Any object in the universe can produce infrared radiation as long as its temperature exceeds zero. In fact, like visible light, its radiation can be refracted and reflected. This produces infrared technology, which uses infrared light detectors because of its unique superiority. It has received extensive attention and has been widely used in military and civilian applications. Militaryly, infrared detection is used for guidance, fire control tracking, warning, target detection, weapon hot sight, ship navigation, etc.; in civil applications, extensive application and industrial equipment monitoring, security surveillance, disaster relief, remote sensing, traffic management, and medicine Diagnostic techniques, etc.

In today's highly developed technology, automatic control and automatic detection are becoming more and more important in people's daily life and industrial control, making people's lives more comfortable and industrial production more efficient. The sensor is an important component in the automatic control. It is an important component of the information acquisition system. The sensor measures the measured or responsiveness into a signal suitable for transmission or detection (generally an electrical signal), and then uses a computer or a circuit device. The signal output by the sensor is processed to achieve the function of automatic control. Since the response time of the sensor is generally short, the industrial production can be controlled in real time through a computer system. Infrared sensors are a common type of sensors. Since infrared sensors are a type of sensor that detects infrared radiation, any object in nature will radiate infrared energy as long as it is stable above absolute zero. Therefore, infrared sensors are called very practical ones. Class sensors, using infrared sensors can design a number of practical sensor modules, such as infrared thermometers, infrared imagers, infrared body detection alarms, automatic door control systems.

The infrared sensor is a sensor that measures with the physical properties of infrared rays. Infrared light, also known as infrared light, has the properties of reflection, refraction, scattering, interference, absorption and the like. It is an invisible light whose spectrum is outside the red color of visible light, so it is called infrared.

In engineering, the position (band) occupied by infrared rays in the electromagnetic spectrum is divided into four bands: near-infrared, mid-infrared, far-infrared, and far-infrared. Any substance, as long as it has a certain temperature (above absolute zero), can radiate infrared rays.

First understand the infrared light. Infrared light is part of the solar spectrum. The most important feature of infrared light is its photothermal effect, which radiates heat, which is the largest photothermal effect region in the spectrum. Infrared light is an invisible light that, like all electromagnetic waves, has the properties of reflection, refraction, scattering, interference, and absorption. The infrared light travels in a vacuum at 300,000 Km/s. Infrared light propagates through the medium and produces attenuation. The attenuation in the metal is very large, but infrared radiation can pass through most semiconductors and some plastics. Most of the liquid absorbs infrared radiation very much.

Different gases have different degrees of absorption, and the atmosphere has different absorption bands for different wavelengths of infrared light. Research and analysis have shown that there is a relatively large "transparency" for infrared light having a wavelength of 1 to 5 μm and a range of 8 to 14 μm. That is, infrared light of these wavelengths can better penetrate the atmosphere. Any object in nature, as long as its temperature is above absolute zero, produces infrared radiation. The photothermal effect of infrared light is different for different objects, and the thermal energy intensity is also different. For example, black bodies (objects that can absorb all of the infrared radiation projected onto their surface), mirrors (objects that can totally reflect infrared radiation), transparent bodies (objects that can penetrate all infrared radiation), and gray bodies (which can partially reflect or Objects that absorb infrared radiation) will produce different photothermal effects.

Strictly speaking, there are no black bodies, mirror bodies and transparent bodies in nature, and most objects belong to gray bodies. These characteristics are important theoretical basis for the use of infrared radiation technology in military and scientific research projects such as satellite remote sensing telemetry and infrared tracking.

The physical nature of infrared radiation is thermal radiation. The higher the temperature of the object, the more infrared radiation is emitted, and the stronger the energy of the infrared radiation. The study found that the thermal effects of various monochromatic lights in the solar spectrum are gradually increasing from purple to red, and the maximum thermal effect occurs in the frequency range of infrared radiation. Therefore, infrared radiation is called thermal radiation or heat ray. .

Kirchhoff's law: At a certain temperature, the ratio of the radiant flux W to the absorptivity per unit area is a constant for any object and equal to the same area blackbody radiant flux W at that temperature. At a given temperature, the emissivity of the object = the absorptivity (same band); the greater the absorptivity, the greater the emissivity.

