Working Principle and Application of Infrared Sensors

Updated: October 2025 - This comprehensive guide covers the latest developments in infrared sensor technology, including recent advances in quantum dot infrared photodetectors (QDIPs), microbolometer arrays, and AI-enhanced thermal imaging systems.

Table of Contents

I. Introduction

Any object in the universe produces infrared radiation as long as its temperature exceeds absolute zero (-273.15°C or 0 Kelvin). Like visible light, infrared radiation can be refracted, reflected, and absorbed, which forms the foundation of infrared technology and its numerous applications.

In today's rapidly advancing technological landscape, automatic control and detection systems play increasingly crucial roles in industrial automation, smart cities, and everyday life. Sensors serve as the critical components in these systems, converting physical phenomena into electrical signals that can be processed by computers and control circuits. The fast response times of modern sensors enable real-time control and monitoring across various applications.

Infrared sensors represent one of the most versatile and widely used sensor categories. They detect infrared radiation emitted by objects, making them extremely practical since any object above absolute zero emits infrared energy. This fundamental principle enables the development of sophisticated sensor modules including infrared thermometers, thermal imagers, motion detectors, automatic door systems, and advanced security systems.

Infrared sensors leverage the physical properties of infrared radiation for measurement purposes. Infrared light exhibits properties of reflection, refraction, scattering, interference, and absorption. As invisible electromagnetic radiation with wavelengths longer than visible red light (typically 0.7 to 1000 micrometers), infrared radiation is categorized into near-infrared (0.7-1.4 μm), short-wave infrared (1.4-3 μm), mid-wave infrared (3-8 μm), long-wave infrared (8-15 μm), and far-infrared (15-1000 μm).

Infrared Radiation Spectrum showing different wavelength ranges

2025 Update: Recent advances in quantum cascade lasers and superconducting nanowire single-photon detectors have extended infrared detection capabilities into the terahertz range, opening new applications in medical imaging and security screening.

II. Performance Parameters of Infrared Sensors

1. Voltage Response

When modulated infrared radiation illuminates the sensitive surface of a sensor, the ratio of output voltage to input infrared radiant power is called the voltage response rate, denoted as R_V.

Where:

• U_s: Output voltage of the infrared sensor

• P₀: Power per unit area projected onto the infrared-sensitive element

• A: Area of the sensitive element of the infrared sensor

2. Response Wavelength Range

Curve 1: Voltage response rate curve of thermal sensors (wavelength-independent)

Curve 2: Voltage response rate curve of photonic sensors (wavelength-dependent)

(1) The response wavelength range (spectral response) represents the relationship between the sensor's voltage response rate and incident infrared radiation wavelength.

(2) The wavelength corresponding to maximum response rate is called the peak wavelength.

(3) The cutoff wavelength occurs when the response rate falls to 50% (0.707) of the peak value, defining the sensor's usable wavelength range.

3. Noise Equivalent Power (NEP)

NEP represents the incident radiant power that produces an output signal equal to the sensor's inherent noise level. It's a critical parameter for determining sensor sensitivity.

Where:

• U_s: Output voltage of the infrared detector

• P₀: Power projected onto unit area of the infrared-sensitive element

• A₀: Area of the infrared-sensitive element

• U_N: Comprehensive noise voltage of the infrared sensor

• R_V: Voltage response rate of the infrared sensor

4. Detectivity

Detectivity is the reciprocal of NEP:

Higher detectivity indicates greater sensor sensitivity and ability to detect weaker infrared signals.

5. Specific Detectivity (D*)

Specific detectivity, or normalized detectivity (D*), represents the signal-to-noise ratio for unit incident power when the sensitive element has unit area and the amplifier bandwidth is 1 Hz. This parameter enables comparison between different sensor designs.

6. Time Constant

The time constant indicates how quickly the sensor output responds to changes in infrared radiation. It represents the time lag between radiation changes and corresponding output signal changes.

Where f_c is the modulation frequency at which the response rate drops to 0.707 (3dB) of maximum value.

Thermal sensors typically have larger time constants (milliseconds) due to thermal inertia, while photonic sensors respond much faster (microseconds to nanoseconds).

III. Basic Laws of Infrared Radiation

1. Kirchhoff's Law

At thermal equilibrium, the ratio of emissive power to absorptive power is constant for all objects at a given temperature and wavelength. This ratio equals the emissive power of a perfect blackbody at the same temperature. Importantly, emissivity equals absorptivity for any given wavelength and temperature.

This law establishes that objects with high absorption rates also have high emission rates, forming the theoretical foundation for infrared remote sensing and thermal measurement.

2. Stefan-Boltzmann Law

The total radiant flux from a blackbody is proportional to the fourth power of its absolute temperature. This relationship means small temperature changes produce significant variations in radiant energy, enabling precise temperature measurements using infrared sensors.

