Cees Draijer, Teledyne Dalsa
Technology advances and improved manufacturing of uncooled thermal detectors are fueling this application expansion.
First developed for military applications in the 1950s, thermal imaging lets humans see objects that are not visible to the eye by detecting the thermal radiation (heat) emitted by these objects. Although our eyes can detect only visible light, several technologies allow us to extract information from the environment by looking at a particular bandwidth of the infrared spectrum.
More recently, thermal imaging has started making significant inroads into many other types of applications, thanks in large part to the development of uncooled microbolometer-based detectors.
Thermal image showing thermal features in gray scale. The darker areas indicate that less radiation is emitted from the object, while lighter areas show high levels of emitted thermal radiation. The darkness of the spectacles indicates that emitted radiation is not transmitted through glass objects.
These sensors work in the long-wave infrared band from 7.5 to 14 µm. Microbolometers offer several advantages over traditional cooled detectors, including smaller size, lighter weight, lower cost, immediate power-up capabilities, decreased power consumption and longer MTBF (mean time between failure).
Cooled vs. uncooled thermal detectors
Unlike microbolometers, cooled thermal detectors require additional equipment to obtain operating temperatures between −70 and −150 °C. This cooling equipment is costly and bulky, uses a lot of power, and requires a cooling-down time, which pushes the time to first image anywhere from 10 to 20 minutes. In certain high-end applications, these constraints are acceptable. For example, the military often demands higher-performance technology than microbolometers can deliver at long distances. These applications will continue using less portable, more expensive cooled thermal imaging systems that do not provide instant-on image acquisition.
False-color thermal image taken with a microbolometer camera shows hotter (redder) and cooler (bluer) areas where temperatures vary by 10 °C, indicating increased blood flow caused by inflammation.
Other applications, however – including some military ones – need uncooled thermal sensor technology. Soldiers on the ground or firefighters battling a five-alarm blaze cannot wait for images to be generated – they need immediate access to this critical information. Because the thermal imaging technology is handheld or mounted on helmets, the devices must also be portable and lightweight with low battery power consumption.
Uncooled thermal detectors have certain disadvantages versus their cooled counterparts: They are less sensitive and are intrinsically slow (the typical time constant is 10 ms). These limitations do not affect their suitability for an ever-increasing number of cost-sensitive commercial applications that simply need acceptable image quality, not high performance or speed. The high cost of cooled infrared detectors, in fact, makes them unattainable for the vast majority of commercial applications.
Advances in uncooled detector technology
As advances in uncooled detector technology continue, the increased value that microbolometers offer outweighs the limitations. One such development is the trend in microbolometer arrays toward increased pixel count and decreased pixel pitch. Currently, 17-µm arrays are in production and, recently, 12-µm-pixel arrays have been introduced, which allow for even smaller detector footprints and/or higher resolutions.
Although this smaller pixel can be manufactured, it may take some time before it is good enough to replace the current-generation 17-µm pixel. We expect that this latest migration will be similar to what we experienced when the 17-µm appeared: It took some time before it was on par with the 25-µm pixel. In this “pixel race,” which is quite common in imaging, developers must work hard to ensure that each new generation of smaller pixel delivers the same performance as its predecessor – or better.
The ability to make a smaller pixel permits either higher resolution or the same resolution with a smaller, lighter and more energy-efficient device. This technology evolution is required to further drive down the cost of microbolometers and open up new commercial markets and applications.
Commercial applications
The automotive industry already uses thermal imaging as part of a safety system that can, for example, identify animals or humans on the road and warn drivers before they encounter these potential hazards. Since human vision sees motion at 60 Hz, a microbolometer-based detector offers more than adequate performance for this application. Although thermal imaging cameras are currently offered only by a luxury carmaker as an expensive option on certain models, they will become more commonplace as the cost of this technology decreases. Rear-view visibility systems (i.e., backup cameras) are now becoming mandatory on all new light vehicles sold in the U.S., and thermal imaging technology may one day be a required safety feature, as well.
