Solar radiation measurements are key to a range of applications in modern climate science, agronomy, renewable energy engineering, and atmospheric research.
Image Credit: Leonid Sorokin/Shutterstock.com
A number of instruments are heavily dependent on the performance of the detector at their core, including pyranometers designed to measure broadband solar irradiance (generally 300 to 3000 nm), and pyrgeometers designed to measure terrestrial longwave radiation (≈4 to 100 µm).1,2
Thermopile detectors remain the industry gold standard among available broadband sensing technologies, thanks to their passive operation, stable DC output, and nearly flat spectral responsivity.
The ST150 thermopile from Dexter Research Center has been specifically designed for these high-accuracy radiometric applications.
Its powerful combination of broadband linearity, high output voltage, low temperature coefficient, and optional on-die PTC thermistor means that the ST150 represents the ideal solution for manufacturers looking to cost-effectively meet or even exceed the performance requirements of ISO 9060:2018 pyranometer classifications.
Why Thermopile Detectors Are Ideally Suited for Use in Solar Radiation Instruments
Thermopiles work by converting differential heating into voltage via the Seebeck effect.3 A measurable DC voltage proportional to irradiance is generated when solar or terrestrial radiation warms the thermopile’s hot junction and its cold junction remains thermally anchored.
Thermopiles are the most suitable choice for pyranometers and pyrgeometers for several reasons:
- Their ability to exhibit a flat spectral response from UV to infrared, meeting the broadband requirements specified in ISO 9060:2018.4
- Their passive operation requires no bias voltage, successfully eliminating electronic noise sources and drift.
- Their capacity to enable truly thermodynamic measurements that are not dependent on photon-energy interactions (as in photodiodes). This enables accurate sensing over 100 nm to >100 µm, effectively matching the spectral domains of both terrestrial and solar radiation.
ST150 Thermopile: A High-Performance Broadband Detector
The Dexter Research ST150 has been engineered to capitalize on the inherent strengths of thermopile technology, while providing improved signal quality, stability, and optional temperature monitoring.5
Broad Spectral Response (100 nm to >100 µm)
The ST150 supports the broadband requirements of pyranometers and pyrgeometers by offering a measured flat spectral response from UV to MIR. This versatile detector is suitable for both terrestrial longwave and solar shortwave flux applications.
High Output and Low NEP
The ST150’s typical output voltage is 295 µV under the standard 500 K blackbody test condition and irradiance geometry. This is combined with a typical responsivity of 39.8 V/W and a low noise-equivalent power (NEP) of 0.91 nW/√Hz.
These values demonstrate the detector’s ability to ensure high sensitivity with a signal-to-noise ratio of 8159 √Hz.
Excellent Temperature Stability
Temperature sensitivity remains one of the primary design challenges of pyranometer and pyrgeometer detectors.
The temperature coefficient of responsivity (TCR) of the ST150 is just -0.04 %/°C, simplifying correction algorithms and minimizing temperature-induced drift in radiometric instruments.
Rapid Response Time
A fast time constant of 42 milliseconds supports rapid signal settling. This is an especially valuable feature in solar resource assessment, dynamic atmospheric monitoring, and tracking irradiance fluctuations in passing-cloud studies.
Large Active Area (1.5 mm × 1.5 mm)
The generous absorber size offered by the ST150 enhances coupling efficiency with diffusers, optical domes, and chopper systems employed in solar measurement devices. This helps ensure robust performance, even in low irradiance conditions.
Robust, Low-Cost TO-5 Package
The ST150’s strong TO-5 housing ensures mechanical stability while allowing the instrument to be efficiently integrated into radiometric modules.
Integrated Thermistor Options: PTC or NTC for Thermal Compensation
Knowing the thermopile’s cold-junction temperature is key to ensuring accurate solar radiation measurements and enabling precise compensation of small, temperature-dependent shifts in responsivity.
