Why can infrared measure temperature?
Infrared radiation exists as electromagnetic waves between visible light and radio waves, with wavelengths roughly from 0.76 μm to 1000 μm. In simple terms, any object above absolute zero (-273.15 °C) continuously emits infrared radiation.
This radiation is widespread because atoms and molecules in matter undergo random motion. Changes in their motion state cause bodies to radiate energy outward, a phenomenon known as thermal radiation. Infrared radiation is the thermal radiation component that commonly appears in everyday objects.
The link between infrared emission and temperature comes from the intensity and spectral composition of the thermal radiation. In many cases, the emitted power and spectral distribution depend on the object's temperature and its material properties. When material differences are small, temperature becomes the dominant factor, and under suitable conditions the emitted radiation can map uniquely to temperature. If a sensor can sensitively capture these infrared signals, we can infer an object temperature by monitoring the emitted power.
Types of medical infrared thermometry and key differences
From the contact perspective, thermometers are generally classified as contact or non-contact. Scientifically, contact thermometry usually refers to mercury or electronic thermometers that measure at locations such as the axilla or oral cavity with resolutions down to 0.1 °C. Non-contact thermometry refers to infrared devices, such as tympanic thermometers, forehead thermometers, and handheld infrared thermometers. Infrared devices offer wide measurement ranges, fast response, and high sensitivity, but they are sensitive to atmospheric conditions, ambient temperature, and surface emissivity. Note that a tympanic infrared thermometer is often called a contact device in daily use because it touches the ear canal during measurement; scientifically it is detecting infrared emission from the tympanic membrane.
Based on infrared measurement principles, infrared thermometers are commonly divided into:
- Full-radiation thermometers, which would measure radiation across the entire spectrum. In practice, no detectors or optical windows cover the entire spectrum, so these are not realized in common devices.
- Monochromatic or brightness thermometers, which measure radiation within a single wavelength band.
- Ratio or dual-band thermometers, which infer temperature from the ratio of radiative energies in two bands.
Practical designs therefore choose a limited wavelength range or two bands as substitutes for full-spectrum measurement.
From a system viewpoint, non-contact infrared devices are also grouped as portable, online, and scanning systems. Portable devices are handheld point-measurement tools such as tympanic and forehead thermometers. When a portable device supports real-time data export via wireless communication, it can function as an online device with additional data-processing capabilities. Scanning thermometers acquire dynamic temperature and imaging data via mechanical scanning methods. In recent years, focal-plane array thermal cameras based on staring arrays have become widely used; compared with scanning optical-mechanical imagers, staring imagers provide wide field of view, automatic focusing, continuous zoom, sharper images, strong analysis features, and simpler operation.
Terminology and basic principles
Planck's law: describes the spectral distribution of blackbody radiation at different temperatures.
Source: ResearchGate
Stefan-Boltzmann law: integrating Planck's law over all wavelengths yields the total radiative power per unit area into a hemisphere. The law states that the radiative exitance of a blackbody is proportional to the fourth power of its absolute temperature. This applies to ideal blackbodies and is a reference for real objects.
Source: Encyclopedic reference
Wien displacement law: Planck's law implies a single peak wavelength for monochromatic radiance at a given temperature. The peak wavelength shifts inversely with temperature, and peak radiance increases sharply with temperature.
Source: Encyclopedic reference
Blackbody: a reference emitter with emissivity approximately 1.0 at an aperture, used as a calibration source.
Body temperature: the temperature measured at a specific human body location. Different measurement sites and environmental conditions produce different temperature readings.
Source: Public discussion platform
Resolution: includes thermal resolution and spatial resolution. Thermal resolution, also called noise-equivalent temperature difference, is the smallest measurable temperature difference and is a key cost driver for thermal imagers. Spatial resolution is the imaging resolution, determined by field of view, instantaneous field of view, and measurement field of view.
Measurement accuracy: the deviation between the device reading and a standard blackbody reference, expressed as absolute value and percentage.
Detector: the front-end component that collects temperature-related signals; it is the critical element of thermometry devices.
Optics: components that focus received thermal radiation onto the detector. High-quality optics are expensive.
Emissivity: an object's ability to emit thermal energy; most surfaces have emissivity below 1.0.
Transmissivity: most solids are opaque to thermal infrared; transmissivity for human body measurement is effectively zero.
