Infrared sensors consist of an optical system, a detection element, and a conversion circuit. Optical systems can be classified into two types based on their structure: transmissive and reflective. Detection elements can be classified into thermal detection elements and photoelectric detection elements based on their working principle. The most commonly 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 by the conversion circuit. The most commonly used photoelectric detection element is the photosensitive element, typically made of materials such as lead sulfide, lead selenide, indium arsenide, antimony arsenide, mercury cadmium telluride ternary alloy, germanium, and silicon doped materials.
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, using infrared sensors to measure the surface temperature of the human body remotely in thermal images can identify areas with abnormal temperatures, enabling timely diagnosis and treatment of diseases (see thermal imager); using infrared sensors on artificial satellites to monitor Earth's clouds can achieve 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.
Types of infrared sensors
Infrared sensors can be categorized based on their operation:
1. A portion of the infrared radiation is converted into heat, and the thermal model is used to extract the output signals such as resistance value change and electromotive force.
2. Quantum type, which utilizes the photoelectric effect of absorbing energy difference by semiconductor migration phenomenon and the photoelectromotive force effect due to PN junction.
The phenomenon of thermal pyrolysis is commonly known as the pyrolysis effect, and the most representative examples include the thermal bolometer, the thermopile, and the thermoelectric element.
Thermal infrared sensor
Advantages: Can operate at room temperature; wavelength dependence (sensitivity varies greatly with different wavelengths) does not exist; low cost.
Disadvantages: Low sensitivity and slow response (mS spectrum).
Quantum infrared sensor
Advantages: High sensitivity and fast response (μS spectrum);
Disadvantages: Requires cooling (liquid nitrogen), wavelength-dependent, and relatively high price;
Infrared sensors, in particular, utilize the sensitivity of the far-infrared range for human detection. The wavelength of infrared light is longer than that of visible light but shorter than that of radio waves. Infrared light may seem to be 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 only difference is the wavelength, which varies depending on the object's temperature.