The Stefan-Boltzmann law is expressed as: j* = σT⁴, where σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸ W·m⁻²·K⁻⁴).

Blackbody radiation curves showing spectral radiance vs wavelength for different temperatures

3. Wien's Displacement Law

As temperature increases, the peak wavelength of maximum radiation shifts toward shorter wavelengths. This law is expressed as: λ_max = b/T, where b is Wien's displacement constant (2.898 × 10⁻³ m·K).

This principle explains why hot objects appear to glow red, then white, then blue-white as temperature increases, and is crucial for designing infrared sensors for specific temperature ranges.

IV. Classification and Working Principle of Infrared Sensors

Infrared sensors are broadly classified into two main categories: thermal sensors and photonic sensors, each operating on different physical principles.

1. Thermal Sensors

Thermal sensors absorb incident radiation across all wavelengths and utilize the thermal effects of radiation. When the detection element receives radiant energy, its temperature increases, changing temperature-dependent properties that can be measured to determine radiation intensity.

Example of a liquid level thermal sensor

(1) Thermistor Sensors

Thermistors are composed of sintered metal oxide mixtures (typically manganese, nickel, and cobalt oxides). When infrared radiation heats the thermistor, its resistance changes predictably. By measuring this resistance change, the intensity of incident infrared radiation and corresponding object temperature can be determined.

Recent Development: Modern thermistor sensors now incorporate nanostructured materials and MEMS technology, achieving response times under 1 millisecond and improved temperature coefficients.

(2) Thermocouple Sensors

Thermocouples consist of two dissimilar metals with different thermoelectric properties. When infrared radiation heats one junction while the other remains at a reference temperature, a thermoelectric voltage is generated proportional to the temperature difference. Due to thermal inertia, modulation frequencies should be limited below 10Hz.

(3) Golay Cells

Golay cells utilize gas expansion when heated by infrared radiation. The sensor contains an air chamber connected to a flexible membrane with a reflective surface. Incident infrared radiation heats the gas, increasing pressure and deflecting the membrane. This deflection is detected optically using a light beam and photocell system.

Schematic diagram of a Golay cell showing optical detection mechanism

Golay cells offer high sensitivity and stable performance but have long response times and complex structures, limiting their use primarily to laboratory applications.

(4) Pyroelectric Sensors

Pyroelectric sensors use ferroelectric crystals that exhibit spontaneous polarization dependent on temperature. When infrared radiation changes the crystal temperature, the polarization changes, generating electrical charges proportional to the rate of temperature change.

Working principle of a pyroelectric sensor showing charge generation

Importantly, pyroelectric sensors only respond to changing infrared radiation, requiring modulation techniques for detecting constant radiation sources. Common materials include lithium tantalate (LiTaO₃), triglycine sulfate (TGS), and lead zirconate titanate (PZT).

2. Photonic Sensors

Photonic sensors utilize the photoelectric effect in semiconductor materials. When photons with sufficient energy interact with the semiconductor, they alter its electrical properties, which can be measured to determine radiation intensity.

Photonic sensors offer high sensitivity, fast response (microseconds to nanoseconds), and high-frequency response. However, they typically require cooling for optimal performance and have narrower spectral response ranges.

(1) External Photoelectric Sensors

These sensors utilize the external photoelectric effect where photons with sufficient energy cause electrons to escape the material surface. Photodiodes and photomultiplier tubes are typical examples, offering extremely fast response times (nanoseconds) but requiring high photon energies, limiting them to near-infrared and visible light applications.

(2) Photoconductive Sensors

When infrared radiation illuminates semiconductor materials, it promotes electrons from bound to free states, increasing conductivity. This photoconductive effect forms the basis for sensors using materials like lead sulfide (PbS), lead selenide (PbSe), indium antimonide (InSb), and mercury cadmium telluride (HgCdTe).

Advanced Materials (2025): Quantum dot infrared photodetectors (QDIPs) and graphene-based sensors now offer enhanced sensitivity and broader spectral response while operating at higher temperatures.

(3) Photovoltaic Sensors

These sensors generate voltage when infrared photons create electron-hole pairs in a PN junction. The built-in electric field separates these carriers, creating a photovoltaic effect. Common materials include indium arsenide (InAs), indium antimonide (InSb), and mercury cadmium telluride (HgCdTe).

Comparison of infrared photoconductive and photovoltaic sensor structures

(4) Photomagnetoelectric Sensors

These sensors combine photoelectric and magnetic effects. When infrared radiation creates charge carriers in a semiconductor under a magnetic field, the carriers deflect to opposite sides, generating a measurable voltage. This photomagnetoelectric effect enables sensors with good stability and no cooling requirements, though with lower sensitivity.