The thermal radiation intensity of the ground object is proportional to the fourth power of the temperature. Therefore, the slight temperature difference of the ground object causes a significant change in the energy of the infrared radiation. This feature constitutes the theoretical basis of infrared remote sensing.

Boltzmann's Law: The total radiant flux of a black body increases rapidly with increasing temperature, which is proportional to the fourth power of the temperature. Therefore, a small change in temperature causes a large change in the radiant flux density. It is the theoretical basis for measuring the temperature of an infrared device.

Wien's displacement law: As the temperature increases, the peak wavelength corresponding to the maximum value of the radiation moves toward the short-wave direction.

The working principle of the infrared sensor is not complicated. The entities of each part of a typical sensor system are:

1. Target to be tested: The infrared system can be set according to the infrared radiation characteristics of the target to be tested.

2. Atmospheric attenuation: When the infrared radiation of the target to be measured passes through the earth's atmosphere, the infrared radiation emitted by the infrared source will be attenuated due to the scattering and absorption of gas molecules and various gases and various sol particles.

3. Optical receiver: It receives part of the infrared radiation of the target and transmits it to the infrared sensor. It is equivalent to a radar antenna and is usually an objective lens.

4. Radiation modulator: The radiation from the target to be measured is modulated to provide the target position information, and the large-area interference signal can be filtered out. Also known as a reticle and chopper, it has a variety of structures.

5. Infrared detector: This is the core of the infrared system. It is a sensor that uses infrared radiation to interact with matter to detect infrared radiation. In most cases, it uses the electrical effects exhibited by this interaction. Such detectors can be divided into two types: photon detectors and heat sensitive detectors.

6. Detector cooler: Since some detectors must work at low temperatures, the corresponding system must have refrigeration equipment. After cooling, the device can reduce response time and improve detection sensitivity.

7. Signal Processing System: Amplifies and filters the detected signals and extracts information from these signals. This information is then converted into the required format and finally delivered to the control device or display.

8. Display device: This is the terminal device of the infrared device. Commonly used displays include oscilloscopes, picture tubes, infrared photographic materials, indicating instruments, and recorders.

According to the above process, the infrared system can complete the corresponding physical quantity measurement. The core of the infrared system is an infrared detector. According to the mechanism of detection, it can be divided into two categories: heat detector and photon detector.

The heat detector absorbs all the radiant energy of various wavelengths incident. It is an infrared sensor that has no choice for infrared light waves. The photon effects commonly used in photon detectors are external photoelectric effect, internal photoelectric effect (photovoltaic effect, photoconductive effect) and photoelectromagnetic effect. The heat detector uses the radiant heat effect to cause the temperature of the detector element to rise after receiving the radiant energy, thereby changing the temperature-dependent performance of the detector. Radiation can be detected by detecting a change in one of the properties. In most cases, radiation is detected by thermoelectric changes. When the component receives radiation, causing a physical change in non-electricity, the corresponding change in power can be measured by appropriate transformation.

The response time of the thermal detector to infrared radiation is much longer than the response time of the photodetector. The response time of the former is generally above ms, while the latter is only of the order of ns. Heat detectors do not require cooling, and photon detectors mostly cool.

Common infrared sensors can be divided into thermal sensors and photon sensors.

Thermal sensor

The thermal sensor uses the incident infrared radiation to cause the temperature change of the sensor, thereby correspondingly changing the relevant physical parameters, and determining the infrared radiation absorbed by the infrared sensor by measuring the change of the relevant physical parameters.

The main advantage of the heat detector is that the corresponding band is wide, it can work at room temperature, and it is easy to use. However, the thermal sensor has a relatively long time and low sensitivity, and is generally used for low frequency modulation.

The main types of thermal sensors are: thermal sensor type, thermocouple type, high-altitude pneumatic type and heat release type.

1, thermistor type sensor

The thermistor is formed by mixing and melting manganese, nickel and cobalt oxides. The thermistor is generally formed into a sheet shape. When the infrared radiation is irradiated on the thermistor, the temperature rises and the resistance value decreases. By measuring the magnitude of the change in the thermistor value, the intensity of the incident infrared radiation can be known, and the temperature at which the infrared radiation object is generated can be judged.

