Common infrared sensors can be divided into thermal sensors and photonic sensors.
1. Thermal sensor
The thermal sensor uses the incident infrared radiation to cause the temperature change of the sensor, and then the relevant physical parameters change accordingly, and the infrared radiation absorbed by the infrared sensor is determined by measuring the changes of the relevant physical parameters.
The main advantage of the heat detector is that it corresponds to a wide wavelength band, can operate at room temperature, and is easy to use. However, thermal sensors have a long response time and low sensitivity, making them typically used in low-frequency modulation applications.
There are four main types of thermal sensors: thermal sensor type, thermocouple type, Colloids pneumatic type and heat release electric type.
1. Thermistor sensor
Thermistor sensors are made by mixing oxides of manganese, nickel and cobalt and then firing. Thermistors are generally made into thin sheets. When infrared radiation hits a thermistor, its temperature increases and its resistance value decreases. By measuring the change in the thermistor value, it is possible to know the intensity of the incident infrared radiation and thus determine the temperature of the object that produces the infrared radiation.
2. Thermocouple sensor
Thermocouple sensors consist of two materials with very different thermoelectric powers. When infrared radiation reaches the junction of the closed loop formed by the two metallic materials, the temperature of the junction increases. The other junction, which is not irradiated by infrared radiation, is at a lower temperature, at which point a thermal current is generated in the closed loop. At the same time, a thermoelectric potential is generated in the loop, and the magnitude of the thermoelectric potential reflects the contact absorption intensity of infrared radiation. Infrared sensors made with thermoelectric potential are called thermocouple infrared sensors. Due to its large time constant, long response time and poor dynamic characteristics, the modulation frequency should be limited to less than 10HZ.
3. Colloids pneumatic sensor
Colloids pneumatic sensors take advantage of the gas' ability to absorb infrared radiation, with an increase in temperature and volume to reflect the intensity of infrared radiation. It has an air chamber that is connected to a flexible sheet by a small tube. On the side of the sheet facing away from the pipe is a mirror. The front of the air battery has an absorber mold attached to it, which is a thin film with a low heat capacity. Infrared radiation is incident on the absorption mode through the window, which transfers the absorbed heat energy to the gas, which increases the temperature and pressure of the gas, thus 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, through which the grating image is projected onto the photocell. When the flexible mirror moves due to a change in pressure, the relative displacement of the grating image and the grating diaphragm changes the amount of light falling on the photocell, and the output signal of the photocell also changes. This change reflects the incident infrared light. Radiant intensity. The sensor has the characteristics of high sensitivity and stable performance. However, the response time is long, the structure is complex, and the strength is poor, so it is only suitable for laboratory use.
4. Pyroelectric sensor
A pyroelectric sensor is a thermal crystal or "ferroelectric" with polarization. The polarization (charge per unit area) of a ferroelectric is temperature-dependent. When infrared radiation irradiates the surface of the polarized ferroelectric sheet, the temperature of the sheet increases, the polarization strength decreases, and the surface charge decreases, which is equivalent to releasing a part of the charge, so it is called pyroelectricity. Sensor. If the load resistor is connected to the ferroelectric sheet, an electrical signal output is generated on the load resistor. The magnitude of the output signal depends on the speed at which the temperature of the sheet changes, thus reflecting the intensity of the incident infrared radiation. It can be seen that the voltage responsivity of the pyroelectric infrared sensor is proportional to the rate of change of the incident radiation. When constant infrared radiation hits a pyroelectric sensor, the sensor has no electrical signal output. Only when the temperature of the ferroelectric is changing can there be an electrical signal output. Therefore, the infrared radiation must be modulated (or chopped) so that the constant radiation becomes alternating radiation, and the temperature change of the sensor is constantly pyroelectric and the output of an alternating signal.
2. Photonic sensors
Photonic sensors use certain semiconductor materials to produce photon effects in response to incident light, thereby altering the electrical properties of the material. By measuring the change in electrical properties, the intensity of infrared radiation can be known. An infrared sensor made using the photon effect. Collectively known as photonic sensors. The main characteristics of photonic sensors are high sensitivity, fast response speed, and high response frequency. However, it generally has to work at low temperatures and has a narrow detection band.
According to the working principle of photonic sensors, they can generally be divided into two types: internal photoelectric and external photoelectric sensors, and the latter can be divided into three types: photoconductive sensors, photogenerated volt sensors and photomagnetoelectric sensors.
1. External photoelectric sensor
When light hits the surface of some material, the electrons of the material can escape from the surface if the photon energy of the incident light is large enough. This phenomenon is called the external photoelectric effect or the photoelectron emission effect. Photodiodes, photomultiplier tubes, etc., belong to this category of electronic sensors. It has a relatively fast response time, usually only a few nanoseconds. However, the escape of electrons requires a large photon energy and is only suitable for use in the near-infrared radiation or visible light range.
2. Photoconductive sensor
When infrared radiation irradiates 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, thereby improving the conductivity of semiconductors. This phenomenon is called the phenomenon of photoconductivity. Sensors made of photoconductivity phenomena are called photoconductive sensors, such as lead sulfide, lead selenide, indium antimonide, mercury telluride and other materials can be made into photoconductive sensors. When using a photoconductive sensor, cooling and a certain bias voltage are required, otherwise the response speed will be reduced, the noise will be large, and the response band will be narrow, resulting in damage to the infrared sensor.
3. Photogenerated volt sensor
When infrared radiation hits the PN junction of some semiconductor materials, the free electrons move towards the N region under the action of the electric field in the junction. If the PN junction is open, an additional electric potential is created on the PN junction called the photogenerated potential. Electromotive force. Sensors or PN junction sensors made using this effect. Commonly used materials are indium arsenide, indium antimonide, mercury telluride, lead tin telluride, etc.
4. Optical-magnetoelectric sensor
When infrared radiation hits the surface of certain semiconductor materials, some of the electrons and holes in the semiconductor materials diffuse inward. If they are affected by a strong magnetic field during diffusion, the electrons and holes will deviate in one direction, resulting in an open-circuit voltage. This phenomenon is called the photomagnetoelectric effect. Infrared sensors made using this effect are called photoelectric sensors.
The photoelectric sensor does not need to be refrigerated, the response bandwidth can reach about 7μM, the time constant is small, the response speed is fast, there is no need for bias voltage, the internal resistance is very low, the noise is small, and it has good stability and reliability. However, its sensitivity is low, and the production of low-noise preamplifiers is difficult, which affects the use.
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