Working Principle and Application of Infrared Sensors

Catalog

I Introduction

Any object in the universe can produce infrared radiation as long as its temperature exceeds zero degrees. In fact, like visible light, its radiation can be refracted and reflected, which makes infrared technology come into life.

Today, with the development of technology, automatic control and detection are playing a more and more important role in industrial control and people's daily lives, and the sensor is an important component in automatic control and the information collection system. The sensor converts the measurand to a signal suitable for transmission or detection (generally an electrical signal), and then uses the computer or circuit equipment to process the signal output by the sensor to achieve automatic control. Since the response time of the sensor is generally relatively short, industrial production can be controlled in real-time through the computer system.

The infrared sensor is a common type of sensor. Infrared sensors are used to detect infrared radiation, and any object in nature will radiate infrared energy to the outside as long as it is above absolute zero, so they are very practical. Besides, many practical sensor modules can be designed such as infrared thermometer, infrared imager, infrared alarms, automatic door control system, etc. based on infrared sensors.

Infrared sensors use the physical properties of infrared to measure. Infrared has the properties of reflection, refraction, scattering, interference, absorption and so on. It is invisible light, and its spectrum lies outside the red in visible light, so it is called infrared light.

Infrared Radiation

II Performance Parameters of Infrared Sensors

1. Voltage Response

When (modulated) infrared radiates on the sensitive surface of the sensor, the ratio of the output voltage to the input infrared radiant power is called the voltage response rate, denoted as RV. In the formula:

Us: output voltage of the infrared sensor

P0: power per unit area projected onto the infrared-sensitive element

A: the area of the sensitive element of the infrared sensor

2. Response Wavelength Range

Curve 1: The voltage response rate curve of the thermoelectric sensor (irrelevant to the wavelength).

Curve 2: The voltage response rate curve of the photonic sensor.

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

(2) The wavelength corresponding to the maximum response rate is generally called the peak wavelength.

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

3. Noise Equivalent Power(NEP)

If the output voltage generated by the radiated power projected on the sensitive element of the infrared sensor is exactly equal to the noise voltage of the sensor itself, then this radiated power is called "noise equivalent power", usually indicated as "NEP".

Among them:

Us: the output voltage of the infrared detector;

P0: the power projected onto the unit area of the infrared-sensitive element;

A0: the area of the infrared-sensitive element;

UN: the comprehensive noise voltage of the infrared sensor;

RV: the voltage response rate of the infrared sensor.

4. Detectivity

The detectivity is the reciprocal of the noise equivalent power, namely:

The higher the detectivity of the infrared sensor, the smaller the minimum radiation power that the sensor can detect, and the more sensitive the sensor.

5. Specific Detectivity

The specific detectivity is also called the normalized detectivity. In essence, it's the ratio of the signal voltage to the noise voltage obtained by the radiation of unit power, when the area of the sensitive element of the sensor is the unit area and the bandwidth Δf of the amplifier is 1 Hz, usually represented by the symbol D*. 

The physical dimension of D*:

6. Time Constant

The time constant indicates the rate at which the output signal of the infrared sensor changes when the infrared radiation changes.

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

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

The thermal sensor has large thermal inertia, RC parameters. Its time constant is larger than the photonic sensor, which is generally in the order of milliseconds, while the time constant of the photon sensor is generally in the order of microseconds.

III Basic Law of Infrared Radiation

1. Kirchhoff's Law

At a certain temperature, the ratio of the radiant flux W per unit area of the ground object to the absorption rate is a constant for any object, which is equal to the radiant flux W of the blackbody of the same area at that temperature. At a given temperature, the emissivity = the absorption rate (same band). That means the greater the absorption rate, the greater the emissivity.

The heat radiation intensity of the ground object is proportional to the fourth power of the temperature. Therefore, a small temperature change will cause a significant change in infrared radiation energy. This feature constitutes the theoretical basis of infrared remote sensing.

2. Boltzmann's Law

The total radiant flux of the blackbody increases rapidly with the increasing temperature, which is proportional to the fourth power of the temperature. Therefore, a small change in temperature will cause a great change in radiant flux density. This is the theoretical basis for the infrared device to measure temperature.

3. Wien's Displacement Law

As the temperature increases, the peak wavelength of the maximum radiation moves in the short-wave direction.

IV Classification and Working Principle of Infrared Sensors

Infrared sensors can be divided into thermal sensors and photonic sensors.

1. Thermal Sensors

The thermal detector absorbs all the incident radiation energy of various wavelengths. It uses the thermal effect of radiation to cause a temperature rise when the detection element receives radiant energy, which in turn changing the temperature-related properties of the detector. By detecting a change in one of these properties, we can also detect the radiation.

