Infrared sensors utilize the physical properties of infrared radiation for measurement. Infrared radiation, also known as infrared light, possesses properties such as reflection, refraction, scattering, interference, and absorption. Any substance with a certain temperature (above absolute zero) can radiate infrared radiation. Infrared sensors do not directly contact the object being measured, thus eliminating friction, and offer advantages such as high sensitivity and fast response. An infrared sensor comprises an optical system, a detection element, and a conversion circuit.
Optical systems can be classified into two categories based on their structure: transmission-type and reflection-type. Detection elements can be classified into thermal detection elements and photoelectric detection elements based on their working principle. The most widely used thermal element is the thermistor. When a thermistor is exposed to infrared radiation, its temperature rises, and its resistance changes (this change may increase or decrease, as thermistors can be classified as positive temperature coefficient thermistors or negative temperature coefficient thermistors). This change is converted into an electrical signal output through a conversion circuit.
Photosensitive elements are commonly used in photoelectric detection, and are typically made of materials such as lead sulfide, lead selenide, indium arsenide, antimony arsenide, mercury cadmium telluride ternary alloy, germanium, and silicon doping. Infrared sensors are commonly used for disease diagnosis and treatment (see thermal imagers); monitoring Earth's clouds using infrared sensors on artificial satellites enables large-scale weather forecasting; and infrared sensors can be used to detect overheating in aircraft engines, etc.
Telescopes equipped with infrared sensors can be used in military operations, such as detecting enemies in dense forests during woodland warfare and detecting enemies behind walls during urban warfare. Both of these methods utilize infrared sensors to measure the surface temperature of the human body in order to determine the location of the enemy.
An active infrared sensor emits a modulated infrared beam from its transmitter, which is received by an infrared receiver, thus forming a warning line composed of infrared beams. The sensor should not alarm if obstructed by leaves, rain, small animals, snow, dust, or fog; however, it will alarm if obstructed by a person or an object of similar size. Active infrared detector technology primarily uses a single transmitter and receiver, providing linear protection. It has evolved from the initial single-beam to multi-beam and even dual-transmitter/dual-receiver capabilities, minimizing false alarm rates and enhancing the product's stability and reliability.
Because infrared light is a detection medium with good environmental incoherence (it has good incoherence to sound, lightning, vibration, various artificial light sources and electromagnetic interference sources in the environment); and it is also a product with good target coherence (only targets that block the infrared beam will trigger an alarm), active infrared sensors will be further promoted and applied.
First, let's clarify a concept: what is infrared radiation? Infrared radiation refers to the range of wavelengths from 0.76 to 400 micrometers in the optical spectrum. Infrared radiation is invisible, and all matter above absolute zero (-273.15℃) emits it. Infrared sensors work on the principle of infrared reflection. When a part of the human body is within the infrared region, the infrared radiation emitted by the infrared emitter is reflected back to the infrared receiver due to the body's obstruction. This signal is then transmitted to a pulse solenoid valve via integrated circuitry. Upon receiving the signal, the solenoid valve controls the valve core according to the specified instructions.
An infrared sensor consists of three main parts: an optical system, a detection element, and a conversion circuit. The optical system can be classified into transmissive and reflective types based on its structure. The detection element can be classified into thermal detection elements and photoelectric detection elements based on its working principle. The most common thermal detection element is the thermistor. When a thermistor is exposed to infrared radiation, its temperature rises, its resistance changes, and this change is converted into an electrical signal output by the conversion circuit.
This sensor specifically utilizes the far-infrared range for human detection. As shown in Figure 1, the wavelength of infrared light is longer than visible light but shorter than that of radio waves. Infrared light is often perceived as being emitted only by hot objects, but this is not the case. All objects in nature, such as humans, fire, and ice, emit infrared light; the wavelengths simply vary depending on the object's temperature. For example, in Figure 1, the human body temperature is approximately 36–37°C, emitting far-infrared light with a peak value of 9–10 μm. Objects heated to 400–700°C emit mid-infrared light with a peak value of 3–5 μm. Thermoelectric infrared sensors utilize the thermoelectric effect, and their materials include dielectric ceramics, single-crystal lithium tantalate (LiTaO3), and organic materials such as PVDF.
Thermoelectric infrared sensors have the following characteristics:
(1) Since the infrared radiation emitted by the object is detected, the temperature of the object's surface can be sensed without direct contact. Therefore, the temperature of the human body and the temperature of the moving object can be measured in a non-contact manner.
(2) Thermoelectric infrared sensors receive infrared radiation emitted by the object being detected, and are therefore passive [see Figure 2(a)]. Since they are not active as shown in Figure (b), they do not require tedious work such as calibrating the optical axis of the transmitter and receiver.
Thermoelectric infrared sensors utilize the thermoelectric effect and are made of materials such as strong dielectric ceramics, single crystals like lithium tantalate, and organic materials like PVDF. Thermoelectric infrared sensors have the following characteristics:
(1) It detects infrared radiation emitted from an object and can sense the temperature of the object's surface without direct contact, so it can measure the temperature in a non-contact manner.
(2) Thermoelectric infrared sensors receive infrared radiation emitted by the object being detected. They are passive and do not require tedious work such as calibrating the optical axis of the projector and receiver.
(3) Thermoelectric effect is produced by temperature change and can only accept energy due to temperature change. Thermoelectric infrared sensor differentiates the voltage and outputs it.
The sensing module uses PZT high-dielectric ceramic. When a high voltage is applied to the sensing module, the positive and negative charges appearing on the module surface combine with the opposite charges in the air to achieve electrical neutralization. When the surface temperature of the module changes, the size of the sensing module's polarization changes with the temperature. Therefore, the charge neutralization state at the steady state collapses. Since the relaxation time of the surface charge of the sensing module and the attracted stray charge are different, an electrical imbalance is formed, resulting in unpaired charges, as shown in Figure (b).
Infrared sensors are commonly used for non-contact temperature measurement, gas composition analysis, and non-destructive testing, and are widely applied in fields such as medicine, military, space technology, and environmental engineering. For example, infrared sensors are used to remotely measure thermal images of human body surface temperature, to monitor Earth's clouds using infrared sensors on artificial satellites, and to detect overheating in aircraft engines.