2, thermocouple type sensor

Thermocouples are made up of two materials with large differences in thermoelectric power. When the infrared radiation reaches the junction of the closed loop formed by the two metal materials, the junction temperature rises. The other contact that is not irradiated by infrared radiation is at a lower temperature, and a temperature difference current will be generated in the closed loop. At the same time, the temperature difference potential is generated in the loop, and the magnitude of the temperature difference potential reflects the intensity of the infrared radiation absorbed by the joint. The infrared sensor made by the thermoelectric potential phenomenon is called a thermocouple type infrared sensor. Because of its large time constant, the corresponding time is long, the dynamic characteristics are poor, and the modulation frequency should be limited to below 10 Hz.

3, Lai pneumatic sensor

The high-altitude aerodynamic sensor uses the characteristics of increasing temperature and increasing volume after the gas absorbs infrared radiation to reflect the intensity of infrared radiation. It has a gas chamber connected to a flexible sheet by a small tube. The side of the sheet facing away from the pipe is a mirror. The front of the chamber is provided with an absorption mold which is a film of low heat capacity. The infrared radiation is incident on the absorption mold through the window, and the absorption mode transmits the absorbed thermal energy to the gas, so that the temperature of the gas rises and the gas pressure increases, thereby moving the flexible mirror. On the other side of the chamber, a beam of visible light is focused on the Mirror through the grating diaphragm, and the grid image reflected by the Mirror is projected onto the photocell through the grid. When the flexible mirror moves due to the pressure change, the grid image and the grating diaphragm are relatively displaced, so that the amount of light falling on the phototube changes, and the output signal of the photocell also changes. This variation reflects the incident infrared. The intensity of radiation. This sensor is characterized by high sensitivity and stable performance. However, the response time is long, the structure is complex, and the strength is poor, and it is only suitable for use in the laboratory.

4, pyroelectric sensor

A pyroelectric sensor is a thermal crystal or a "ferroelectric" having a polarization phenomenon. The polarization of ferroelectrics (charge per unit area) is related to temperature. When the infrared radiation is irradiated onto the surface of the ferroelectric sheet which has been polarized, the temperature of the sheet is raised, the polarization thereof is lowered, and the surface charge is reduced, which is equivalent to releasing a part of the electric charge, so it is called a pyroelectric type sensor. If the load resistor is connected to the ferroelectric sheet, an electrical signal output is produced on the load resistor. The size of the output signal depends on how fast the temperature of the sheet changes, reflecting the intensity of the incident infrared radiation. It can be seen that the voltage response rate of the pyroelectric infrared sensor is proportional to the rate of change of the incident radiation. When constant infrared radiation is incident on the pyroelectric sensor, the sensor has no electrical signal output. The electrical signal is output only when the ferroelectric temperature is changing. Therefore, the infrared radiation must be modulated (or quenched), so that the constant radiation becomes a transactional radiation, which constantly causes the temperature change of the sensor to cause pyroelectric generation and output an alternating signal.

Photon sensor

Photon sensors use some semiconductor materials to produce photon effects under the illumination of incident light, which changes the electrical properties of the material. The intensity of infrared radiation can be known by measuring changes in electrical properties. An infrared sensor made using photon effects. Collectively referred to as photon sensors. The main features of the photon sensor are high sensitivity, fast response, and high response frequency. However, it generally has to work at low temperatures and the detection band is narrow.

According to the working principle of the photon sensor, it can be generally divided into two types: internal photoelectric and external photoelectric sensor. The latter is divided into three types: photoconductive sensor, photovoltaic sensor and magneto-optical sensor.

1, external photoelectric sensor

When the light is radiated on the surface of some materials, if the photon energy of the incident light is sufficiently large, the electrons of the material can escape from the surface. This phenomenon is called external photoelectric effect or photoelectron emission effect. Photodiodes, photomultiplier tubes, etc. belong to this type of electronic sensor. Its response speed is relatively fast, usually only a few nanoseconds. However, electron emission requires a large photon energy and is only suitable for use in the near-infrared or visible range.

2, photoconductive sensor

When infrared radiation is irradiated on the surface of some semiconductor materials, some electrons and holes in the semiconductor material can change from the originally non-conductive bound state to the conductive free state, which increases the conductivity of the semiconductor. This phenomenon is called photoconductivity. phenomenon. A sensor made by photoconductivity is called a photoconductive sensor. For example, lead sulfide, lead selenide, indium antimonide, mercury and other materials can be used to make photoconductive sensors. When using a photoconductive sensor, it is necessary to cool and add a certain bias voltage, otherwise the response rate will be reduced, the noise will be large, and the response band will be narrow, so that the infrared sensor is damaged.

