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Nov 22 2019

Deep Analysis of Infrared Sensor

Ⅰ. Introduction

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, which leads to infrared technology. Infrared detector is widely used in military and civil fields because of its unique advantages. In military, infrared detection is used for guidance, fire control tracking, alert, target detection, weapon thermal sight, ship navigation, etc.; in civil field, it is widely used in industrial equipment monitoring, safety monitoring, disaster relief, remote sensing, traffic management, medical diagnosis technology, etc.

With the development of science and technology, the proportion of automatic control and automatic detection in people's daily life and industrial control is more and more heavy, which makes people's life more and more comfortable and the efficiency of industrial production more and more high. The sensor is an important component of the automatic control, and an important component of the information acquisition system. Through the sensor, the feeling or response is measured and converted into a signal suitable for transmission or detection (generally electrical signal), and then the computer or circuit equipment is used to process the signal from the sensor to achieve the function of automatic control.Because the response time of the sensor is generally short, the real-time control of industrial production can be carried out through the computer system. Infrared sensor is a common type of sensor. Because infrared sensor is a kind of sensor to detect infrared radiation, and any object in nature will radiate infrared energy as long as its stability is higher than absolute zero, so infrared sensor is called a very practical type of sensor. Many practical sensor modules can be designed by using infrared sensor, such as infrared temperature measurement Instrument, infrared imager, infrared human detection alarm, automatic door control system, etc.

Ⅱ. What is infrared sensor?

Infrared sensor is a sensor which uses the physical properties of infrared to measure. Infrared light, also known as infrared light, has the properties of reflection, refraction, scattering, interference, absorption, etc. It is a kind of invisible light, its spectrum is located outside red in visible light, so it is called infrared.

In engineering, the position (band) of infrared ray in electromagnetic spectrum is divided into four bands: near infrared band, mid infrared band, far infrared  band and extremely far infrared band. Any substance can radiate infrared ray, as long as it has a certain temperature (higher than absolute zero).

ir sensor module for arduino 2f rasberry pi 500x500

Ⅲ. How does infrared sensor work?

First of all, let's learn about infrared light. Infrared light is a part of the solar spectrum. The biggest characteristic of infrared light is its photothermal effect and radiant heat. It is the largest photothermal effect area in the spectrum. An invisible light, like all electromagnetic waves, having the properties of reflection, refraction, scattering, interference, absorption, etc. The propagation speed of infrared light in vacuum is 300000 km / s. The transmission of infrared light in the medium will produce attenuation, and the transmission attenuation in the metal is very large, but the infrared radiation can pass through most semiconductors and some plastics, and most liquids absorb the infrared radiation very much.

How Infrared Sensor Works

Different gases have different absorption levels, and the atmosphere has different absorption bands for different wavelengths of infrared light. The results show that the infrared light with the wavelength of 1-5 μ m and 8-14 μ m has a relatively large "transparency". That is to say, these wavelengths of infrared light can penetrate the atmosphere well. Any object in nature, as long as its temperature is above absolute zero, can produce infrared radiation. The photothermal effect of infrared light is different for different objects, and the intensity of heat energy is also different. For example, blackbody (an object that can fully absorb the infrared radiation projected on its surface), mirror body(an object that can fully reflect the infrared radiation), transparent body(an object that can fully penetrate the infrared radiation) and gray body (an object that can partially reflect or absorb the infrared radiation) will produce different photothermal effects.

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

The physical essence of infrared radiation is thermal radiation. The higher the temperature of an object, the more infrared it radiates, the stronger the energy of the infrared radiation. It is found that the thermal effect of various monochromatic light in the solar spectrum increases gradually from violet light to red light, and the largest thermal effect occurs in the frequency range of infrared radiation, so people call infrared radiation as thermal radiation or thermal ray.

Ⅳ. The basic law of infrared radiation

① Kirchhoff's Law: at a certain temperature, the ratio of the radiation flux W per unit area of the ground object to the absorption rate is a constant for any object, and is equal to the radiation flux w of blackbody of the same area at that temperature. At a given temperature, the emissivity of the object = the absorptivity (the same band); the higher the absorptivity, the higher the emissivity.

The thermal radiation intensity of the ground object is directly proportional to the fourth power of the temperature, so the small temperature difference of the ground object will cause the obvious change of the infrared radiation energy. This feature constitutes the theoretical basis of infrared remote sensing.

Kirchhoff‘’s Law

② Boltzmann's Law: that is, the total radiation flux of blackbody increases rapidly with the increase of temperature, which is proportional to the fourth power of temperature. Therefore, a small change in temperature will cause a great change in radiation flux density. It is the theoretical basis of measuring temperature with infrared device.

③ Wien displacement law: with the increase of temperature, the peak wavelength corresponding to the maximum radiation value moves to the short wave direction.

Wien displacement law

Ⅴ. The working principle of infrared sensor

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

  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 from the infrared source will be attenuated due to the scattering and absorption of gas molecules, various gases and various colloidal particles.

