Infrared sensor technology is now very mature and has been integrated into people's daily lives, playing a significant role. Before understanding infrared sensors, we should first understand what infrared radiation, or infrared light, is.
We know that light is also a type of electromagnetic radiation. In terms of human experience, it usually refers to the range of light waves that are visible to the naked eye, from 400nm (violet light) to 700nm (red light), which is the range that can be perceived by the human eye.
As shown in the figure, we call radiation outside of red light, with wavelengths between 760nm and 1mm, infrared light. Infrared light is invisible to the naked eye, but we can still perceive it through some special optical equipment.
Infrared radiation is a type of light invisible to the human eye, and therefore possesses all the characteristics of light. However, it also has a very significant thermal effect. All matter above absolute zero (-273°C) emits infrared radiation.
Therefore, simply put, an infrared sensor is a type of sensor that uses infrared light as a medium to process data.
Types of infrared sensors
Infrared radiation is a type of light invisible to the human eye, and therefore possesses all the characteristics of light. However, it also has a very significant thermal effect. All matter above absolute zero (-273°C) emits infrared radiation.
Depending on the method of emission, infrared sensors can be divided into two types: active and passive.
Working principle and characteristics of active infrared sensors
An active infrared sensor emits a modulated infrared beam from its transmitter, which is received by an infrared receiver, thus forming a warning line composed of infrared beams. The sensor should not trigger an alarm if obstructed by leaves, rain, small animals, snow, dust, or fog; however, it will trigger an alarm if obstructed by a person or an object of similar size.
Active infrared detector technology mainly uses a single transmitter and receiver, which is a linear protection method. It has now evolved from the initial single beam to multiple beams, and can even transmit and receive simultaneously, minimizing the false alarm rate and thus enhancing the stability and reliability of the product.
Because infrared light is a detection medium with good environmental incoherence (it has good incoherence with environmental sound, lightning, vibration, various artificial light sources and electromagnetic interference sources); and it is also a product with good target coherence (only targets that block the infrared beam will trigger an alarm), active infrared sensors will be further promoted and applied.
Working principle and characteristics of passive infrared sensors
Passive infrared sensors work by detecting infrared radiation emitted by the human body. The sensor collects external infrared radiation and focuses it onto the infrared sensor itself. Infrared sensors typically use pyroelectric elements, which release electrical charges when they receive infrared radiation and their temperature changes; these charges are then detected, processed, and used to generate an alarm.
This type of sensor is designed to detect radiation emitted by the human body. Therefore, the radiation-sensitive element must be extremely sensitive to infrared radiation with a wavelength of around 10 μm. To ensure sensitivity to infrared radiation from the human body, its irradiated surface is typically covered with a special filter, which significantly controls environmental interference.
A passive infrared sensor contains two pyroelectric elements connected in series or parallel. Furthermore, the two electrodes are manufactured with opposite polarization directions. The ambient background radiation has almost the same effect on both pyroelectric elements, causing their pyroelectric effects to cancel each other out, resulting in no signal output from the detector.
Once an intruder enters the detection area, the infrared radiation from the human body is focused by a partial mirror and received by a pyroelectric element. However, the heat received by the two pyroelectric elements is different, and the pyroelectricity is also different, so they cannot cancel each other out. After signal processing, an alarm is triggered.
Depending on the energy conversion method, infrared sensors can be divided into two types: photonic and pyroelectric.
Photonic infrared sensor
Photonic infrared sensors are sensors that operate based on the photonic effect of infrared radiation. The photonic effect refers to the interaction between the photons in the infrared radiation and the electrons in the semiconductor material when infrared radiation is incident on certain semiconductor materials, which changes the energy state of the electrons and thus causes various electrical phenomena.
By measuring changes in the electronic properties of semiconductor materials, the intensity of corresponding infrared radiation can be determined. The main types of photon detectors include internal photodetectors, external photodetectors, free-carrier detectors, and QWIP quantum well detectors.
The main characteristics of photon detectors are high sensitivity, fast response speed, and high response frequency, but the disadvantages are that the detection band is narrow and they generally operate at low temperature (in order to maintain high sensitivity, liquid nitrogen or thermoelectric cooling is often used to cool the photon detector to a lower operating temperature).
