Infrared sensors are sensors that use the physical properties of infrared rays to measure. Infrared light has the properties of reflection, refraction, scattering, interference, and absorption. Any substance, as long as it has a certain temperature (above absolute zero), can radiate infrared rays. The infrared sensor does not directly contact the measured object during measurement, so there is no friction, and it has the advantages of high sensitivity and fast response.

The infrared sensor includes an optical system, a detection element, and a conversion circuit. Optical systems can be divided into two types: transmission type and reflection type according to different structures. The detection element can be divided into thermal detection elements and photoelectric detection elements according to the working principle. The most widely used thermal element is a thermistor. When the thermistor is exposed to infrared radiation, the temperature increases, and the resistance changes (this change may be larger or smaller because thermistors can be divided into PTC thermistors and NTC thermistors), which is converted into an electrical signal output by a conversion circuit. Photodetection elements are commonly used photosensitive elements, usually made of lead sulfide, lead selenide, indium arsenide, antimony arsenide, mercury cadmium telluride ternary alloy, germanium and silicon doping, and other materials.

Infrared sensors are commonly used in non-contact temperature measurement, gas composition analysis, and non-destructive testing, and are widely used in medicine, military, space technology, and environmental engineering. For example, using infrared sensors to measure the thermal image of the human body surface temperature from a distance, you can find the parts with abnormal temperature and diagnose and treat the disease in time (see thermal imager). A wide range of weather forecasts can be realized by using infrared sensors on artificial satellites to monitor the earth’s clouds. It can also be used to detect overheating of running engines on aircraft.

Telescopes with infrared sensors can be used in military operations, in forest warfare to detect enemies in dense forests, and in urban warfare to detect enemies behind walls, all of which use infrared sensors to measure the surface temperature of the human body to know where the enemy is.

Infrared Sensor Working Principle

In the following part, Easybom will elaborate on how infrared sensor works?

According to different emission methods, infrared sensors can be divided into two types: active and passive.

Active infrared sensor

The transmitter of the active infrared sensor emits a modulated infrared beam, which is received by the infrared receiver, thereby forming a warning line composed of infrared beams. When it is blocked by leaves, rain, small animals, snow, sand, and fog, the alarm should not be reported, and the alarm will be issued when people or objects of considerable size are blocked.

Active infrared detector technology mainly adopts one transmitter and one receiver, which belongs to linear prevention. Now it has developed from the original single beam to multi beams, and it can also double transmitter and receiver to minimize the false alarm rate, thereby enhancing the stability and reliability of the product.

Since infrared is a detection medium with good incoherence of environmental factors (for sound, lighting, vibration, various artificial light sources, and electromagnetic interference sources in the environment, it has good incoherence); it is also a product with good coherence of target factors (Only the target that blocks the infrared beam will trigger the alarm), so the active infrared sensor will be further promoted and applied.

Passive infrared sensor

Passive infrared sensors work by detecting infrared rays emitted by the human body. The sensor collects the infrared radiation from the outside world and concentrates it on the infrared sensor. Infrared sensors usually use pyroelectric elements, which will release charges when receiving infrared radiation temperature changes, and generate an alarm after detection and processing.

This sensor is aimed at detecting human radiation. Therefore, the radiation-sensitive element must be very sensitive to infrared radiation with a wavelength of around 10 μm. In order to be sensitive to the infrared radiation of the human body, its radiation surface is usually covered with a special filter, so that the interference of the environment is obviously controlled.

A passive infrared sensor consists of two pyroelectric elements connected in series or parallel to each other. Moreover, the polarization directions of the two electrodes are opposite, and the background radiation of the environment has almost the same effect on the two pyroelectric elements so that the electric discharge effects cancel each other out, so the detector has no signal output.

Once the intruder enters the detection area, the infrared radiation of the human body is focused by part of the mirror and received by the pyroelectric element, but the heat received by the two pyroelectric elements is different, and the pyroelectricity is also different, which cannot be offset. An alarm is generated after signal processing. Passive infrared sensors are widely used in human infrared detectors.

According to the different energy conversion methods, infrared sensors can be divided into two types: photonic type and pyroelectric type.

Photonic infrared sensor

Photonic infrared sensors are sensors that work by utilizing the photon effect of infrared radiation. The so-called photon effect refers to that when infrared rays are incident on some semiconductor materials, the photon flow in the infrared radiation interacts with the electrons in the semiconductor material, changing the energy state of the electrons, thereby causing various electrical phenomena.

By measuring changes in electronic properties in semiconductor materials, the intensity of the corresponding infrared radiation can be known. The main types of photon detectors are internal photodetectors, external photodetectors, free carrier detectors, QWIP quantum well detectors, etc.

The main features of photon detectors are high sensitivity, fast response speed, and high response frequency, but the disadvantage is that the detection band is narrow and generally works at low temperature (to maintain high sensitivity, liquid nitrogen or thermoelectric refrigeration is often used, etc.) Cool the photon detector to a lower operating temperature) by the way of liquid nitrogen or thermoelectric cooling.

Pyroelectric infrared sensor

Pyroelectric infrared sensors use the thermal effect of infrared radiation to cause the temperature change of the component itself to detect certain parameters, and its detection rate and response speed are not as good as photon sensors.

However, since it can be used at room temperature and its sensitivity is independent of wavelength, it has a wide range of applications. Pyroelectric infrared sensors utilizing the pyroelectric effect of ferroelectrics have high sensitivity and are widely used.

