON Project broadcasts. "The camera detected the array of underground lava tubes you can't normally see from the air because the lava's crusted over. Geologists mapped and tracked the whole network of lava tubes."
Heat-critical molding. When Brian Johnson got a FLIR thermal-imaging camera, it was love at first use. Johnson, a manufacturing engineer with STC Submarine Systems, Portland, OR, practiced using the portable camera all around the plant. "I found bearings that were bad, couplings that were misaligned, heaters that weren't consistently heating up to the proper temperature," Johnson recalls.
STD makes under-sea fiber-optic communications systems, which range from 1,000 to 3,000 km in length and consist of cable and repeaters. They connect two pieces of cable using a joint that's about 7 inches in diameter and 18 inches long. The joints are critical and expensive.
Especially critical is getting a smooth continuation of the polymer jacketing between the cable and the joint that doesn't let any water get inside. STC uses injection molding for the job. However, if you have cold spots in the mold then the polymer doesn't mold to itself very well. With hot spots, it molds too well, and you get a thin wall. To control the process, you must keep the temperature constant.
To do that, STC uses FLIR's thermal camera to analyze new mold blocks. "We used to have a dummy joint that had multiple thermocouples in it," says Johnson. "We'd put it in the block and take readings as it ran. Now we've found it easier to bring the mold up to heat, throw it open, and take a picture of the surface with the thermal camera." The image gives engineers an immediate surface temperature across the board, instead of just where the thermocouples sit.
Keeping PCs cool. Geert De Vries, analytical lab manager of Intel's Customer Quality Support Group, Swindon, UK, reports that thermal imaging can measure the temperature of components inside systems, especially in PCs. As microprocessors, such as the Pentium, become denser and run hotter, thermal management inside the case becomes more important. "It's no longer a case of just putting a fan in," says De Vries. "The way the components and additional boards in the case are laid out affects the way the cooling air movement takes place inside. This directly affects the ability of the CPU to get rid of heat."
Intel's current FLIR camera doesn't have a digital output. But storing images on PCMCIA cards would make the process more convenient and less dependent on one set of equipment, says De Vries.
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