Reflectivity: the fraction of thermal energy reflected by a surface. When emissivity is low, reflections can bias temperature measurements, so reflected temperature compensation is often required.
Ear and forehead thermometers
Tympanic thermometers detect infrared radiation emitted from the eardrum. Forehead thermometers measure surface temperature of the forehead and infer core body temperature using empirical relationships. Forehead devices are more distinctly non-contact and have been widely used for rapid screening and personal use.
Hardware-wise, both tympanic and forehead thermometers include a probe, control unit, signal processing and compensation circuitry, a display, and a power supply. Two common detector configurations are:
- waveguide coupled thermopile sensor plus a temperature-sensitive resistor for ambient compensation;
- waveguide coupled pyroelectric sensor plus a temperature-sensitive resistor for ambient compensation.
For accuracy, the national standard GB/T 21417.1-2008 specifies that tympanic thermometers should have resolution of 0.1 °C or better and a maximum permissible error of ±0.2 °C within the display range 35.0 °C to 42.0 °C. Forehead thermometer standards are less consistent; many devices claim ±0.3 °C accuracy.
Typical handheld thermopile-based infrared thermometer block diagrams show the thermopile sensor feeding analog front-end amplification, compensation, and microcontroller processing.
Common thermopile sensor models used in handheld thermometers include MLX90614 and MLX90615SSG. These integrate infrared sensing and signal conditioning, often with dedicated measurement ICs for improved performance.
Pyroelectric sensor solutions are also common for handheld tympanic and forehead thermometers. They offer fast response, wide spectral response, broad operating frequency, and sensitivity largely independent of wavelength. Typical pyroelectric sensor designs use devices like LN074B, LHI878, or similar parts from established suppliers.
Thermal imagers
Thermal imagers are available as optical-mechanical scanning imagers and as non-scanning focal-plane array (FPA) imagers. Scanning instruments build images by scanning a field; non-scanning FPA cameras capture a full image at once and are analogous to CCD or CMOS imagers, making them suitable for high-throughput screening.
A block diagram of a dynamic scanning radiometric thermometer shows mechanical scanning optics feeding a detector and signal chain for image reconstruction.
FPA detectors are categorized into cooled and uncooled types. For high-volume human temperature screening in ambient environments, uncooled FPAs are preferred due to room-temperature operation, lower cost, and lower failure rates.
An FPA detector typically comprises an N×M array of photosensitive pixels and a matching readout integrated circuit. These parts are bonded using flip-chip techniques with indium bumps or similar interconnects.
The pixel array collects thermal radiation and converts it to photo-generated current. The readout circuit integrates that current across a capacitor to produce a voltage per pixel, which is then amplified, filtered, and digitized by an ADC. Subsequent processing reconstructs the thermal image.
Several vendors have released FPA-based thermal imaging solutions for fever screening, offering rapid large-scale filtering of individuals with elevated skin temperature.
Three major challenges and mitigation strategies
1. Amplifying weak signals and interference rejection
Thermal signals collected by sensors are typically very small and further attenuated during conversion. The measured signal is always contaminated by noise and background interference. The quality of the analog amplification and interference suppression circuits directly affects signal-to-noise ratio and device cost.
Advances in analog front-end design and integration have improved performance and supply availability. Algorithmic signal processing and on-chip compensation can also improve noise performance while reducing size and power consumption.
2. Nonlinear mapping from signal to actual temperature
Measured signals often relate to physical temperature in nonlinear ways. Calibration procedures and algorithmic compensation are common methods to correct nonlinearities and improve accuracy.
3. Influence of sensor package temperature on measurements
Sensor temperature changes cause measurement drift determined by the sensor materials and construction. These effects are intrinsic and cannot be fully eliminated. Typical mitigation involves modeling the measurement environment and applying algorithmic environmental compensation based on sensor temperature monitoring.
Recommendations and closing remarks
During infectious outbreaks, for household use either tympanic or forehead thermometers are acceptable. For infants, forehead thermometers are often preferred. For larger venues or organizations with resources, thermal imaging systems provide contactless, high-speed screening.
Some criticism suggests FPA imagers are overkill. In practice, useful imaging depends on appropriate algorithms and operational deployment rather than the mere presence of an image source.
Regarding safety concerns, passive infrared thermography does not harm the human body. It merely detects emitted infrared radiation and is safe for screening applications.