  3. Optical receiver: it receives part of the infrared radiation of the target and transmits it to the infrared sensor. Equivalent to radar antenna, usually objective lens.

  4. Radiation modulator: it can modulate the changed radiation light from the target to be tested, provide the target orientation information, and filter out large-area interference signals. Also known as modulation disk 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 the physical effect of the interaction between infrared radiation and matter to detect infrared radiation. In most cases, it uses the electrical effect of the interaction. These detectors can be divided into two types: photon detector and heat sensitive detector.

  6. Detector Cooler: because some detectors must work at low temperature, the corresponding system must have refrigeration equipment. After refrigeration, the equipment can shorten the response time and improve the detection sensitivity.

  7. Signal processing system: amplify and filter the detected signals, and extract information from these signals. Then, this kind of information is transformed into the required format, and finally transmitted to the control equipment or display.

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

IR sensor object detection working principle

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

The thermal detector absorbs all the radiant energy of all kinds of incident wavelengths. It is an infrared sensor with no choice for infrared light wave. The common photon effects of photon detectors are external photoelectric effect, internal photoelectric effect (photovoltaic effect, photoconductive effect) and photoelectromagnetic effect. The thermal detector uses the radiation heat effect to make the temperature rise after the detector receives the radiation energy, and then the temperature dependent performance of the detector changes. Radiation can be detected by detecting a change in one of the properties. In most cases, radiation is detected by thermoelectric changes. When the element receives radiation and causes the physical change of non electric quantity, the corresponding electric quantity change can be measured after appropriate transformation.

The response time of thermal detector to infrared radiation is much longer than that of photodetector. The response time of the former is generally more than MS, while that of the latter is only ns. Thermal detectors do not need to be cooled, most photon detectors need to be cooled.

Ⅵ. Types of infrared sensors

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

1. Thermal sensor

The thermal sensor uses the incident infrared radiation to change the temperature of the sensor, and then make the relevant physical parameters change accordingly. The infrared radiation absorbed by the infrared sensor is determined by measuring the changes of the relevant physical parameters.

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

The main types of thermal sensors are: thermal sensor type, thermocouple type, gaolai pneumatic type and heat release electric type.

① Thermistor sensor

The thermistor is made of manganese, nickel and cobalt oxides. The thermistor is usually made into thin sheet. When the infrared radiation irradiates the thermistor, its temperature increases and the resistance decreases. By measuring the change of the thermistor value, we can know the intensity of the incident infrared radiation, thus we can judge the temperature of the object generating the infrared radiation.

Thermistor sensor

② Thermocouple sensor

Thermocouples are made of two materials with great difference in thermal power. When infrared radiation reaches the contact of the closed circuit composed of these two metal materials, the contact temperature increases. The other contact which is not irradiated by infrared radiation is at a lower temperature, at this time, the temperature difference current will be generated in the closed circuit. At the same time, thermoelectric potential is generated in the loop, and the magnitude of thermoelectric potential reflects the strength of infrared radiation absorbed by the contact. The infrared sensor made of thermoelectric potential is called thermocouple infrared sensor. Because of its large time constant, long corresponding time and poor dynamic characteristics, the modulation frequency should be limited below 10Hz.

Thermocouple sensor

③ Lai pneumatic sensor

After absorbing the infrared radiation, the temperature and volume of the gas are increased to reflect the intensity of the infrared radiation. It has an air chamber connected to a flexible sheet by a small pipe. One side of the back pipe of the sheet is a reflector. The front of the gas chamber is attached with an absorption mode, which is a thin film with low heat capacity. The infrared radiation is incident on the absorption mode through the window, and the absorption mode transmits the absorbed heat energy to the gas, which makes the gas temperature and pressure increase, so that the flexible mirror moves. On the other side of the chamber, a beam of visible light is focused on the flexible mirror through the grating light bar, and the grating image reflected by the flexible mirror is projected onto the photoelectric cell through the grating light bar. When the flexible mirror moves due to the change of pressure, the relative displacement between the grating image and the grating light bar will change the amount of light falling on the photocell, and the output signal of the photocell will also change, which reflects the intensity of the in out infrared radiation. This sensor is characterized by high sensitivity and stable performance. But the response time is long, the structure is complex and the intensity is poor, so it is only suitable for laboratory use.

④ Pyroelectric sensor

Pyroelectric sensor is a kind of thermal crystal or ferroelectric with polarization phenomenon. The polarization intensity (charge per unit area) of ferroelectrics is related to temperature. When infrared radiation irradiates the surface of the polarized ferroelectric sheet, the temperature of the sheet increases, the polarization intensity decreases, and the surface charge decreases, which is equivalent to releasing part of the charge, so it is called pyroelectric sensor. If the load resistor is connected to a ferroelectric sheet, an electrical signal output is generated on the load resistor. The size of the output signal depends on the speed of the temperature change of the chip, which reflects the intensity of the incident infrared radiation. It can be seen that the voltage response rate of pyroelectric infrared sensor is directly proportional to the change rate of incident radiation. When the constant infrared radiation irradiates on the pyroelectric sensor, the sensor has no electrical signal output. Only when the temperature of ferroelectrics is in the process of change can the electrical signal be output. Therefore, it is necessary to modulate the infrared radiation (or chopping light) so that the constant radiation becomes the alternating radiation, which constantly causes the temperature change of the sensor, so as to generate pyroelectric and output the alternating signal.

