Accurate temperature measurement is fundamental to scientific pursuits in many industries, be it military, medical, or industrial. The most common forms of non-contact thermal measurement are thermal cameras and infrared thermometers, both of which rely on the emissivity of an object or person to determine temperature.
Thermal cameras and infrared thermometers are useful in military to medical diagnostics, measuring temperature accurately from a distance. Image used under Adobe Stock license
However, in situations that require extremely high-accuracy measurements, both solutions fall short due to the unknown emissivity of materials. In response to these challenges, a research team at the University of Houston has recently developed a novel thermal imaging technique using near-infrared (NIR) spectroscopy to achieve more accurate surface temperature measurements.
Researchers Devise NIR Non-Contact Thermometer
The University of Houston research team recently set out to address the limitations of conventional thermal cameras and infrared thermometers in accurately measuring the surface temperature of photothermal catalysts. These conventional methods can struggle to attain accurate measurements due to the varying emissivity of targets, which changes with temperature and wavelength.
To overcome this, the researchers developed a non-contact thermometer using a near-infrared (NIR) spectrometer. This device collects thermal radiation with an optical fiber and fits the collected spectrum to the ideal black-body radiation formula. This determines an accurate temperature without precise emissivity values.
Graphical abstract of the researcher’s setup. Image used courtesy of Cell
The experimental setup included an NIR spectrometer equipped with an InGaAs array detector, which captures the NIR spectrum from 800 nm to 1700 nm. The researchers used this setup to measure the surface temperature of a silver heating stage, with a resulting error margin of less than 2°C.
Applying this technique to photothermal catalysts, the researchers synthesized Ru-Cu/Al2O3 powder and calibrated the NIR thermometer for this specific catalyst. They found significant temperature gradients on the catalyst’s surface under laser heating, with differences exceeding 200°C at high laser powers. Conventional IR cameras underestimated the surface temperature by 30°C–40°C, largely due to inherently inaccurate emissivity settings.
To validate their findings, the researchers conducted COMSOL multiphysics heat transfer simulations, which confirmed the observed temperature gradients and the influence of laser power, thermal conductivity, and optical penetration depth on temperature distribution. Their study ultimately found that their NIR thermometer provides a more accurate and reliable method for measuring high surface temperatures.
A Solution to Emissivity Challenges
Emissivity is an inherent property of materials that describes how efficiently a surface emits thermal radiation compared to an ideal black body. As a good indicator of thermal radiative efficiency, emissivity is commonly exploited to determine temperature from thermal radiation measurements. However, accurately determining emissivity is complex because it is not a constant value; it changes with the material’s temperature and the specific wavelength of the emitted radiation.
Emissivity impacts temperature readings. Image used courtesy of Optotherm
Thermal cameras typically use single-band techniques, focusing on a narrow range of wavelengths (8–14 micrometers) to capture thermal images. They require an input value for emissivity, which can lead to erroneous temperature readings if the emissivity is not precisely known and adjusted for each measurement condition. The variation in emissivity due to surface conditions, oxidation, and other factors further complicates accurate temperature determination.
Multi-spectral techniques attempt to address this by measuring thermal radiation at multiple wavelengths to model emissivity as a function of wavelength and temperature. While this approach improves accuracy, it still depends heavily on the accuracy of the emissivity models used, which are often complex and require solving multiple coupled equations.
Using the Entire Spectrum for Higher Precision
The Houston team’s method leverages the entire spectrum rather than relying on a single wavelength or a narrow band, thereby mitigating the dependence on emissivity. By fitting the collected spectrum to the ideal black-body radiation formula and using a simple calibration step, the researchers effectively eliminated the need for precise emissivity values. According to the team, their findings will be particularly valuable in applications where accurate surface temperature measurements are needed, such as photothermal catalysis.