The main types of thermal sensors are thermal sensitive sensor type, thermocouple type, Golay cell type, and pyroelectric type.

Liquid Level Thermal Sensor

(1) Thermistor Sensors

The thermistor is made of mixed oxides of manganese, nickel, and cobalt after sintering. The thermistor is generally made into a thin sheet. When infrared radiates on the thermistor, its temperature increases, and the resistance value decreases. By measuring the resistance change of the thermistor, we can obtain the intensity of the incident infrared radiation, so that the temperature of the object can be determined.

(2) Thermocouple Sensors

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

(3) Golay Cell

When the gas absorbs infrared radiation, its temperature rises, and the volume increases. By using this characteristic of gas, the Golay cell can reflect the intensity of infrared radiation. The Golay cell has an air chamber connected to a flexible sheet through a small pipe. The side of the sheet away from the pipe is a mirror. There is an absorption film attached to the front of the air chamber, which is a thin film with low thermal capacity.

The infrared radiates on the absorption film through the window, and the absorption film transfers the absorbed heat energy to the gas, increasing the temperature of the gas and the air pressure, 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 grid-like diaphragm, and the grid-like image reflected by the flexible mirror is projected onto the photocell through the diaphragm.

When the flexible mirror moves due to the pressure changes, there will be a relative displacement between the grid-like image and the grid-like beam, so that the amount of light falling on the photocell changes, and the output signal of the photocell also changes. This variation reflects the intensity of the incident infrared radiation.

The Golay Cell is characterized by high sensitivity and stable performance. However, it has a long response time, complex structure, and low intensity, so it is only suitable for the laboratory.

Schematic of a Golay cell

(4) Pyroelectric Sensors

The pyroelectric sensor is a kind of thermal crystal or “ferroelectric” with the polarization phenomenon. The polarization strength (charges per unit area) of ferroelectrics is related to the temperature. When infrared radiates on the surface of the ferroelectric sheet that has been polarized, the temperature of the sheet will increase, reducing the polarization intensity and the number of charges on the surface. This is equivalent to releasing a part of the charge, so it is called a pyroelectric sensor.

If the load resistor is connected to the ferroelectric sheet, an electrical signal output will be generated on the load resistance. The size of the output signal depends on the speed of the temperature change of the sheet, which reflects 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 change rate of the incident radiation. The sensor has no electrical signal output when constant infrared radiates on the pyroelectric sensor. Only when the temperature of the ferroelectric is changing, there will be an electric signal output. Therefore, the infrared radiation must be modulated to make the constant radiation into alternating radiation, continuously changing the temperature so as to cause the pyroelectricity to generate the alternating signal output.

Working of a Pyroelectric Sensor

2. Photonic Sensors

An infrared sensor made with the photon effect is a photonic sensor. Photonic sensors use some semiconductor materials to generate the photonic effect under the incident light, changing the electrical properties of the materials. By measuring the changes in electrical properties, we can know the intensity of infrared radiation.

The photonic sensor has high sensitivity, fast response speed, and high response frequency. However, it is generally required to work at low temperatures and the detection band is relatively narrow.

According to the working principle of the photonic sensor, it can be generally divided into two types: the internal photoelectric sensor and the external photoelectric sensor. The latter is subdivided into the photoconductive sensor, photovoltaic sensor, and photomagnetoelectric sensor.

(1) External Photoelectric Sensors

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 material can escape the surface. This phenomenon is called the external photoelectric effect or photoelectron emission effect. Photodiodes and photomultipliers are typical types of external photoelectric sensors.

External photoelectric sensors have fast response speed, generally only a few nanoseconds. However, electron escape requires large photon energy and is only suitable in the range of near-infrared radiation or visible light.

(2) Photoconductive Sensors

When infrared is irradiated on the surface of some semiconductor materials, some electrons and holes in the semiconductor material can be changed from the non-conductive restrained state to a conductive free state, which increases the conductivity of the semiconductor. This phenomenon is called the photoconductive phenomenon, and sensors made with the photoconductive phenomenon are called photoconductive sensors. Common materials are lead sulfide, lead selenide, indium antimonide, and mercury telluride. When we use a photoconductive sensor, it is necessary to cool and add a certain bias voltage, otherwise, the response rate will be reduced, resulting in loud noise and narrow response band, damaging the infrared sensor.

(3) Photovoltaic Sensors

When the infrared radiates on the PN junction of semiconductor materials, under the action of the electric field in the junction, free electrons move to the N region. If the PN junction is open, an additional potential is generated at both ends of the PN junction, called the photo-electromotive-force effect. Photovoltaic sensors are made with this effect. Commonly used materials are indium arsenide, indium antimonide, mercury telluride, lead-tin telluride.

