An Introduction to Thermopile Detectors

A thermopile detector is a passive radiation sensing voltage-generating device. It does not emit any radiation and require cooling or bias. Dexter Research Center (DRC) provides stable, high output radiation sensing thermopile detectors covering linear dynamic range from the UV to long wave IR.

The spectral absorption of DRC detectors is flat from the ultraviolet to the far infrared. Based on target size, radiance and temperature, the output of thermopiles is typically in the range of microvolts to millivolts.

Key Components of Thermopile Detectors

Thermopile detectors consist of an array of thermocouple junctions linked in series as differential pairs. These differential pairs form the hot and cold junctions as shown in Figure 1.

Alternating n-type and p-type materials called ‘Arms’ connect these junctions and generate a Seebeck effect between them. A voltage is generated in proportion to the temperature gradient between the cold and hot junctions.

Key features of the Model 2M Thin Film thermopile detector

Figure 1. Key features of the Model 2M Thin Film thermopile detector

Bismuth and antimony are the arm materials for thin film-based thermopiles. Alternating n-type and p-type poly-silicon or n-type with aluminum or gold are the arm materials for silicon thermopiles. The cold junctions and the detector package are normally thermally connected. These junctions are positioned around the perimeter of the substrate opening.

The hot junctions have a coating of an energy absorber and are positioned in the center of the detector pattern. The detector’s active area is defined by these hot junctions, which are thermally isolated from the rest of the package by means of a thin membrane.

It is necessary to know the detector cold junction temperature to perform a radiometrically calibrated measurement with a thermopile detector. This can be done by determining the temperature of the detector package using a thermistor or active device like a LM20 from National Semiconductor.

Most accurate temperature measurements are possible when the thermistor or other device is thermally connected to the detector package and is in the proximity to the detector.

Thermopile detectors have very low noise at the level of a resistor of equal resistance. They generate only the Johnson noise of their resistance and yield a consistent output for DC radiation up to a frequency restricted by the time constant. In addition, they do not require chopper.

DRC Thermopile Detectors

DRC thermopile detectors are in tiny TO-18, TO-5, or TO-8 transistor type packages. The ambient air is removed from the detector package and one of the four encapsulating gases is then filled in prior to hermetically sealing the package. The encapsulating gas presents one of the key thermal paths to dissipate energy from the active area.

DRC detectors have a flat spectral response over the ultraviolet to the far infrared owing to the use of unique energy absorbing materials. The selection of optical band-pass filters decides spectral sensitivity depending on the application of the detector.

Besides having a variety of optical filters and window materials, DRC can customize them depending on the detector application. Internal heatsinks, optional internal apertures, and different options of package aperture sizes are also offered by Dexter Research to address the design requirements of customers.

Types of Thermopile Detectors

Bismuth-Antimony silicon-based and thin film-based are the types of thermopile detectors offered by DRC. The resistance and noise voltage of thin film-based thermopiles are lower when compared to silicon-based thermopiles, thus providing a higher signal-to-noise ratio.

The time constant of a thin film thermopile with an output equivalent to a silicon-based thermopile is comparatively slower. The active area of thin film thermopiles is typically large. The following table compares the two types of thermopiles:

Parameter Thin Film Silicon
Output Voltage Higher Lower
Signal-to-Noise Ratio Higher Lower
Temperature Coefficient of ℜ -0.36%/°C -0.04%/°C
Noise Voltage Lower Higher
Time Constant Slower Faster
Cost Higher Lower
Operating Temperature 100°C 125°C*

* Specific configurations to 225°C

An internal compensating element is available in most of the thin film thermopiles and is blinded. It is generally linked in opposition to the active element to reduce the effect of an unexpected change in ambient package temperature.

This temperature compensation is useful for roughly the first few seconds of thermal shock to the detector package. Compensated silicon thermopiles are also available from DRC.

DRC also supplies different kinds of thermopile detector modules with digital output. The company’s silicon thermopile detector technology is the cornerstone of its Temperature Sensor Module (TSM) , which consists of an integrated ASIC in the detector package to yield a calibrated digital output for precise non-contact temperature measurements.

Applications of Thermopile Detectors

Thermopile detectors find use in the following applications:

  • Non-contact temperature measurements in process control and industrial applications
  • Hand-held non-contact temperature measurements
  • Thermal line scanners
  • Tympanic Thermometers Infrared Radiometry Refrigerant Leak Detection
  • Automotive exhaust gas analysis of HC, CO2 and CO
  • Commercial building HVAC and lighting control
  • Security human presence and detection
  • Black ice detection and early warning
  • Blood glucose monitoring
  • Horizon sensors for satellites, aircraft, and hobbyist applications
  • Medical gas analysis such as blood alcohol breathalyzers, incubator CO and CO2, and anesthetic
  • Automotive occupancy sensing
  • Automotive HVAC control
  • Aircraft flame and fire detection
  • Fire detection in transportation tunnels
  • Hazard detection including flame and explosion
  • Household appliance temperature measurement

This information has been sourced, reviewed and adapted from materials provided by Dexter Research.

For more information on this source, please visit Dexter Research.

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