Infrared thermometers are devices that measure infrared radiation—an invisible form of electromagnetic energy located just below the visible spectrum. Though we can’t see it, infrared radiation behaves much like visible light: it can be absorbed, reflected, and focused.
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These thermometers allow for non-contact temperature measurement, meaning there's no need to physically touch the object. This capability makes them particularly useful in scenarios where traditional sensors might be impractical. Today, infrared thermometers come in various forms, from portable handheld models to fixed-mount units designed for specific tasks.
Types of Infrared Thermometers
Infrared thermometers fall into a few main categories:
- Spot Infrared Thermometers: These devices measure the temperature at a spot on a surface.
- Infrared Scanning Systems: These systems scan a broader area by aiming the spot thermometer at a rotating mirror. They're commonly used in industrial conveyor systems, such as for monitoring materials like metal or glass exiting an oven.
- Infrared Thermal Imaging Camera: These more complex systems measure temperatures across many points to create a thermographic image or thermogram. They rely heavily on both hardware and software to generate detailed heat maps.
Based on their intended use, infrared thermometers are also categorized as clinical or industrial.
- Industrial models are designed to cover a wide temperature range across different surfaces.
- Clinical models are optimized for body temperature, offering higher accuracy within a narrower range. Both types are available as handheld or fixed-mount devices.1
How Infrared Thermometers Work
Infrared thermometers operate on the principle of black body radiation. According to this principle, any object with a temperature above absolute zero emits infrared radiation as a result of molecular motion. The hotter the object, the faster its molecules move—and the more radiation it emits. That’s why heated metal begins to glow red or white at high temperatures.
Here’s how the measurement process works:
- A lens focuses infrared radiation from the target onto a detector, often a thermopile.
- The thermopile absorbs this radiation and converts it into heat.
- This heat is then converted into an electrical signal.
- The detector analyzes the signal to calculate the object’s temperature.
An important factor in this process is emissivity, which describes how efficiently an object emits infrared energy. It’s measured on a scale from 0 to 1, where 1 represents a perfect emitter (a "black body"). Shiny or reflective materials typically have low emissivity, which can affect measurement accuracy.1
Some infrared thermometers are fixed at an emissivity setting of 0.95, which limits their use on low-emissivity surfaces like polished aluminum (around 0.05). More advanced models offer adjustable emissivity settings, making them suitable for a wider range of materials.1
Understanding Distance-to-Spot Ratio (D:S)
To get accurate readings with an infrared thermometer, it’s not just about pointing and clicking—you also need to understand how much area the device is measuring. That’s where the distance-to-spot ratio (D:S) comes in. It tells you how the size of the measured area (the “spot”) changes with distance from the target.
The D:S ratio refers to the distance between the thermometer and the object compared to the diameter of the area being measured. As the distance increases, the measured spot becomes larger—which can affect accuracy if the target is small or surrounded by materials of a different temperature.1
- A low D:S ratio is better for close-up readings of larger surfaces.
- A high D:S ratio allows for more precise measurement of small areas from farther away.
By understanding how infrared thermometers work—including principles like emissivity, D:S ratio, and sensor design—users can select the right model for their specific needs and ensure accurate readings. With this foundation in place, we can now look at how these tools are applied across different industries.
Applications of Infrared Thermometers
Infrared thermometers are widely used across industries thanks to their quick, non-invasive measurement capabilities.2
- HVAC and building inspection: Used to detect insulation breakdown, air leaks, furnace performance, and heat loss—key for energy conservation.
- Industrial and electrical applications: Monitor motor and boiler systems, steam pipes, and detect hotspots in electrical components. They also help spot internal corrosion or wear in petrochemical infrastructure.
- Food safety: Ensure proper storage and processing temperatures, contributing to quality and sanitation.
- Agriculture and forestry: Help monitor plant stress, soil conditions, crop moisture, and irrigation needs—vital for optimizing yield and resource use.
- Healthcare: Non-contact infrared thermometers (NCITs) are commonly used for quick and hygienic body temperature checks. They improve clinical efficiency, reduce patient discomfort, and are less affected by environmental factors compared to infrared thermography (IRT), though the difference in accuracy is minor.3
New Developments
Infrared thermometry continues to evolve, driven by the demand for cost-effective, accurate, and user-friendly solutions—especially in public health and embedded system applications. Recent innovations focus on making these tools more accessible for real-time monitoring, automation, and integration into low-power devices.
For example, researchers have developed iThermowall, a low-cost, open-source infrared thermometer designed for automated fever screening in public spaces. It operates without an operator and uses proximity sensing to trigger body temperature measurements. Validation tests showed its readings were statistically consistent with those of commercial-grade reference thermometers.4
In another advancement, a project presented at the Second International Conference on Physics, Photonics, and Optical Engineering (ICPPOE 2023) showcased an Arduino-based digital infrared thermometer. This system, built with a MLX90614 sensor, display, and buzzer, demonstrated reliable performance with an accuracy of ±0.3 °C—comparable to commercial models—across a diverse test group.5
These efforts reflect a broader trend toward open-source, customizable infrared devices suitable for public health monitoring, DIY applications, and education. As sensor technology, microcontrollers, and embedded systems become more affordable, infrared thermometry is likely to see continued innovation in both consumer and industrial spaces.
Want to Learn More?
Infrared thermometers offer a practical and efficient way to measure temperature across a range of environments and industries. Their ease of use, portability, and non-contact functionality make them indispensable in fields from healthcare to energy to agriculture. That said, understanding their limitations—especially around surface-only readings and emissivity—is crucial for proper use.
Interested in how other non-contact sensors are changing industry and healthcare? You might also want to explore:
References and Further Reading
- What are Infrared Thermometers? [Online] Available at https://assist.asta.edu.au/sites/assist.asta.edu.au/files/Science%20ASSIST%20Information%20sheet%20Infrared%20Thermometers.pdf
- Yongqing, W., Zongqing, G., Shuonan, W., Ping, H. (2017). The temperature measurement technology of infrared thermal imaging and its applications review. 2017 13th IEEE International Conference on Electronic Measurement & Instruments (ICEMI), 401-406. DOI: 10.1109/ICEMI.2017.8265833, https://ieeexplore.ieee.org/abstract/document/8265833
- Zhao, Y., Bergmann, J. H. (2023). Non-Contact Infrared Thermometers and Thermal Scanners for Human Body Temperature Monitoring: A Systematic Review. Sensors, 23(17), 7439. DOI: 10.3390/s23177439, DOI: 10.3390/s23177439, https://www.mdpi.com/1424-8220/23/17/7439
- Abuzairi, T., Imaniati Sumantri, N., Irfan, A., Maulana Mohamad, R. (2021). Infrared thermometer on the wall (iThermowall): An open source and 3-D print infrared thermometer for fever screening. HardwareX, 9, e00168. DOI: 10.1016/j.ohx.2020.e00168, https://www.sciencedirect.com/science/article/pii/S2468067220300778
- Lin, Y. (2024). Non-contact infrared thermometer based on arduino. Second International Conference on Physics, Photonics, and Optical Engineering (ICPPOE 2023), 13075, 635-643. DOI: 10.1117/12.3026720, https://www.spiedigitallibrary.org/conference-proceedings-of-spie/13075/1307527/Non-contact-infrared-thermometer-based-on-arduino/10.1117/12.3026720.short
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