Source：Shenzhen Kai Mo Rui Electronic Technology Co. LTD2026-07-27
Principles of Infrared Temperature Measurement
Fundamental Working Principle
Any object whose temperature is higher than absolute zero (-273.15°C) constantly emits thermal radiation outward and simultaneously absorbs thermal radiation from surrounding objects. Objects with different temperatures radiate varying levels of energy at distinct wavelengths, among which infrared radiation is always included.For objects below 1000°C, infrared waves constitute the dominant component of their thermal radiation. Therefore, precise surface temperature can be acquired by measuring the infrared radiation emitted by the target object itself.
Kirchhoff's Law of Thermal Radiation states that for objects under thermal equilibrium, the ratio between radiated energy and absorptivity is independent of the object’s inherent physical properties, and is only determined by wavelength and thermodynamic temperature.
If the ratio of radiated energy to absorbed energy of a standard object is defined as a benchmark, we can calculate the surface temperature of nearby targets inversely by detecting their thermal radiation output (multiple calculation algorithms are available for this deduction process). This reference standard object is known as an absolute blackbody, shortened to blackbody.
Definition of a blackbody: It can fully absorb all radiant energy of any wavelength incident on its surface at any temperature. Accordingly, the ratio of its radiated heat to absorbed heat equals exactly 1.
Leveraging this characteristic, blackbodies serve as calibration references for temperature measurement in practical applications. Blackbody calibration can be implemented during product manufacturing, or performed online on-site in actual working environments.
Nevertheless, theoretical ideal conditions cannot be fully replicated in real scenarios, and multiple factors will impair temperature detection accurac
Factors Affecting the Accuracy of Human Body Temperature Measurement
Overall measurement precision is jointly influenced by three core aspects: the human body itself, thermal radiation transmission medium, and infrared detector hardware.
Ambient temperature directly alters superficial body temperature;Physical activity status impacts readings: body temperature rises noticeably right after strenuous exercise such as running;
Natural diurnal temperature fluctuation: human body temperature hits the lowest point in the early morning, peaks in the afternoon, then gradually declines. Females generally have a slightly higher average body temperature than males;Uneven surface temperature across bodily regions: the forehead is the optimal measurement site, featuring unobstructed skin, dense capillary distribution and uniform temperature spread.
The emissivity of an ideal blackbody is 1, while the human body has an emissivity of approximately 0.98, extremely close to that of a blackbody (a perfect blackbody with emissivity = 1 cannot be manufactured physically). For simplified measurement workflows, human skin is commonly treated as a blackbody equivalent, which is a feasible practical workaround.
The thermal energy emitted by an object is governed solely by its temperature and wavelength, with infrared bands carrying the vast majority of radiant energy. Hence only infrared spectral bands need to be analyzed for temperature detection, rather than the full electromagnetic spectrum.
The transmissivity of infrared radiation through the atmosphere varies drastically with wavelength. Several transmission bands named infrared atmospheric windows allow infrared waves to pass through air with minimal attenuation. The primary atmospheric windows are: 2μm–2.6μm, 3μm–6μm, and 8μm–14μm. Thermal imaging temperature measurement technology relies entirely on these atmospheric windows.Many infrared detectors adopt germanium glass lenses for this reason: germanium glass delivers excellent light transmittance across the 2–16μm spectrum alongside stable chemical properties.
Even within atmospheric windows, transmittance never reaches 100%, so thermal radiation gradually attenuates with transmission distance. For short-range measurement, this attenuation factor can be ignored for simplification; over long distances, infrared electromagnetic waves are absorbed by atmospheric carbon dioxide and other gases, resulting in energy loss that requires dedicated temperature compensation algorithms to correct readings.
Two mainstream types of infrared detectors convert incoming thermal radiation into electrical signals:A. Thermopile (thermocouple)-based detectorsB. Bolometer micro-radiation thermal detectors
Working Principle of Bolometer Detectors
Infrared irradiation heats the built-in thermistor, causing its resistance value to change. The magnitude of infrared radiant energy is quantified by measuring the corresponding voltage fluctuation induced by resistance variation.
Most thermal imaging cameras adopt this bolometer architecture
- Uncooled thermal imagers: widely deployed in industrial and medical scenarios
- Cooled thermal imagers: predominantly used for military applications
Based on thermistor materials, bolometer detectors are further categorized into two mainstream solutions: Vanadium Oxide (VOₓ) and Amorphous Silicon (α-Si).
Key Challenges & Optimization Solutions
The electrical signal output from thermal imaging detectors has a non-linear correlation with actual object temperature, which introduces complexity and makes highly precise human body temperature readings difficult to achieve. The following optimized schemes are commonly adopted to improve accuracy:
AI Algorithm-Assisted CalibrationTreat human skin as a blackbody equivalent, utilize artificial intelligence algorithms to lock optimal temperature measurement areas, and formulate real-time online calibration strategies. This solution is applied in most AI thermal imaging temperature screening cameras.
On-Site Blackbody Real-Time CalibrationIntegrate a physical blackbody reference into the measurement scene for continuous live calibration. Though this method is more cumbersome and entails higher costs, it delivers superior detection accuracy up to ±0.3°C.
This rigorous calibration solution has been deployed at several entrances of Guangzhou Metro stations for passenger temperature screening.
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