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Article Excerpt
Infrared (IR) thermopile sensors-which measure infrared radiation and consist of a serially interconnected array of thermocouples, each of which is comprised of two dissimilar metals-have been finding expanding use and opportunities in varied applications where the thermopile's ability to provide non-contact measurement of temperature (or presence or position) offers clear advantages over competing and more invasive sensing technologies.
The thermopile sensor's ability to perform non-contact measurement safeguards against any damage to the sensed target or possible measurement error due to contact with the target. Furthermore, non-contact temperature measurement protects individuals against the possibility of contamination from infectious diseases. In this vein, last year, demand for thermopile-based fever (ear or forehead) thermometers was particularly brisk, due to the SARS (Severe Acute Respiratory Syndrome) epidemic.
The IR thermopile responds to a broad infrared spectrum; and does not require a source of bias (electrical) voltage or current, since the infrared thermopile is a voltage generating device. To improve or optimize its performance for particular applications, the IR thermopile uses a bandpass filter, which ensures that only the desired IR radiation strikes the IR absorbing material.
Because the IR thermopile emits radiation as a function of its own temperature, the amount of radiation absorbed by the sensor is a function of the difference in temperature between the target object and the sensor. Since the thermopile (or any other radiation sensor) generates a signal proportional to the difference in temperature between the sensor and target object, in remote temperature measurement it is necessary to register the sensor's temperature and the increase in temperature of the IR absorber material. A thermistor is typically used for registering the sensor's temperature, recording ambient temperature, and facilitating temperature compensation.
There are essentially two basic types of IR thermopiles available: thin-film (bismuth and antimony) thermopiles, and fully CMOS-compatible silicon micromachined thermopiles. Bismuth/antimony thermopile sensors, which come either on a thin film substrate or on a micromachined silicon chip with a membrane, generally have a higher signal-to-noise ratio, higher output sensitivity, low thermal conductivity and resistivity. Fully CMOS compatible silicon thermopiles (e.g., those that have a silicon substrate and also use silicon or polysilicon in the thermocouple sensing elements/junctions) have a lower temperature coefficient and faster time constant. However, they have a lower responsivity, and reach lower thermoelectric power, compared to their Bi-Sb counterparts. The CMOS technology is designed to ensure a long lifetime and stability of all thermal and electrical properties.
The expanding opportunities for thermopiles have, in particular, been driven by the application of silicon micromachining technology to allow for producing numerous (e.g., 100 or more) thermocouples on a minute area of several square millimeters. Silicon micromachining technology allows for mass producing thermopile sensors and arrays that offer small size, low power, high responsivity, rapid response time, and low-cost (through taking advantage of automated mass production techniques).
Key existing high-volume applications for silicon micromachined thermopiles at present include, for example, fever thermometers (primarily ear thermometers), industrial non-contact IR thermometers/temperature sensors, automotive climate control, microwave ovens, and home air conditioners. Moreover, thin-film thermopile sensors and silicon thermopile sensors are used in NDIR (non-despersive infrared) gas detection instruments.
The output signal of silicon thermopile sensor is typically in the sub-millivolt/microvolt range and, therefore, requires signal amplification and processing. It is desirable to locate the electronics as close to the sensor as possible to ensure an adequate signal-to-noise level. A thermopile module containing the thermopile sensor and signal amplification, processing, and calibration electronics can be more easily and cost-effectively integrated into the user's product or system. Such a plug-and play, programmable module has key potential for simplification the OEM's design-in efforts, thereby expanding the served markets/applications for thermopiles and opening up new markets.
PerkinElmer Optoelectronics (Fremont, CA, 510-979-6500)(www.optoelectronics.perkinelmer.com)(whose thermopile business is centered in Wiesbaden, Germany (++49-611-492-247) is helping to proliferate the served and future applications for thermopiles by spearheading the use of programmable and pre-calibrated thermopile modules equipped...
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