Pyroelectric infrared sensor
Pyroelectric infrared sensors use the thermal effect of infrared radiation to cause temperature changes in the element itself to detect certain parameters. However, their detection rate and response speed are not as good as those of photonic sensors.
However, because it can be used at room temperature and its sensitivity is independent of wavelength, it has a wide range of applications. Pyroelectric infrared sensors utilizing the pyroelectric effect of ferroelectric materials have high sensitivity and have been widely used.
The pyroelectric effect occurs when certain insulating materials are heated; as the temperature rises, equal but opposite charges are generated at both ends of the crystal. This polarization phenomenon caused by thermal change is called the pyroelectric effect. The pyroelectric effect has been used in pyroelectric infrared sensors in the last decade. Crystals capable of producing the pyroelectric effect are called pyroelectric materials, also known as thermoelectric elements. Commonly used materials for thermoelectric elements include single crystals, piezoelectric ceramics, and polymer thin films.
Structure of a Pyroelectric Infrared Sensor A pyroelectric infrared sensor consists of the following four main parts:
① The aluminum substrate and field-effect transistor (FET) that make up the circuit;
② Ceramic materials exhibiting pyroelectric effect;
③ Window material that restricts the incident infrared wavelength;
④ Outer shell TO-5 type tube cap and tube seat.
Because individual detector elements have drawbacks such as short detection range and difficulty in processing the acquired signal in subsequent circuits, infrared assemblies are currently preferred for detection. An infrared assembly consists of a pyroelectric infrared sensor, a lens, a measurement conversion circuit, and a sealed housing. The lens expands the detection range and improves measurement sensitivity; the measurement conversion circuit performs signal processing such as filtering and amplification; and the sealed housing prevents erroneous operation caused by external noise. This type of assembly is small, low-cost, and versatile, making it widely used.
Applications of infrared sensors
Based on current applications, infrared sensors have the following advantages:
1. It has better environmental adaptability than visible light, especially its ability to work at night and in inclement weather;
2. It has good concealment, generally passively receiving signals from targets, making it safer and more secure than radar and laser detection, and less susceptible to interference;
3. It detects camouflaged targets better than visible light because it uses infrared radiation characteristics formed by the temperature and emissivity differences between the target and the background.
4. Compared with radar systems, infrared systems are smaller, lighter, and consume less power;
Based on the performance characteristics of infrared sensors described above, we can develop a variety of different infrared detectors.
Utilizing its optical effect:
1. Photoconductive Detector: Also known as a photoresistor. When a semiconductor absorbs photons with sufficiently high energy, some charge carriers within it transition from a bound state to a free state, thereby increasing the semiconductor's conductivity. This phenomenon is called the photoconductive effect. Photoconductive detectors made using the photoconductive effect are divided into two types: polycrystalline thin-film and single-crystal.
2. Photovoltaic Detectors: Primarily utilize the photovoltaic effect of pn junctions. Infrared photons with energy greater than the bandgap excite electron-hole pairs in and around the junction region. The existing junction electric field causes holes to enter the p-region and electrons to enter the n-region, creating a potential difference between the two regions, resulting in a voltage or current signal in the external circuit. Compared to photoconductive detectors, photovoltaic detectors have a 40% higher background-limit detectivity, do not require an external bias electric field or load resistor, consume no power, and have high impedance.
3. Light emission - Schottky barrier detector: When the metal and semiconductor are in contact, a Schottky barrier is formed. Infrared photons pass through the Si layer and are absorbed by PtSi, which gives electrons the energy to jump to the Fermi level. Holes are left to cross the barrier and enter the Si substrate. Electrons in the PtSi layer are collected, and infrared detection is completed.
4. Quantum Well Detector (QWIP): Two semiconductor materials are grown alternately in thin layers using artificial methods to form a superlattice. There is a band break at the interface, which confines electrons and holes in a low potential well, thereby quantizing energy to form a quantum well.