3, photovoltaic sensor

When infrared radiation is applied to the PN junction of some semiconductor materials, the free electrons move to the N region under the action of the electric field inside the junction. If the PN junction is open, an additional potential is generated at both ends of the PN junction, called photogeneration. Electromotive force. A sensor or PN junction sensor made with this effect. Commonly used materials are indium arsenide, indium antimonide, mercury telluride, antimony tin and lead.

4, magneto-optical sensor

When infrared radiation is irradiated on the surface of some semiconductor materials, some electrons and holes in the semiconductor material will diffuse to the inside. If a strong magnetic field is applied during diffusion, electrons and holes are biased to one side, thus generating an open circuit voltage. This phenomenon is called the magneto-optical effect. An infrared sensor made by this effect is called a magneto-optical sensor.

The magneto-optical sensor does not need to be cooled, the response band can reach about 7μM, the time constant is small, the response speed is fast, no bias is applied, the internal resistance is extremely low, the noise is small, and the stability and reliability are good. However, its sensitivity is low, and the low-noise preamplifier is difficult to manufacture, thus affecting the use.

Infrared radiation thermometer structure

It consists of optical systems, modulators, infrared sensors, amplifiers and indicators;

The optical system can be transmissive or reflective. The components of the transmissive optical system are made of infrared optical materials.

Infrared thermometer block diagram:

High-temperature (above 700 °C) measuring instruments, the useful band is mainly in the near-infrared region of 0.76-3μm, and materials such as general optical glass or quartz can be selected.

Medium temperature (100-700 ° C) measuring instrument, the useful band is mainly in the mid-infrared region of 3-5 μm, and hot-pressing optical materials such as magnesium fluoride and magnesium oxide are often used.

Measuring low temperature (below 100 °C) instruments, the useful band is mainly in the mid-range and far-infrared bands of 5-14μm, and most of them are materials such as germanium, silicon and hot-pressed zinc sulfide.

A modulator is a device that modulates infrared radiation to modulate the radiation. Generally, a micro-motor is used to drive a gear plate or an equidistant hole plate, and the gear plate or the perforated disk is rotated to cut the incident radiation and the radiation signal projected onto the infrared sensor is changed. Because the system is easy to process alternating signals, and can achieve a higher signal to noise ratio.

Gaolai pneumatic sensor structure

It has a gas chamber connected to a flexible sheet by a small tube. The side of the sheet facing away from the pipe is a mirror. An absorbing film is attached to the front of the gas chamber, which is a film of low heat capacity.

On the other side of the chamber, a beam of visible light is focused on the flexible mirror through the grating aperture, and the grid image reflected by the flexible mirror is projected onto the photocell through the grating aperture.

The high-altitude aerodynamic sensor uses the characteristics of increasing temperature and increasing volume after the gas absorbs infrared radiation to reflect the intensity of infrared radiation.

The infrared radiation is incident on the absorption film through the window, and the absorption film transmits the absorbed thermal energy to the gas, so that the gas temperature rises and the gas pressure increases, thereby moving the flexible mirror.

On the other side of the chamber, a beam of visible light is focused on the flexible mirror through the grating aperture, and the grid image reflected by the flexible mirror is projected onto the photocell through the grating aperture.

When the flexoscope moves due to pressure changes, the grid image and the grid diaphragm are relatively displaced, so that the amount of light falling on the phototube changes, and the output signal of the photocell changes. This amount of change reflects the intensity of incident infrared radiation. The flaw in this sensor is high sensitivity and stable performance. However, the response time is long, the structure is complex, and the strength is poor, and it is only suitable for use in the laboratory.

1, voltage response

When (modulated) infrared radiation is incident on the sensitive surface of the sensor, the ratio of the output voltage of the sensor to the input infrared radiation power is called the voltage response rate of the sensor and is recorded as RV.