Infrared Photoconductive&Photovoltaic Sensors

(4) Photomagnetoelectric Sensors

When infrared radiates on the surface of some semiconductor materials, some electrons and holes in the semiconductor material will diffuse inward. If a strong magnetic field is applied during the diffusion, the electrons and holes will separately deflect to one side, thus generating an open circuit voltage. This is called the photomagnetoelectric effect, and infrared sensors made based on this are called photomagnetoelectric sensors.

The photomagnetoelectric sensor does not need to be refrigerated, and the response band can reach about 7μM. Besides, it has a small time constant, fast response speed, low internal resistance, small noise, good stability and reliability, and no bias voltage is required. However, its sensitivity is low and the low-noise pre-amplifiers are difficult to produce.

V Application of Infrared Sensors

The infrared sensors have the following advantages in applications:

• Its environmental adaptability is better than visible light, especially working at night and in bad weather;

• It has good concealing performance, generally passively receiving the target signal, which is safer and more confidential than radar and laser detection, and is not easy to be interfered;

• It detects with the infrared radiation characteristic formed by the temperature difference and emissivity difference between the target and the background, thus, its ability to identify the camouflaged target is better than that of the visible light;

• Compared with the radar system, the infrared system has small size, lightweight, and low power consumption.

According to the above-mentioned performance characteristics of infrared sensors, we can develop a variety of different infrared detectors, which are mainly used in the following fields according to the application functions and places.

1. Radiation and Spectrum Measurement

Infrared sensors are widely used in these fields. The pyrgeometer based on medium infrared radiation measurement can be used for observations of climate change such as global warming; infrared space telescope based on far-infrared radiation measurement can be used for astronomical observation of cosmic bodies; Meteorological satellites with infrared spectrum scanning radiometer can help analyze the meteorological observation of clouds.

A Pyrgeometer

In industrial and mining enterprises, infrared thermometers based on radiant quantity measurement and infrared analyzers based on infrared spectra measurement are commonly used.

2. Search and Track System

The familiar short-range combat air-to-air missile carried by fighter planes uses the infrared track system. It is based on the electromagnetic radiation waves emitted by the target in the infrared spectrum to search and track the spatial position and movement path of the infrared target.

The image quality of the infrared searcher tracker depends on the spatial resolution related to the pixel size and number of pixels. In other words, the larger the number of pixels, the smaller the pixel size, leading to a clearer displayed image and farther searchable distance.

Infrared Search and Tranck System

3. Thermal Imaging System

The thermal imager uses infrared detectors and the optical objective lens to receive the infrared radiation energy of the target and reflect the energy distribution pattern on the photosensitive element of the infrared detector, thereby obtaining infrared thermography. This thermography is corresponding to the heat distribution on the surface of the object. Generally speaking, the thermal imager is to convert the invisible infrared energy emitted by the object into visible thermography. The different colors on the thermography represent the different temperatures of the measured object.

Any object with a temperature will emit the infrared. The thermal imager receives the infrared emitted by the object, and displays the temperature distribution on the surface of the object through a colored picture. According to the slight difference in temperature, we can find the abnormal point of the temperature for maintenance.

Thermal imagers were first developed for military purposes, and in recent years they have rapidly expanded into the civil and industrial fields as long as there are temperature differences. For example:

• In the construction field, they're used for checking hollows, defects, falling off of ceramic tiles, damping, heat bridges, etc.;

• In the field of fire protection, they're applied to find the fire source, determine the cause of the accident, and find the injured in the smoke;

• In the public security system, it can find people hiding at night;

• In the field of automobile production, it can detect the performance of tires, thermal fuse in the air conditioner, engine, exhaust pipes, etc.;

• In medicine, it can detect the effect of acupuncture, and find out diseases such as nasopharyngeal cancer and breast cancer in the early stage;

• In electric power, it can check the wiring, the junction, quick-closing valve, and substation, etc.

Basic Components of a Thermal Imaging System

4. Infrared Communication System

It is a system that uses the modulated infrared radiation beam to transmit the encoded data, and then the silicon photodiode converts the received infrared radiation signal into an electrical signal to achieve short-range communication. It does not interfere with the normal operation of other nearby equipment, and is especially suitable for indoor communication in densely populated areas. In addition, the communication system has low power consumption and low cost, which is safety and reliability.

Infrared sensor technology is also widely used in other applications such as the alarm of access control system, lighting control, fire detection, toxic and hazardous gas leak detection, infrared distance measurement, heating and ventilation.

Infrared Communication Network

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