Pyroelectric effect

When some insulating materials are heated, as the temperature rises, electric charges of equal numbers and opposite signs will be generated at both ends of the crystal. This phenomenon of electrical polarization due to thermal changes is called the pyroelectric effect. The pyroelectric effect has been used in pyroelectric infrared sensors for the past decade. The crystal that can produce a pyroelectric effect is called the pyroelectric body, also known as a pyroelectric element. Commonly used materials for thermoelectric elements include single crystals, piezoelectric ceramics, and polymer films.

The structure of the pyroelectric infrared sensor

The pyroelectric infrared sensor consists of the following four main parts:

①Aluminum substrate and field-effect transistor (FET) constituting the circuit;

② Ceramic materials with pyroelectric effect;

③ Window materials that limit incident infrared wavelengths;

④ Shell TO-5 type cap and socket.

When the detector element is used alone, the detection distance is short, and the obtained signal is not easy to be processed by the subsequent circuit, so at present, infrared assemblies are mostly used for detection. The infrared assembly is composed of a pyroelectric infrared sensor, a lens, a measurement conversion circuit, and a sealed tube shell. The lens can expand the detection range and improve the sensitivity of the measurement; the measurement conversion circuit can complete the signal processing processes such as filtering and amplification; the sealed tube shell can prevent wrong actions caused by external noise. This assembly is small in size, low in cost and versatile in functions, so it is widely used.

Infrared temperature sensor

In nature, when the temperature of an object is higher than absolute zero, due to the existence of its internal thermal motion, it will continuously radiate electromagnetic waves around it, including infrared rays with a wavelength range of 0.75 to 100 μm. The infrared temperature sensor is produced by the principle.

Infrared ray

An infrared ray is a kind of light invisible to the human eye, but in fact, it is an objective substance like any other light. Any object as long as its temperature is higher than thermodynamic zero will have infrared radiation to the surrounding. Infrared is the light outside the red light in visible light, so it is called infrared. Its wavelength range is roughly in the spectral range of 0.75 to 100 μm.

Infrared radiation

The physical nature of infrared radiation is thermal radiation. The higher the temperature of the object, the more infrared radiation it radiates, and the stronger the energy of the infrared radiation. Studies have found that the thermal effects of various monochromatic lights in the solar spectrum gradually increase from violet light to red light, and the largest thermal effect occurs within the frequency range of infrared radiation, so people call infrared radiation thermal radiation.

Infrared motion sensor

In nature, all objects radiate infrared rays, so using the detector to measure the infrared difference between the target itself and the background, different infrared images can be obtained. The motion sensor can detect the infrared rays emitted by the moving person or animal, and output the switch signal, which can be applied to various occasions that need to detect the moving human body.

The human body has a constant body temperature, which is generally 37 degrees, so it emits infrared rays with a specific wavelength of about 10UM. The infrared probe built into the motion sensor can just detect the infrared rays of about 10UM emitted by the human body. About 10UM of infrared light emitted by the human body is enhanced by the Fresnel filter and then concentrated on the infrared induction source. The infrared induction source usually adopts a pyroelectric element, which will lose its charge equilibrium when the temperature of the infrared radiation received by the human body changes. The charges are released outwards, thus producing electrical signals. The infrared rays radiated by the human body are detected to achieve the purpose of detecting human movement.

Infrared proximity sensor

The realization of proximity sensing is to judge the approaching or leaving by the infrared energy emitted by the sensor detective IRLED and the amount of infrared energy reflected back by the external blocking. The following figure shows the structure of the general mobile phone proximity sensor.

Infrared distance sensor

An infrared distance sensor is a kind of sensor using infrared rays as the medium. It features a wide measurement range and a short response time. It is widely used in modern scientific technology, national defense, as well as industrial and agriculture fields. The infrared distance sensor has a pair of infrared signals emitting and receiving diodes. The infrared distance sensor LDM301 is used to emit infrared light, forming a reflection process after it radiates the object. This reflection is received by the sensor, and then CCD is used to process the data. The distance is calculated by the signal processor.

Infrared Sensor Applications

Flame detector

The flame sensor uses a special infrared receiver tube to detect the flame based on the infrared rays being very sensitive to flames and then converts the brightness of the flame into a level signal with high and low changes, which is input to the central processing unit. The CPU conducts the corresponding process according to the change of the signal.

The flame sensor can detect infrared light with a wavelength in the range of 700 nanometers to 1000 nanometers, and the detection angle is 60°. The sensitivity of the infrared wavelength can reach the maximum value of around 880 nanometers.

The far-infrared flame probe converts the change of the intensity of the external infrared light into the change of the current, which is reflected as the change of the value in the range of 0 to 255 through the A/D converter. The stronger the external infrared light, the smaller the value; the weaker the infrared light, the larger the value.

The infrared distance sensor uses the principle that the intensity of the reflection of the infrared signal is different at different distances from the obstacle to detect the distance of the obstacle. The infrared distance sensor has a pair of infrared signals transmitting and receiving diodes. The transmitting tube emits infrared signals of a specific frequency, and the receiving tube receives the infrared signals of this frequency. When the infrared detection direction encounters obstacles, the infrared signals are reflected back and received by the tube receives, after processing, they return to the central processing unit through the digital sensor interface, and the central processing unit can use the infrared return signal to identify changes in the surrounding environment.

Infrared thermometer

The composition of the infrared thermometer mainly consists of the optical system, modulator, infrared sensor amplifier, indicator, and other parts. Infrared sensors are devices that receive target radiation and convert them into electrical signals.

Infrared imaging

In many cases, people not only need to know the average temperature of the surface of the object, but also need to know the temperature distribution of the object to analyze, study the structure of the object, and detect internal defects. Infrared imaging can visually display the temperature distribution of an object in the form of an image.

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