Are the movies all lies? Does the red laser net that the flying thief "passes through" really exist?

We must have seen this scene in the movies. A flying thief sneaks into a highly guarded secret base, which is surrounded by an intricate network of red lasers. The laser lines are intertwined in the narrow passages, and the slightest touch will trigger a sharp alarm. So in reality, does such security technology really exist? Can these detections really be avoided through agile technology?

In fact, this security technology is similar to the real-life interrupted beam detection technology. This is a photoelectric detection method commonly used in security monitoring and intrusion detection. The core principle is to determine whether there is an intruder or an object moving by detecting whether the light beam is blocked by an object.

Figure 1 Interrupted beam detection technology

First of all, the system consists of two main components: a light source and a photosensitive element. To ensure that the detection beam is not seen by intruders, the light source is covered with an infrared filter, emitting infrared pulse beams, which form invisible rays intertwined in space. We know that light is a form of electromagnetic waves, and different wavelengths of electromagnetic waves correspond to different types of light or radiation. Arranged in order from short to long wavelengths, the electromagnetic spectrum includes gamma rays, X-rays, ultraviolet rays, visible light, infrared, microwaves, and radio waves.

Visible light is electromagnetic waves that the human eye can perceive, with a wavelength range of approximately 380 nanometers (violet light) to 760 nanometers (red light). The wavelength range of infrared light is longer than that of visible light, beyond the range of visible light for the human eye, so humans cannot observe red "infrared light" with their naked eyes like the characters in the movie.

Figure 2 Electromagnetic spectrum

Under normal working conditions of the system, the infrared beam emitted by the light source will pass through the space to the photosensitive element on the opposite side. The photosensitive element is a device that can detect light signals. When the beam is not blocked, the photosensitive element will continue to receive light signals from the light source, which means that the path is clear and there are no obstacles.

However, when an intruder enters the beam path, the beam will be blocked. At this time, the photosensitive element will not be able to receive the beam, causing signal interruption. After the signal is interrupted, the detection system will determine that an object has entered the monitoring area, and the system will immediately trigger the alarm mechanism.

In order to expand the monitoring range and improve the detection accuracy, multiple light sources and photosensitive elements are usually used, or the light beam is reflected multiple times in space through a reflector. This can form a complex network of beams to cover a larger area. Reflectors can guide the light beams in different directions, forming cross beams in multiple locations, ensuring that any intruder will find it difficult to avoid these beams.

This technology is effective at distances of several hundred feet, but it should be noted that the light intensity is reduced each time the beam bounces off a reflector, and the location of the light source and photosensor could result in an intruder crawling under or over the installation, as in the movies.

Similarly, there are laser detectors, whose alarm principle is similar to that described above, but the laser beam has excellent directionality and high power density, which makes its transmission efficiency extremely high, with less attenuation during transmission, and can remain effective within a distance of hundreds of meters or even kilometers. This makes laser detectors particularly suitable for large-scale monitoring needs, and their high penetration ability enables them to work reliably even in adverse weather conditions, reducing the interference of environmental factors such as rain and fog on the signal, thereby reducing the false alarm rate. In addition, lasers have excellent anti-interference capabilities and can effectively avoid interference from external light sources, such as sunlight or other artificial light sources, thereby ensuring the stable operation of the system.

Figure 3 Laser detector

However, with the advancement of technology, security systems have more and more combinations, and interrupt detection technology is only one of them. If a thief really attacks, the existing detection technology will leave him nowhere to hide. Among them, passive infrared detection technology is an important technology widely used in security systems, called PIR (Passive Infrared) detectors. Due to its low cost, strong concealment and stable technology, passive infrared detectors have been widely used and recognized in the market.

Passive infrared detection technology uses the thermal radiation of the human body for detection. It is known that the average body temperature of the human body is about 37°C, and it emits infrared radiation with a wavelength of 8 to 12 microns. In the 1980s, the emergence of pyroelectric infrared sensors made it possible to detect human infrared thermal radiation without contact. This sensor converts the detected infrared thermal radiation into a voltage signal. When the human body moves, the infrared radiation heat is focused on the pyroelectric sensor by the optical system. The two elements receive different amounts of heat, and the charges cannot offset each other, generating an alarm signal. After processing, it can drive various control circuits to trigger anti-theft alarms and automatic detection systems.

Figure 4 Passive infrared detector schematic

However, infrared detectors still suffer from a high rate of false alarms, which are alarms that are triggered when there is no actual danger. This is usually due to the detector's inability to find a balance between sensitivity and accuracy, especially in the presence of disturbing factors such as pets or thermals.

Foreign companies, such as ADEMCO International, Siemens Security of Germany and PARADOX SECURITY SYSTEMS LTD of Canada, have launched intelligent PIR detectors by combining intelligent algorithms and advanced microprocessing technology. These detectors are not only optimized in physical structure to prevent false alarms and missed alarms, but also use advanced digital signal processing technology to significantly improve anti-interference ability and detection reliability. The latest high-performance detectors, such as the products of SUREN Company in the United States, have further improved performance by improving lens design and signal processing, and have applied for a number of patents.

So how do researchers solve the problem of false alarms? How do they make infrared detectors recognize the difference between humans and pets? In the application of passive infrared detectors, although some anti-interference methods are used in the physical design, due to the presence of various infrared radiation heat sources in the detection environment, the signals received by the detectors are still mixed with many interference signals, especially interference signals with infrared wavelengths similar to human radiation, such as infrared radiation from pets. Therefore, in order to improve the reliability of the detector, it is necessary to deeply analyze the signal characteristics of these interference sources and distinguish them from human infrared radiation signals in order to find the unique characteristics of human infrared radiation signals and effective signal analysis methods. Researchers will collect a large amount of different data, such as specific path experiments of the human body, random movement experiments of the human body, random movement experiments of animals, etc., and then use computer methods to further classify and identify data features through these data.

With the continuous advancement of technology and the application of intelligent algorithms, future security detectors will be more reliable, further reduce false alarms and improve safety. Although the thrilling moments in movie scenes are thrilling, the real security system is constantly developing to ensure our safety.

References

He Qi'ao. Research on active infrared intrusion sentinel auxiliary police system[D]. South China University of Technology, 2017.

Liang Guangqing. Research on human recognition technology based on passive infrared detector[D]. Chongqing University, 2009.

The article is produced by Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting.

Author: Cai Wenchui, a postgraduate student at Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences

Reviewer: Li Ming, Researcher, Institute of High Energy Physics, Chinese Academy of Sciences