V. Applications of Infrared Sensors

Infrared sensors offer several key advantages:

• Superior environmental adaptability - Excellent performance in darkness and adverse weather conditions

• Passive operation - Enhanced security and stealth capabilities compared to active radar systems

• Camouflage detection - Ability to identify concealed targets through thermal signatures

• Compact design - Smaller, lighter, and more energy-efficient than radar systems

• Cost-effectiveness - Lower manufacturing and operational costs

1. Radiation and Spectrum Measurement

Infrared sensors enable precise measurement across various applications:

• Climate monitoring - Pyrgeometers measure atmospheric radiation for climate change research

• Astronomical observation - Space-based infrared telescopes study cosmic phenomena

• Meteorological satellites - Infrared scanning radiometers analyze cloud formations and weather patterns

• Industrial applications - Non-contact temperature measurement and gas analysis

A pyrgeometer used for measuring atmospheric longwave radiation

2025 Applications: Advanced infrared spectrometers now enable real-time monitoring of greenhouse gases, air quality assessment, and precision agriculture through drone-mounted sensors.

2. Search and Track Systems

Military and civilian applications utilize infrared sensors for target detection and tracking:

• Missile guidance systems - Heat-seeking missiles track aircraft engine signatures

• Surveillance systems - Border security and perimeter monitoring

• Search and rescue - Locating people in disaster scenarios

• Wildlife monitoring - Tracking animal movements and population studies

Modern infrared search and track system with advanced signal processing

Modern systems incorporate AI-enhanced target recognition, multi-spectral fusion, and improved spatial resolution through larger detector arrays and advanced optics.

3. Thermal Imaging Systems

Thermal imagers convert invisible infrared radiation into visible images representing temperature distributions. These systems have expanded from military origins into numerous civilian applications:

Industrial Applications:

• Predictive maintenance - Early detection of equipment failures through thermal anomalies

• Electrical systems - Identifying overheating components and connections

• Building inspection - Detecting insulation defects, moisture, and structural issues

• Process monitoring - Temperature control in manufacturing

Medical Applications:

• Fever screening - Non-contact temperature measurement for health monitoring

• Diagnostic imaging - Early detection of inflammation and circulation issues

• Surgical guidance - Real-time tissue temperature monitoring

Security and Safety:

• Fire detection - Early fire identification and hotspot monitoring

• Perimeter security - Intrusion detection in all weather conditions

• Search and rescue - Locating people in smoke or darkness

Basic components of a modern thermal imaging system

Latest Developments (2025): AI-powered thermal imaging now enables automated anomaly detection, predictive analytics, and integration with IoT systems for smart building management and Industry 4.0 applications.

4. Infrared Communication Systems

Infrared communication systems transmit data using modulated infrared radiation, detected by silicon photodiodes. These systems offer several advantages:

• No electromagnetic interference - Safe operation near sensitive equipment

• High security - Line-of-sight transmission prevents eavesdropping

• Low power consumption - Energy-efficient operation

• Cost-effective - Simple implementation and low component costs

Infrared communication network topology for indoor applications

Modern Applications:

• Consumer electronics - Remote controls, wireless data transfer

• Industrial automation - Machine-to-machine communication

• Medical devices - Non-invasive data transmission

• Automotive systems - Vehicle-to-infrastructure communication

Emerging Applications (2025):

• Smart homes and IoT - Integration with voice assistants and automated systems

• Autonomous vehicles - LiDAR systems and night vision capabilities

• Healthcare monitoring - Continuous vital sign monitoring and telemedicine

• Environmental monitoring - Air quality sensors and climate research

• Food safety - Temperature monitoring in cold chain logistics

• Energy efficiency - Smart HVAC systems and building automation

Future Trends in Infrared Sensor Technology:

• Quantum sensors - Enhanced sensitivity using quantum effects

• Flexible sensors - Wearable and conformable infrared detectors

• Multi-spectral imaging - Combined visible and infrared sensing

• Edge AI integration - On-sensor processing and decision making

• Improved materials - Perovskites and 2D materials for better performance

Article Update Information

Last Updated: October 2025

Key Updates Made:

• Corrected "Boltzmann's Law" to "Stefan-Boltzmann Law" for accuracy

• Added latest developments in quantum dot infrared photodetectors (QDIPs)

• Updated applications to include AI-enhanced thermal imaging and IoT integration

• Included recent advances in flexible and wearable infrared sensors

• Added information about emerging applications in autonomous vehicles and smart cities

• Enhanced mobile responsiveness and improved visual formatting

• Updated technical specifications to reflect current industry standards

• Added future trends and emerging technologies section

Sources: IEEE Sensors Journal, Nature Photonics, Applied Physics Reviews, and leading sensor manufacturers' technical documentation (2023-2025)

Recommended Further Reading:

• What are Types and Application of Gas Sensors?

• Humidity Sensor: Classification, Package and Application

• All You Need to Know about Ultrasonic Sensors

• Introduction to Temperature Sensors