In the formula:

US: Output voltage of the infrared sensor

P0: power projected onto the unit area of ​​the infrared sensitive component

A: area of ​​the infrared sensor sensitive component

2. Response wavelength range:

Curve 1: The voltage response rate of the pyroelectric sensor (independent of wavelength).

Curve 2: Voltage response rate curve of the photon sensor.

(1) The response wavelength range (or spectral response) is the relationship between the voltage response rate of the sensor and the wavelength of the incident infrared radiation, which is generally represented by a curve (see the figure above).

(2) Generally, the wavelength corresponding to the maximum response rate is referred to as the peak wavelength.

(3) The wavelength corresponding to the response rate falling to half of the response value is called the cutoff wavelength, which indicates the wavelength range used by the infrared sensor.

3, noise equivalent power

If the output voltage generated by the radiant power projected onto the sensitive element of the infrared sensor is exactly equal to the noise voltage of the sensor itself, then this radiant power is called "noise equivalent power". Usually indicated by the symbol "NEP".

Where: Us is the output voltage of the infrared detector; P0 is the power projected onto the unit area of ​​the infrared sensitive component; A0 is the area of ​​the infrared sensitive element; UN is the integrated noise voltage of the infrared detector; RV is the voltage response rate of the infrared detector .

4, detection rate

The detection rate is the reciprocal of the noise equivalent power, ie:

The higher the detection rate of the infrared sensor, the smaller the minimum radiant power that the sensor can detect, and the more sensitive the sensor.

5, the ratio of detection

The specific detection rate is also called the normalized detection rate, or the detection sensitivity. In essence, when the area of ​​the sensitive component of the sensor is a unit area, and the bandwidth Δf of the amplifier is 1 Hz, the ratio of the signal voltage obtained by the unit power radiation to the noise voltage. Usually indicated by the symbol D*.

Physical dimensions of D*: cmHz1/2W-1 (300 K)

6, time constant

The time constant represents the rate at which the output signal of the infrared sensor changes with infrared radiation.

The time when the output signal lags behind the infrared radiation, called the time constant of the sensor, is numerically:

Ï„=1/2Ï€fc

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

The thermal inertia and RC parameters of the thermal sensor are large, and the time constant is larger than that of the photon sensor, which is generally millisecond or longer; and the time constant of the photon sensor is generally microsecond.

The application and prospect of infrared sensor

The application of infrared sensors is mainly reflected in the following aspects:

1. Infrared radiometer: used for radiation and spectral radiation measurements.

2. Search and Tracking System: Used to search and track infrared targets, determine their spatial location and track their motion.

3. Thermal imaging system: An infrared radiation distribution image that can form the entire target.

4. Infrared ranging system: measure the distance between objects. (Using the principle of non-diffusion when transmitting infrared rays, because infrared rays have a small refractive index when passing through other substances, so long-range rangefinders will consider infrared rays)

5. Communication system: Infrared communication is a way of wireless communication.

6. Hybrid system: refers to two combinations of the above two types of systems.

Infrared sensor applications can be used for non-contact temperature measurement, gas composition analysis, non-destructive testing, thermal image detection, infrared remote sensing, and reconnaissance, search, tracking, and communication of military targets. The application prospect of infrared sensors will be more extensive with the development of modern science and technology. In the future development, the performance and sensitivity of the infrared sensor will be greatly improved.

1. Intelligent: The current infrared sensor is mainly used in combination with peripheral devices, and the smart sensor has a built-in microprocessor, which can realize two-way communication between the sensor and the control unit, and has the advantages of miniaturization, digital communication, simple maintenance, etc., and can be used as a single The module works independently.

2. Miniaturization: An inevitable trend in sensor miniaturization. In the current application, due to the volume problem of the infrared sensor, the degree of its use is far less than that of the thermoelectric device. Therefore, the impact of miniaturization of infrared sensors on their future is not negligible.

3, high sensitivity and high performance: In medicine, human body temperature test, infrared sensor has been applied quite well due to the rapid measurement, but limited to its low accuracy and can not replace the existing body temperature measurement method. Therefore, the high sensitivity and high performance of infrared sensors is an inevitable trend in its future development.

Although there are still many shortcomings of infrared sensors at this stage, infrared sensors have played a huge role in modern production practices. With the improvement of detection equipment and other parts of the technology, infrared sensors can have more performance and Better sensitivity will also have a wider range of applications.

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