Infrared detectors can be made using the principle of electron transitions in quantum wells. However, because only the polarization vector perpendicular to the superlattice growth surface plays a role in the incident radiation, the photon utilization rate is low; the concentration of ground-state electrons in the quantum well is limited by doping, resulting in low quantum efficiency; the response spectrum is narrow; and the low-temperature requirements are stringent.
Utilizing its thermal effect:
1. Liquid mercury thermometer and pneumatic Golay cell: These utilize the thermal expansion and contraction effect of materials.
2. Thermocouples and thermopile: These utilize the thermoelectric effect, which allows different materials to generate a thermoelectric electromotive force due to a temperature gradient.
3. Quartz resonator uncooled infrared imaging array: infrared detection is achieved by utilizing the principle that the resonant frequency is sensitive to temperature.
4. Thermal calorimeters: These utilize the thermistor effect of a material's resistance or dielectric constant—radiation causes a temperature rise that changes the material's resistance—to detect thermal radiation. Semiconductor resistors are the most widely used due to their high temperature coefficient; thermal calorimeters are often called "thermostats."
Furthermore, the emergence of high-temperature superconducting materials has drawn attention to superconducting detectors that utilize the sharp change in resistance near the transition temperature. If room-temperature superconductivity becomes a reality, it will be one of the most remarkable types of detectors of the 21st century.
5. Pyroelectric detectors: Some crystals, such as triglycine sulfate and barium strontium niobate, will cause a change in spontaneous polarization intensity when exposed to infrared radiation and their temperature rises. As a result, a small voltage is generated between the two outer surfaces of the crystal perpendicular to the spontaneous polarization direction, which can be used to measure the power of infrared radiation.
Based on their application functions and locations, infrared sensor applications can be broadly categorized as follows:
Radiation measurement, spectral measurement
These types of measuring instruments have a wide range of applications. For example, ground-based radiometers based on mid-infrared radiation measurements can be used to observe climate change such as global warming; infrared space telescopes based on far-infrared radiation measurements can be used for astronomical observation of celestial bodies; and meteorological satellites equipped with infrared spectral scanning radiometers can achieve meteorological observation and analysis of clouds and other phenomena. In industrial and mining enterprises, infrared thermometers based on radiation measurement and infrared analyzers based on infrared spectral measurement are more commonly used.
As we know, the close-range air-to-air missiles carried by fighter jets use infrared tracking systems. These systems search for and track infrared targets based on the electromagnetic radiation waves emitted by the target within the infrared spectrum, determine their spatial position, and track their trajectory.
The image quality of an infrared search and tracker depends on its spatial resolution, which is related to pixel size and pixel count. In other words, the higher the pixel count and the smaller the pixel size, the clearer the image and the greater the search distance.
Sofradir, a French company specializing in MCT (mercury cadmium telluride) cooled infrared detector technology, recently launched a high-performance IRST infrared search and track system based on MWIR mid-infrared with a 10μm image distance. This system can be used by pilots or soldiers to effectively identify, distinguish, and locate small targets up to 10km away, whether in daylight or at night, even in smoke or fog conditions.
thermal imaging system
A thermal imager uses an infrared detector and an optical imaging lens to receive the infrared radiation energy distribution pattern of the target object and reflect it onto the photosensitive element of the infrared detector, thus obtaining an infrared thermal image. This thermal image corresponds to the heat distribution field on the object's surface. Simply put, a thermal imager converts the invisible infrared energy emitted by an object into a visible thermal image. Different colors in the thermal image represent different temperatures of the object being measured.
Any object with a temperature emits infrared radiation. A thermal imager receives this infrared radiation and displays the temperature distribution on the surface of the object in a colored image. By detecting minute temperature differences, it identifies temperature anomalies, thus playing a role in maintenance. It is also commonly known as an infrared thermal imager.
Thermal imagers were initially developed for military purposes, but in recent years they have rapidly expanded into civilian industrial applications. Their applications are very broad, found wherever there are temperature differences.
