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Thermal Sensor Technology
Uncooled Thermal Sensor Technology
A paradigm shift has occurred in the commercial infrared camera industry as a result of classified military thermal sensor development in the 1980’s. In the past most high performance infrared cameras used in commerce had photon-detection sensors that needed to be cooled to liquid nitrogen temperature (77K). Cryogenic coolers were used to cool the detectors requiring about 10 minutes turn-on time to achieve this very low temperature. New high performance cameras use thermal sensors that eliminate the need for cryo-coolers. Their turn-on time is 15 seconds or less.
Cryo-coolers are expensive and have wear-out issues. The elimination of the cryo-cooler and advances in electronics has made the cameras smaller, more reliable and less expensive.
Thermal Sensors
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The principle of a thermal infrared sensor is shown below. A tiny thin plate (which I call a “platelet”) is made on a silicon wafer in a silicon foundry by a micro-machining process. The platelet is typically 50 m (microns) square by 0.5 m thick. Even smaller sizes are under development. Long thin support legs and a vacuum environment thermally isolates it from the substrate. Small thermal radiation from the target focused onto the platelet heats it. The higher the target temperature, the greater the focused radiation is and therefore the higher the platelet temperature.
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The temperature of the platelet and therefore the intensity of the radiation can be measured by the change in resistance of an electrical resistor deposited on the platelet -- the microbolometer sensor. It can be measured by a thermocouple with the hot junction on the platelet and the reference junction on the substrate -- the thermoelectric sensor. Or it can be measured by an electrical capacitance effect -- the pyroelectric sensor. The microbolometer was developed by Honeywell and is used in Infrared Solutions, Inc. cameras.
Microbolometer Technology
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Individual sensor elements use the change in electrical resistance of a VOx resistor deposited onto the tiny “platelets” fabricated by silicon micro-machining in a silicon foundry. Incoming target radiation heats the VOx causing a change in electrical resistance, which is readout by measuring the resulting change in bias current. 80,000 and more sensors can be fabricated together into a two-dimensional array. The structure can be dimensioned to operate at 30 Hertz. That is, the thermal conductance of the isolating legs can be adjusted to match the time-constant for 30-hertz operation. An example of a microbolometer element is pictured on the left.
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It consists of a two-layer structure. An interconnecting readout circuitry is applied to the silicon process wafer and then the microbolometer structure is built on top of the readout circuitry. First a pattern of islands _ wavelength thick are deposited on the readout circuitry. The islands are made of a material that can be selectively etched away later to form a bridge structure. Three layers -- silicon nitride, vanadium oxide, and silicon nitride -- are deposited over the sacrificial islands. The sacrificial islands are then etched away leaving the thermally isolated bridge structure of vanadium oxide. A photo of an early Honeywell microbolometer element is shown in the picture below followed by a photo of one corner of a 320 by 240 microbolometer array.
Most of today’s camera manufacturers use the 320 by 240 microbolometer array. However there is an excellent alternative for many commercial applications – the 160 by 120 array. The smaller array and its resulting camera can be produced at a much lower cost. Far more arrays can be produced on a single wafer and the yield is higher for the smaller array. In addition, one of the most expensive components of an infrared camera is the lens and its cost is proportional to the array size.
The only advantage of the larger array is field of view (FOV). With the same f# and focal length lens and the same detector size, a camera with 320 by 240 or 160 by 120 will have identical spatial resolution. But the target size for a fixed distance between the camera and target will be twice as large in both dimensions for the camera with the larger array. For many commercial applications the cost savings of the smaller array size over shadows the advantage of a larger FOV.
Some Historical Facts
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During the 80’s the Department of Defense gave both Honeywell and Texas Instruments (TI) large classified contracts to develop uncooled infrared sensor technology. The military wanted a sensor that had very short turn-on time. Both programs were very successful – the pyroelectric sensor by TI and the microbolometer by Honeywell.
In 1992 the US Government de-classified the use of Infrared Technology for commercial products but maintained control of the technology. TI developed commercial imagers with their sensor technology and Honeywell licensed their microbolometer sensor technology to other companies.
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Four companies originally purchased the microbolometer license. Loral, the company that purchased the Honeywell Electro Optics Division, received a license. Santa Barbara Research that was part of Hughes, Amber Engineering, and Rockwell purchased licenses. All four of these licenses have changed hands, some more than once. Loral was acquired by Lockheed Martin who later put the microbolometer activity in their Sanders operation which was later acquired by British Aerospace. Raytheon acquired Santa Barbara Research from Hughes and acquired Amber Engineering. Boeing acquired Rockwell’s infrared activity and after a few years sold it to DRS Technologies. So the original four licensees are now three, British Aerospace, Raytheon and DRS Technologies.
Three additional companies have purchased licenses, Indigo Systems, InfraredVision Technologies Corporation and the latest, NEC. In addition to these six licensees a seventh, Institut National d’Optique (INO), has a limited license. Honeywell continues to supply microbolometer arrays to Infrared Solutions.
Infrared Basics
Infrared radiation is electromagnetic radiation whose wavelengths are greater than those of visible light but shorter than those of microwaves.
