According to the classification, infrared sensors can be divided into thermal sensors and photon sensors.

Thermal sensor

The thermal detector uses the detection element to absorb infrared radiation to produce a temperature rise, and then accompanied by changes in certain physical properties. Measuring the changes in these physical properties can measure the energy or power it absorbs. The specific process is as follows: The first step is to absorb infrared radiation by the thermal detector to cause a temperature rise; the second step is to use some temperature effects of the thermal detector to convert the temperature rise into a change in electricity. There are four types of physical property changes commonly used: thermistor type, thermocouple type, pyroelectric type, and Gaolai pneumatic type.

# Thermistor type

After the heat-sensitive material absorbs infrared radiation, the temperature rises and the resistance value changes. The magnitude of the resistance change is proportional to the absorbed infrared radiation energy. Infrared detectors made by changing the resistance after a substance absorbs infrared radiation are called thermistors. Thermistors are often used to measure thermal radiation. There are two types of thermistors: metal and semiconductor.

R(T)=AT−CeD/T

R(T): resistance value; T: temperature; A, C, D: constants that vary with the material.

The metal thermistor has a positive temperature coefficient of resistance, and its absolute value is smaller than that of a semiconductor. The relationship between resistance and temperature is basically linear, and it has strong high temperature resistance. It is mostly used for temperature simulation measurement;

Semiconductor thermistors are just the opposite, used for radiation detection, such as alarms, fire protection systems, and thermal radiator search and tracking.

# Thermocouple type

Thermocouple, also called thermocouple, is the earliest thermoelectric detection device, and its working principle is pyroelectric effect. A junction composed of two different conductor materials can generate electromotive force at the junction. The end of the thermocouple receiving radiation is called the hot end, and the other end is called the cold end. The so-called thermoelectric effect, that is, if these two different conductor materials are connected into a loop, when the temperature at the two joints is different, current will be generated in the loop.

In order to improve the absorption coefficient, black gold foil is installed on the hot end to form the material of the thermocouple, which can be metal or semiconductor. The structure can be either a line or a strip-shaped entity, or a thin film made by vacuum deposition technology or photolithography technology. Entity type thermocouples are mostly used for temperature measurement, and thin-film type thermocouples (consisting of many thermocouples in series) are mostly used to measure radiation.

The time constant of the thermocouple type infrared detector is relatively large, so the response time is relatively long, and the dynamic characteristics are relatively poor. The frequency of the radiation change on the north side should generally be below 10HZ. In practical applications, several thermocouples are often connected in series to form a thermopile to detect the intensity of infrared radiation.

# Pyroelectric type

Pyroelectric infrared detectors are made of pyroelectric crystals or “ferroelectrics” with polarization. Pyroelectric crystal is a kind of piezoelectric crystal, which has a non-centrosymmetric structure. In the natural state, the positive and negative charge centers do not coincide in certain directions, and a certain amount of polarized charges are formed on the crystal surface, which is called spontaneous polarization. When the crystal temperature changes, it can cause the center of the positive and negative charges of the crystal to shift, so the polarization charge on the surface changes accordingly. Usually its surface captures floating charges in the atmosphere and maintains an electrical equilibrium state. When the surface of the ferroelectric is in electrical equilibrium, when infrared rays are irradiated on its surface, the temperature of the ferroelectric (sheet) rises rapidly, the polarization intensity drops quickly, and the bound charge decreases sharply; while the floating charge on the surface changes slowly. There is no change in the internal ferroelectric body.

In a very short time from the change in the polarization intensity caused by the temperature change to the electrical equilibrium state on the surface again, excess floating charges appear on the surface of the ferroelectric, which is equivalent to releasing a part of the charge. This phenomenon is called the pyroelectric effect. Since it takes a long time for the free charge to neutralize the bound charge on the surface, it takes more than a few seconds, and the relaxation time of the spontaneous polarization of the crystal is very short, about 10-12 seconds, so the pyroelectric crystal can respond to rapid temperature changes.

# Gaolai pneumatic type

When the gas absorbs infrared radiation under the condition of maintaining a certain volume, the temperature will increase and the pressure will increase. The magnitude of the pressure increase is proportional to the absorbed infrared radiation power, so the absorbed infrared radiation power can be measured. Infrared detectors made by the above principles are called gas detectors, and the Gao Lai tube is a typical gas detector.

Photon sensor

Photon infrared detectors use certain semiconductor materials to produce photoelectric effects under the irradiation of infrared radiation to change the electrical properties of the materials. By measuring the changes in electrical properties, the intensity of infrared radiation can be determined. The infrared detectors made by the photoelectric effect are collectively called photon detectors. The main features are high sensitivity, fast response speed and high response frequency. But it generally needs to work at low temperatures, and the detection band is relatively narrow.

According to the working principle of the photon detector, it can be generally divided into an external photodetector and an internal photodetector. Internal photodetectors are divided into photoconductive detectors, photovoltaic detectors and photomagnetoelectric detectors.

# External photodetector (PE device)

When light is incident on the surface of certain metals, metal oxides or semiconductors, if the photon energy is large enough, the surface can emit electrons. This phenomenon is collectively referred to as photoelectron emission, which belongs to the external photoelectric effect. Phototubes and photomultiplier tubes belong to this type of photon detector. The response speed is fast, and at the same time, the photomultiplier tube product has a very high gain, which can be used for single photon measurement, but the wavelength range is relatively narrow, and the longest is only 1700nm.

# Photoconductive detector

When a semiconductor absorbs incident photons, some electrons and holes in the semiconductor change from a non-conductive state to a free state that can conduct electricity, thereby increasing the conductivity of the semiconductor. This phenomenon is called the photoconductivity effect. Infrared detectors made by the photoconductive effect of semiconductors are called photoconductive detectors. At present, it is the most widely used type of photon detector.

# Photovoltaic detector (PU device)

When infrared radiation is irradiated on the PN junction of certain semiconductor material structures, under the action of the electric field in the PN junction, the free electrons in the P area move to the N area, and the holes in the N area move to the P area. If the PN junction is open, an additional electric potential is generated at both ends of the PN junction called the photo electromotive force. Detectors made by using the photo electromotive force effect are called photovoltaic detectors or junction infrared detectors.

# Optical magnetoelectric detector

A magnetic field is applied laterally to the sample. When the semiconductor surface absorbs photons, the electrons and holes generated are diffused into the body. During the diffusion process, the electrons and holes are offset to both ends of the sample due to the effect of the lateral magnetic field. There is a potential difference between both ends. This phenomenon is called the opto-magnetoelectric effect. Detectors made of photo-magnetoelectric effect are called photo-magneto-electric detectors (referred to as PEM devices).