For example, in the construction industry, it can be used to inspect for hollow spots, defects, tile detachment, dampness, and thermal bridging; in the fire protection industry, it can be used to locate the source of fire, determine the cause of an accident, and find injured people in smoke; in the public security system, it can be used to find people hiding at night; in the automobile manufacturing industry, it can be used to test the performance of tires, air conditioning heating elements, engines, exhaust pipes, etc.; in medicine, it can be used to test the effects of acupuncture and detect diseases such as nasopharyngeal carcinoma and breast cancer in their early stages; and in the power industry, it can be used to inspect wires, connections, quick-closing switches, and transformer cabinets.
Infrared communication system
This system uses a modulated infrared radiation beam to transmit encoded data, and then a silicon photodiode converts the received infrared radiation signal into an electrical signal to achieve short-range communication. It has the advantages of not interfering with the normal operation of other nearby devices, making it particularly suitable for indoor communication in densely populated areas. Furthermore, this communication system is characterized by low power consumption, low cost, and high security and reliability.
other
Infrared sensing technology is also widely used in access control alarms and controls, lighting control, fire detection, toxic and harmful gas leak detection, infrared ranging, heating and ventilation, and other comprehensive applications. At the "SENSOR+TEST" exhibition in Nuremberg, Germany, the French company ULIS showcased its latest product, the Micro80P infrared thermal sensor array, which utilizes the latest on-chip innovations (such as the I2C standard interface and low-power management).
This sensor array is based on highly reliable amorphous silicon technology and has a sensitivity of up to 80×80 image distance. Its performance far exceeds that of single-element or four-element thermal sensors used in current motion detectors, greatly improving the capabilities of industrial-grade infrared thermal detection sensors.
This product can be used not only to detect temperature points or surfaces and to detect movement, but also to count, locate, and classify targets or human activities. For example, in HVAC applications, it can be used to count people in a room and measure the temperature of the room walls, thereby enabling automatic and precise adjustment of indoor heating and air conditioning systems to achieve maximum energy savings in buildings.
Development of infrared sensors
With advancements in science and technology, the development of computer microprocessor technology, the improvement of modern digital signal processing technology, the introduction of new semiconductor materials, and progress in manufacturing processes, infrared sensors have experienced rapid growth in recent years. According to a forecast by a foreign research institution, global sales of infrared sensors will increase from $ 152 million in 2010 to $286 million in 2016 .
In recent years, the development trend of infrared sensors has been mainly reflected in the following four aspects:
Firstly, the use of new materials and processing technologies improves the infrared detectivity of sensors, increases the response wavelength, shortens the response time, increases pixel sensitivity and pixel density, enhances anti-interference performance, and reduces production costs. For example, Pyreos and Irisys have introduced a new pyroelectric sensing technology that combines thin films and ceramics, enabling the arraying of sensing elements.
Secondly, sensors are becoming larger and more multifunctional. With the development of microelectronics technology and the continuous expansion of sensor application fields, infrared sensors are evolving from small, single-function sensors to larger, more multifunctional ones.
Large infrared sensors (16×16 to 64×64 pixels) developed abroad can not only measure temperature fields, but also have advanced human detection functions that small infrared sensors do not have (that is, they can accurately locate an individual's position in space and identify the person even if they are not moving) or large-area security monitoring functions, making them very suitable for applications such as home automation, healthcare, and security protection.
In addition, the development of new multispectral sensors has greatly improved the functionality of infrared imaging arrays.
Thirdly, the sensors are becoming more intelligent. New intelligent infrared sensors typically have multiple built-in microprocessors, possessing advanced digital signal processing or compensation functions such as Fourier transform and wavelet transform, as well as self-diagnostic functions and two-way digital communication functions, which greatly improves the stability, reliability, signal-to-noise ratio, and convenience of the sensors.
Fourth, sensors are becoming increasingly miniaturized and integrated. By employing on-chip integration technology (including blind cell replacement, non-uniformity correction, and some image processing functions) and other new device structures and manufacturing processes, driven by MEMS (microelectromechanical systems) and even NEMS (nanoelectromechanical systems) based on nanotechnology, the size of infrared sensors has been greatly reduced, power consumption has been significantly lowered, and integration has been significantly improved.
Due to the superior performance of infrared sensors, many mainstream instrument research institutions and manufacturers are investing more and more in their research and development.