Pyroelectric sensor

2. Photon sensor

The photon sensor uses some semiconductor materials to produce photon effect under the illumination of incident light, which changes the electrical properties of materials. By measuring the change of electrical properties, we can know the intensity of infrared radiation. The infrared sensors made by photon effect are called photon sensors. The main characteristics of photon sensor are high sensitivity, fast response speed and high response frequency, but generally it must work at low temperature and the detection band is narrow.

According to the working principle of photon sensor, it can be divided into internal photoelectric sensor and external photoelectric sensor. The latter is divided into photoconductive sensor, photovoltaic sensor and magnetoelectric sensor.

① External photoelectric sensor

When the light radiates on the surface of some materials, if the photon energy of the incident light is large enough, the electrons of the materials can escape from the surface. This phenomenon is called external photoelectric effect or photoelectron emission effect. Photodiode, photomultiplier tube and so on belong to this type of electronic sensor. Its response speed is relatively fast, generally only a few nanoseconds. However, electron escape requires a large amount of photon energy, which is only suitable for near-infrared radiation or visible light.

Photoelectric sensors

② Photoconductive sensor

When infrared radiation irradiates on the surface of some semiconductor materials, some electrons and holes in the semiconductor materials can change from the original non-conductive bound state to the conductive free state, which increases the conductivity of the semiconductor. This phenomenon is called photoconductivity phenomenon. The sensors made of photoconductive phenomena are called photoconductive sensors. For example, lead sulfide, lead selenide, indium antimonide, mercury telluride and other materials can be used to make photoconductive sensors. When using photoconductive sensor, we need to cool and add a certain bias voltage, otherwise the response rate will be reduced, the noise will be large, the response band will be narrow, and the infrared sensor will be damaged.

Photoconductive sensor

③ Photovoltaic sensor

When the infrared radiation irradiates on the PN junction of some semiconductor materials, the free electrons move to the N-region under the action of the electric field in the junction. If the PN junction is open, an additional potential will be generated at both ends of the PN junction, which is called the photogenerated electromotive force.  The sensors or PN junction sensors  based on this effect are usually made of materials such as indium arsenide, indium antimonide, mercury telluride, lead tin telluride, etc.

④ Magnetoelectric sensor

When the infrared radiation irradiates on the surface of some semiconductor materials, some electrons and holes in the semiconductor materials will diffuse to the interior. If the diffusion is affected by a strong magnetic field, the electrons and holes will each deviate to one side, resulting in an open circuit voltage. This phenomenon is called the optical magnetoelectric effect. The infrared sensor made of this effect is called magnetoelectric sensor.

The response band is about 7 μ m, the time constant is small, the response speed is fast, there is no bias, the internal resistance is very low, the noise is small, and it has good stability and reliability. However, its sensitivity is low and it is difficult to make low noise preamplifier, which affects its use.

Magnetoelectric Sensor Module

Ⅶ. Application and Prospect of infrared sensor

 (1) The application of infrared sensor is mainly reflected in the following aspects:

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

    2. Search and tracking system: used to search and track infrared target, determine its spatial position and track its motion.

    3. Thermal imaging system: it can form the infrared radiation distribution image of the whole target.

    4. Infrared ranging system: to measure the distance between objects. (it uses the non-proliferation principle of infrared propagation, because the refractive index of infrared is very small when it passes through other substances, so infrared will be considered in long-distance distance distance rangefinders.)

    5. Communication system: infrared communication as a way of wireless communication.

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

Infrared radiometer

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

 (2) Development trend

    1. Intellectualization: at present, the infrared sensor is mainly used in combination with peripheral equipment. The built-in microprocessor of the intelligent sensor can realize the two-way communication between the sensor and the control unit. It has the advantages of miniaturization, digital communication, simple maintenance, etc., and it can work independently as a module.

    2. Miniaturization: an inevitable trend of sensor miniaturization. Now in application, because of the volume problem of infrared sensor, its use degree is far worse than that of thermoelectric corner. Therefore, whether the infrared sensor is miniaturized and portable or not can't be ignored.

    3. High sensitivity and high performance: in medicine, the infrared sensor has been widely used for the measurement of human body temperature, but it can not replace the existing temperature measurement method due to its low accuracy. Therefore, the high sensitivity and high performance of infrared sensor is the inevitable trend of its future development.

Ⅷ. Summary

Although there are many deficiencies in the current infrared sensor, the infrared sensor has played a huge role in the modern production practice. With the improvement of detection equipment and other parts of the technology, the infrared sensor can have more performance and better sensitivity, and will have a broader application range.


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