Analysis of image monitoring system based on infrared

1 Introduction

Vision is the most important sensory organ of human beings. Image information is the main source for people to obtain information from the objective world, accounting for 80% of the total amount of information that people rely on facial features to obtain from the outside world. Using image technology to monitor important objects is undoubtedly a monitoring method Among them, getting the most information is also the most intuitive way. In recent years, with the further development of urban commercialization, institutions such as hotels, large supermarkets, stock exchanges, and high-rise buildings have sprung up in every city. Although the businesses they conduct are very different, none of them An exception is the use of image monitoring systems. The current monitoring system generally works continuously for 24 hours, and there is a lot of redundant information in the image information recorded by the recorder. This paper proposes a pyroelectric infrared sensor and introduces it into the camera channel of the image monitoring system. Using it to sense the characteristics of trace infrared rays emitted by a moving human body in the form of telemetry, the image monitoring system is used to shoot when someone appears or walks, thereby effectively reducing redundant information and making truly practical information for the system administrator to observe And processing.

2. Pyroelectric infrared sensor

2.1 Infrared Infrared is an electromagnetic wave. The position in the electromagnetic spectrum is at the infrared end, and the wavelength range is roughly 0.76um-1000um. Infrared rays, along with visible light, ultraviolet rays, X-rays, gamma rays and radio waves, constitute infinite continuous electromagnetic waves.

Theoretical analysis and experimental research show that not only infrared light in sunlight, but any object, as long as the temperature is higher than absolute zero, it will continuously radiate infrared light. Therefore, infrared rays are ubiquitous in nature. Our human body is also an infrared radiation source. It is not only different in infrared radiation energy from other indoor objects in the room, but also the peak wavelength of the infrared radiation radiated by each body. The infrared peak wavelength of human radiation is about 10um.

2.2 Structure and its main components The pyroelectric infrared sensor is composed of a case, a Fresnel lens, a PIR infrared sensor, a dedicated IC (WT8072) and other circuits.

l Fresnel lens Fresnel lens is a precision optical device, usually a thin sheet made of polyethylene injection molded with precise and detailed texture. At present, infrared plastic lenses with excellent performance are often used-Fresnel lenses with multi-layer beam structure. The structure of the three-layer multi-field Fresnel lens group is shown in Figure 1

The lens is composed of three layers. It uses twenty-four small lenses to divide the field of view of the warning area into twenty-four small fields of view (also called sensitive areas). Each sensitive area is separated by a blind area. The infrared radiation from each small field of view is converged onto the infrared sensor located below the concave surface of the lens. Although the size of the blind zone is much smaller than the size of the sensitive zone, when the human body moves in the guard zone, from the blind zone to the sensitive zone or from the sensitive zone to the blind zone, the infrared radiation emitted by the human body is repeatedly The blind zone is blocked, and only infrared radiation that passes through the sensitive zone can be received by the infrared thermoelectric element.

l PIR infrared sensor
PIR infrared sensor (also known as infrared pyroelectric sensor), this material has a pyroelectric effect. The so-called pyroelectric effect refers to the effect that the intensity of spontaneous polarization in this kind of material changes with temperature. Under the action of an external electric field, the dielectric will be polarized, that is, one surface of the dielectric will be positively charged, while the other surface will be negatively charged. For pyroelectric materials, the phenomenon that they can still maintain this kind of polarization state after the applied voltage is removed is called "spontaneous polarization". The intensity of this spontaneous polarization depends on the temperature. As the temperature increases, the polarization intensity decreases, that is, the charge per unit area decreases. Pyroelectric materials are used to make extremely thin sheets and make them form spontaneous polarization. When it is irradiated by an infrared radiation source, the temperature of the sheet will increase, the polarization intensity will weaken, and the surface charge will decrease. This part of the charge released is usually called pyroelectric charge. Because the amount of pyroelectric charge can reflect the temperature change of the material, the output voltage converted by the pyroelectric charge through the circuit can also reflect the temperature change of the material, thereby detecting the change of infrared radiation energy.

The optical system of the infrared sensor can concentrate the infrared radiation energy from multiple directions through reflection or special lens transmission, and then concentrate it on the infrared transmitter. In this way, the infrared detector can detect the change of radiation in a certain three-dimensional defense space. When there is no moving human body and other targets in the protection area, because all background objects have relatively small infrared radiation energy at room temperature and are basically stable, the camera cannot be triggered, that is, the camera does not work. When a human body moves in the detection area, it will cause changes in the infrared thermal radiation energy. The infrared sensor converts the changes in the received infrared thermal radiation energy between the moving human body and the background object into corresponding electrical signals, which are sent to the camera after proper processing, and then the image is taken. Using Fresnel lens group to detect moving human body is shown in Figure 2.

The detection wavelength range of the PIR infrared sensor is 8-14um. Since the infrared radiation wavelength of the human body is within this detection wavelength range, it can detect the moving human body better.

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