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.
PCB Design Considerations in Medical Infrared Thermometry
Medical infrared thermometers rely on weak analog signals captured from infrared sensors and thermopile elements, so PCB design has a direct impact on measurement stability and clinical accuracy. The layout must ensure that low level sensor outputs are not affected by switching noise from digital processing or power regulation stages.
A common approach is to physically separate the analog front end from digital circuits and route sensitive signal traces away from high frequency clock lines. Ground plane continuity is important, and a single point grounding strategy is often used to reduce noise coupling into the measurement chain. These groud planing practices help maintain signal integrity under fast sampling conditions and varying environmental temperatures.
Thermal behavior is another important factor because infrared measurement depends on stable reference conditions. Poor heat distribution across the PCB can introduce drift in sensor readings, especially in compact handheld devices. Designers often use copper pours and thermal vias to spread localized heat from processors and power management components more evenly across the board.
From a manufacturing perspective, infrared thermometer PCBs are usually built with multilayer structures to improve routing efficiency and reduce electromagnetic interference between sensor and control domains. Stackup planning must balance compact form factor requirements with stable impedance control for signal lines.
Power integrity also plays a key role because these devices operate on small batteries and must maintain consistent voltage levels throughout the measurement cycle. Decoupling placement close to sensor ICs and careful routing of power rails help prevent transient drops that could affect infrared sampling accuracy. When combined, these PCB level considerations directly determine the reliability of medical infrared thermometry in real clinical use cases.
Single-Chip Solution for Infrared Forehead Thermometers
A forehead thermometer, also called an infrared thermometer, measures body temperature by detecting the infrared radiation emitted by the human body. Any object above absolute zero (-273°C) emits infrared radiation. The thermometer's sensor receives that infrared radiation and converts it into an electrical signal. An infrared temperature signal processing chip converts the electrical signal into a digital signal to obtain the sensed temperature value.
Three key factors that determine accuracy
- Sensor quality. Thermopile infrared sensors are MEMS-based devices; their design and manufacturing conditions affect accuracy and measurement consistency.
- ADC precision and stability. The accuracy of the infrared temperature signal processing chip depends on the ADC performance. The SD8709-based single-chip solution integrates a high-precision ADC and MCU to perform signal measurement, A/D conversion, data processing, built-in LCD/LED driving, and serial communication functions on a single chip. See Figure 3.
- Compensation algorithms. The quality of sensor-signal and ambient-temperature compensation algorithms directly affects measurement accuracy and consistency. Portable forehead and ear thermometer solutions require mature algorithms and calibration procedures to meet medical certification requirements. Reported production devices meet China CFDA or foreign FDA and CE medical certifications.
Quick Design Guide for Infrared Thermometers
The diagram below shows a reference system that combines an MSP430 microcontroller with TI power-management, amplifier, and temperature-sensor components for an infrared thermometer.
Peripherals Relevant to Thermometer Design
- SAR ADCs or high-resolution sigma-delta ADCs on MSP430 devices, together with TI amplifiers such as the TLV333, can sample analog output from infrared temperature sensors and convert it into digital temperature values. The MCU can also monitor battery voltage in real time.
- On-chip LCD drivers enable rapid implementation of LCD displays. For example, the MSP430FR4133 includes up to a 4×36 or 8×32 segment LCD driver with flexible segment and COM pin configuration, which can simplify PCB layout.
- I2C interfaces support high-precision digital temperature sensors, digital infrared sensors, and digital proximity sensors for auxiliary sensing inputs.
- Integrated timer modules can generate multiple PWM signals to drive indicator LEDs and buzzers.
- GPIO interrupts in low-power modes provide quick button response for battery-powered devices in standby.
Low-Power and Memory Options
MSP430 devices are designed for ultra-low power and include low-power intelligent peripherals. Since infrared thermometers are often used frequently and are battery powered, low-power operation is a key design consideration. The MSP430 family offers various memory sizes from 16 KB up to 512 KB, enabling migration between devices with minimal software changes. Recommended MCU part numbers are shown in the image below.
Infrared Thermometer Accuracy Design Factors
When evaluating or designing infrared temperature devices, focus on the sensor's signal-to-noise ratio, temperature coefficient, spectral response and uniformity; the optical system's field of view, spot size, and stray radiation suppression; and robust calibration against traceable temperature references. Errors can arise from emissivity differences, ambient reflections, device-to-skin distance, sensor drift, and inadequate calibration procedures.
Key Factors Affecting Accuracy
From the upstream supply chain perspective, the three most critical factors affecting infrared thermometer accuracy are:
- Infrared sensor performance
- Optical system design
- Calibration and verification procedures
Power Design Options for Battery-Powered Thermometers
Most handheld infrared thermometers are wireless and rely on batteries as the system power source. Common battery configurations include:
- One or two dry cell alkaline batteries in series, producing 1.5 V or 3.0 V system voltage.
- A 9 V snap battery.
- Rechargeable lithium battery packs.
NTC Thermistors and RFID for Medical Applications
NTC (Negative Temperature Coefficient) thermistors are resistive devices whose resistance decreases as temperature rises. They are made from sintered non-oxide ceramics composed of manganese, nickel, cobalt, and other elements, with electrodes formed on the ceramic body. Common package types include leaded styles and SMD chip types. Resistance changes with temperature and can range roughly from 1% to 5% per °C, making NTC thermistors a common choice for electronic temperature sensing.
Murata's NTC thermistors are offered with high accuracy and fast thermal response. The product range includes various package shapes (SMD chips and leaded types) and targets applications such as temperature monitoring, overheat detection, circuit protection, current control, and heating. The NXF series is a flexible leaded type suited for temperature sensing. NXF parts are compact and fast-responding, with flexible lead lengths available from 25 to 150 mm. Murata's ceramic processing allows these thermistors to meet a range of application requirements.