Building inspectors and energy auditors also rely upon infrared thermography to quickly detect issues like water leaking through roofs and foundations, electrical and plumbing problems, structural defects, moisture accumulation and mold growth, missing insulation and heat loss. A handheld device that uses thermal imaging technology benefits from a microbolometer, because this device needs to be compact, portable and inexpensive.
Another market that offers potential for the adoption of infrared technology and microbolometers is security and video surveillance. As the cost of uncooled technology continues to decrease, thermal imaging cameras for night-vision applications, such as detecting the movement of individuals, animals and objects, will become more accessible.
Instrument or tool?
Commercial applications demand compact, cost-effective thermal imaging technology. Microbolometers provide an excellent solution, because they support smaller dimensions and low power consumption, while still delivering adequate image performance. Looking to the future, the expectation is that technology advances will result in microbolometers entering the consumer market as they evolve from an instrument to a tool. One such example would be a homeowner who purchases an inexpensive thermography device from a neighborhood big-box do-it-yourself store for repair and maintenance projects around the house.
Thermal imagers also have the potential to be integrated into ubiquitous devices such as cellphones. However, significant challenges must be overcome before this technology can be widely used in high-volume consumer applications. We have to look at manufacturing yields, as it will take a lot of effort to bring costs down. Today, the average cost of a midrange detector far exceeds that of a smartphone, so significant pressure must be applied to bring down the detector’s cost – to the sub-$10 level, some believe – before true integration in cellphones can happen.
It is interesting to note that thermal imaging is following a path very similar to that of digital imaging. Initially, owing to its high cost, digital technology was only integrated in instruments. As it became more affordable, it started appearing in commercial applications and consumer devices such as cellphones. Today, everyone has a physical camera integrated in their phone, and the cost of the imager is very low: less than $2.
Vanadium oxide vs. amorphous silicon
Unlike uncooled technology development, the efforts spent on cooled technology in the past couple of decades have been focused mainly on improving performance, with less consideration for cost-down breakthroughs to enable higher-volume applications. As uncooled arrays began to migrate out of research labs and into production in the 1990s, the industry started focusing its attention on funding and further developing the detector materials used in microbolometers, with the intention of addressing higher-volume, lower-cost markets.
Today, the two main types of microbolometer materials are vanadium oxide and amorphous silicon. Although these two absorbing elements are designed to work the same way, some differences exist between them in terms of performance (i.e., sensitivity, noise, resolution and frame rate). They also differ in the ease and cost of manufacturing and exporting, and in access to R&D funding. A combination of these factors has resulted in manufacturers and customers now demonstrating a clear preference for vanadium oxide, provided that the price is similar to that of amorphous silicon.
The importance of better manufacturing
While performance improvements play a vital role in the acceptance of uncooled detectors by mainstream commercial markets, better manufacturability – especially the ability to produce high yields while maintaining a low defect rate – is also critical.
The importance of refining manufacturing processes for optimal yield and minimal defects is recognized by manufacturers like Teledyne Dalsa, which is now entering the infrared market with the assistance of a $13 million grant from the province of Quebec. The funds are being used to increase the production capacity for microelectromechanical systems at the company’s Bromont, Quebec, plant and to develop advanced infrared imaging technology.
The first prototype of Teledyne Dalsa’s microbolometer long-wave infrared imager with 640 × 512 pixels.
The company has chosen vanadium oxide as its detecting technology, believing it offers the best chance to reach the performance and high production volumes needed. The key to success is not only being able to make a microbolometer, but also having a stable enough manufacturing process to successfully produce this device in large volumes. The facility is completely integrated, allowing production of both the microbolometer pixel area and a wafer-scale vacuum package within a single process flow. This is made possible by the unique configuration of tools and capabilities that is uncommon in the microbolometer business as it stands now.
Looking to the future
Thermal imaging will continue to make significant inroads into commercial and consumer applications, due in large part to the technology advances and improved manufacturing of uncooled microbolometer-based detectors. These developments are significantly lowering the cost of this technology and improving its overall performance, while ensuring that thermal imaging devices and systems become smaller and lighter, and consume less power.
Meet the author
Cees Draijer is a senior program manager for infrared development at Teledyne Dalsa in Waterloo, Ontario; email: [email protected]