In its most familiar form, it is radiated heat which can be sensed by our skin, yet cannot be seen by our eyes. All objects, whatever their temperature, emit infrared radiation. The intensity emitted depends upon the fourth power of the absolute temperature of the object. It also depends upon a material property of the object, termed “emissivity”. An ideal infrared emitter, said to be a “blackbody,” has an emissivity of unity. Most real objects have emissivities less than unity, and therefore emit less intense infrared radiation than a blackbody at the same temperature. In summary, the infrared emitting properties of an object are characterized by its temperature and its emissivity.
Imaging Radiometer Temperature Measurement
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If it is necessary to measure the temperature of an object, one way is by directly contacting the object with a thermometer or similar device. In some instances, however, this is not practical. In such cases, it is possible to measure the temperature of an object from a distance using an instrument sensitive to infrared radiation. If the emissivity is known or can be estimated, then the temperature of the object can be determined. Such instruments capable of remote temperature measurement are termed “radiometers.” Because infrared radiation is similar to visible light, only having a longer wavelength, it is possible to “see at night” by electronic cameras which respond to infrared wavelengths. Such systems, termed “thermal imaging systems,” find widespread military use. For example, they enable our armed forces to view scenes at night which are displayed on TV screens. Such devices were widely used in the Gulf War, and are beginning to be used by law enforcement agencies.
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It is possible to combine the concept of a radiometer with that of a thermal imaging system to provide an imaging radiometer. In this way, it is possible to determine the temperature of any and all points in a scene by measuring the infrared radiation emitted by every object in the scene. Such systems have found widespread commercial use in predictive and preventive maintenance in factories. For example, a hot spot in an electrical junction box which is detected by an imaging radiometer can warn of an impending failure. This is only one of the many applications addressed with Infrared Solutions’ infrared cameras.
* The discussion above is concerned with what are termed “continuous sources” of infrared radiation. Other sources, which emit only at selected infrared wavelengths, are termed “discrete sources.” For simplicity, a discussion of discrete sources has not been included.
Camera Technology
Infrared cameras historically used sensors made of exotic materials that required cooling with liquid nitrogen to a temperature of -320°F. A technology breakthrough achieved through military research now makes possible the production of uncooled high performance detectors capable of sensing and measuring infrared energy. Fueled by the commercial availability of this new uncooled technology, significant opportunity in the application of IR technology has emerged.
The technology makes possible the development of new, lower cost, and more easily operated infrared imaging devices using a two-dimensional array of detectors. This permits “seeing” the entire scene at once and can produce real-time pictures at TV rates by products such as Infrared Solutions' IR-InSight® and FlexCam® series of cameras.
Sample Images
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Electrical Inspection
Electrical connections become hot due to overload or high resistance caused by corrosion or a poor connection. This image shows that the left and right fuse clips are worn and need to be replaced. Catching a problem like this during routine inspections or after maintenance can catch a problem early, before it causes expensive downtime.
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Quality Control
Examining prototypes and parts for potential heat related design deficiencies enables companies to release better and more durable products to the market.
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Condition Monitoring
Industrial pumps are known to work continuously for many hours. If a pump stops working or starts to wear, this can cost a factory much needed time. Using an IR camera to monitor the temperature of these type pumps, a maintenance worker can spot troubled pumps BEFORE the failure and address with little or no down time - saving you time and money.
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Process Monitoring
Many processes require careful monitoring of process temperatures to maximize yields and minimize scrap. Here a plastic molding machine shows temperature variations on the metal die. By adjusting process parameters, these variations can be controlled, maximizing your yields, and your profits!
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Predictive Maintenance
Monitor temperatures of bearings and other moving components. Friction is a machine’s nightmare, and an engineer or technician would do anything to avoid it. As bearings wear with time, they begin to slow machinery and create unnecessary heat. Monitoring changes over time can help predict when maintenance needs to be performed. Neither too soon, nor too late. Saving the costs of unnecessary maintenance.
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Preventive Maintenance
This is an image of the inlet to a water heater. As you can see, at this joint there is a significant temperature change in the corner. A technician used this information to determine that the L-shaped fitting was blocked. This type of blockage has been known to cause corrosion and overheating with possible extreme circumstances. Early detection of this problem saved the owner money and peace of mind.
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R & D Heat Flow
New development is occurring at an ever increasing pace. Understanding heat flow for new products going into production can give your company the edge over the competition. Heat flow analysis can also be used in the electrical circuit board industry. Know what you need to know - now.
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Substation Maintenance
Power companies’ business is to produce power. IR imagery allows you the ability to monitor failing insulators or powerlines creating excess heat and ultimately - power outages. A maintenance worker armed with an IR camera would be able to predict or prevent failures - giving you more up time.
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Roofing Inspection
Infrared inspection of this industrial roof reveals water saturation due to leaks or condensation under the roof membrane. The heat generated by sunlight dissipates much slower where moisture saturated insulation exists. After nightfall, a slight temperature difference will exist between the dry insulation and the water saturated insulation that can be seen with a highly sensitive infrared camera.
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