Dexter Research Center accommodates this need with two temperature-sensing options:
1. Standard Internal NTC Thermistor
The base ST150 features an internal 30 kΩ (±5 %) NTC thermistor suitable for ambient package temperature measurement. This allows direct thermal correction within the device itself, which is especially important when using pyranometers across wide environmental ranges.5
2. ST150R Poly-Silicon PTC Thermistor
The ST150R version features a 75 kΩ (±20 %) poly-silicon PTC thermistor that offers a stable temperature coefficient of 0.11 %/°C. This low-cost temperature monitoring solution is ideally suited for use in applications requiring precision thermal compensation without the strict need for ultra-tight tolerances.5,6
Supporting Product Variants for Multichannel or Filtered Solar Applications
Dexter Research Center offers the ST150 in both Dual and Quad configurations, suitable for projects requiring differential channels. It is possible to pair these versions with optional optical filters such as HC, CH4, CO2, and CO, affording users the necessary flexibility to fine-tune performance for:7
- Atmospheric composition monitoring
- Multi-band radiometry
- Selective spectral measurements
The experience behind the ST150 is what truly sets this apart, however. Dexter Research Center began designing and manufacturing precision infrared sensors in 1977 and has since become widely recognized for delivering reliable, consistent performance across a diverse array of industrial, scientific, and environmental applications.
This depth of experience ensures that OEM designers developing robust solar measurement platforms can be confident in terms of critical factors such as stable calibration behavior, device-to-device uniformity, and long-term performance.
Summary: The ST150 Thermopile Detector
The ST150 thermopile detector from Dexter Research Center successfully integrates reliable performance into a cost-effective and compact TO-5 package.
It delivers the essential performance features for accurate solar measurements, including broadband linearity, high signal quality, stable temperature response, and built-in thermal compensation.
The ST150’s performance aligns well with the requirements of pyranometers, pyrgeometers, and a growing range of tools and instruments employed in renewable energy analysis and atmospheric monitoring.
Solar measurement instruments that require the use of a detector able to strike the right balance between manufacturing practicality, scientific precision, and long-term reliability can only benefit from the robust combination of the ST150 and Dexter Research Center’s expertise and experience in thermopile technology.
References and Further Reading
- Tapakis, R., and Charalambides, A. G. (2013). Equipment and methodologies for cloud detection and classification: A review. Solar Energy, 95, 392–430. DOI: 10.1016/j.solener.2012.11.015. https://linkinghub.elsevier.com/retrieve/pii/S0038092X12004069.
- Maghrabi, A.H., et al. (2019). The influence of atmospheric water content, temperature, and aerosol optical depth on downward longwave radiation in arid conditions. Theoretical and Applied Climatology, 138(3-4), pp.1375–1394. DOI: 10.1007/s00704-019-02903-y. https://link.springer.com/article/10.1007/s00704-019-02903-y.
- Graydon, O. (2016). Seebeck analysis. Nature Photonics, 10(4), 207–207. DOI: 10.1038/nphoton.2016.59. https://www.nature.com/articles/nphoton.2016.59.
- ISO 9060:2018. (2018). Solar energy - Specification and classification of instruments for measuring hemispherical solar and direct solar radiation. Available at: https://www.iso.org/standard/67464.html.
- Dexter Research Center. (2019). ST150 Silicon Based Thermopile Detector: 1 Channel - Dexter Research Center. (online) Available at: https://dexterresearch.com/product-finder/st150-silicon-based-thermopile-detector-1-channel/#.
- Dexter Research Center. Datasheet: Thermistor Options. Available at: https://dexterresearch.com/product-finder/st150-silicon-based-thermopile-detector-1-channel/#
- Dexter Research Center. Datasheet: Gas Filter Detector Availability. Available at: https://dexterresearch.com/product-finder/st150-silicon-based-thermopile-detector-1-channel/#
Acknowledgments
Produced from materials originally authored by the Dexter Research Center.
This information has been sourced, reviewed, and adapted from materials provided by Dexter Research Center, Inc.
For more information on this source, please visit Dexter Research Center, Inc.