Figure 2. The principle of a thermal infrared sensor.
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The temperature also can be measured by a thermocouple with the hot junction on the plate and the reference junction on the substrate. This is the basis for the thermoelectric sensor. Temperature also can be measured by an electrical capacitance effectthe basis for the pyroelectric sensor.
Microbolometer technology
Individual sensor elements measure the change in electrical resistance of a vanadium oxide resistor deposited on the platelets. Incoming target radiation heats the vanadium oxide, causing a change in electrical resistance, which is read by measuring the resulting change in bias current.
The two-dimensional array on the platelet can have 80,000 sensors or more. The camera's refresh rate of 30 Hz produces an image similar to what's seen in a video.
Most cameras feature the 320 by 240 microbolometer array. However, the 160 by 120 array is an alternative for many applications. The smaller array and its camera can be produced at a lower cost, providing an opportunity for cost-sensitive purchasers to implement this technology.
The advantage of the larger array is its field of view. For a given f-stop and lens configuration, the 320 by 240 array will have identical spatial resolution as the 160 by 120 array. But the target size will be twice as large in both dimensions for the camera with the larger array. For some commercial applications, the cost savings of the smaller array may overshadow this shortcoming.
Thermoelectric technology
In the thermoelectric sensor, individual thermocouples measure the temperature of the platelet. Each platelet can carry as many as five series-connected thermocouples that produce a small voltage in response to focused radiation from the target. Moving the sensor array in the focal plane of the camera lens produces a two-dimensional image consisting of 14,400 pixels (120 by 120).
A silicon nitride pattern deposited on the silicon produces a thermally isolating bridge structure with selectively etched well pits in the silicon wafer under the bridge structure. Dissimilar metals A and B are deposited on the bridge structure. Electrical contacts are added along the sensor. Each element has three series thermocouples. Because this array is linear, the electrical connections can be located to the side, permitting a large fill factor.
Advances in infrared technology reduce infrared camera complexity and price, which produces more widespread applications for the technology. Infrared cameras are an integral part of predictive maintenance inspection at large companies, and small- to medium-size companies are embracing it, as well.
Roger Schmidt is the founder of Infrared Solutions, Inc. He can be reached at [email protected], (763) 551-0003 or (800) 760-4523.
Figures: Infrared Solutions, Inc.