Infrared waves can go through walls
Seeing and sensing objects
Classic vision works in such a way that electromagnetic radiation (usually light; electromagnetic radiation behaves at the same time as a wave) hits an object, is completely or partially reflected by it, and part of this reflected radiation hits a surface with the help of optics is projected, which is sensitive to the radiation in question in some form. In the case of the eye, the optics consist of a lens and a vitreous body, while a large number of light-sensitive cells distributed on the retina represent the sensitive surface. In the case of a camera, these functions are performed by the lens or film (or in the case of digital cameras, the digital image sensor). In any case, the actual imaging process through the optics is a relatively simple process that works without particularly complex information processing. In principle, the human nervous system or the camera processor only has to read out the individual image points one after the other from the relevant light-sensitive visual cells or sensor pixels.
If the radiation source, viewed from the eye or camera, is behind the object that is completely or partially impermeable to this radiation, it casts a shadow that can also be depicted with the aid of the optics.
Another form of "seeing", which can also be called "sensing", is based on the fact that rays, waves or fields are influenced in some form by objects. If it is sufficiently well known in which way this influence (e.g. reflection, refraction, weakening, concentration, deflection, absorption) takes place, a powerful data processing system can be used to calculate backscattered rays or waves or a change in the field density from suitable measured values, where objects of which type are located.
Some animals have mastered such methods of sensing. As is well known, bats are able to very quickly obtain very detailed information about their environment by evaluating backscattered ultrasound sounds. The "evaluation" of course happens unconsciously. Electric eels can generate electrical fields and draw conclusions about their properties from their deformation by the environment. A number of technical devices are also available today that can be used to accomplish similar things. Such devices are logically particularly helpful when the environment behind opaque walls is to be explored. For police task forces, for example, it would often be important to know how many perpetrators and hostages are in a building and where and what they are doing.
For any type of seeing or sensing through closed obstacles (usually walls), two requirements must be met: The radiation, the waves or the field must be able to penetrate the wall material to some extent, and they must be somehow influenced by the objects to be depicted. In the following, some types of rays, waves and fields are briefly discussed with regard to their suitability for depicting objects located behind walls. The list does not claim to be complete.
Infrared radiation ("infrared light") can be used for vision by means of special cameras. These cameras differ from conventional cameras only in that their objective lenses consist of materials that are permeable to the relevant spectral range and that their image sensors are sensitive to the relevant spectral range. So-called thermal imaging cameras (WBKs), for example, are able to use the thermal infrared rays emitted by each object at normal ambient temperatures to image objects, with each object shining more or less by itself in the spectral range of a thermal imaging camera, depending on the surface material and temperature. This is useful, for example, to find missing people in the forest or bushes, as they are usually warmer than the surroundings, have correspondingly more intense rays and can be seen as bright images on the thermal image, even through not too thick foliage.
However, infrared radiation is only suitable to a very limited extent for seeing or feeling something through walls, as it cannot penetrate the usual wall materials. Even glass is impervious to thermal infrared. Only an indirect method of "looking" behind walls can be implemented with a thermal imaging camera. People hidden in the trunk of a car, for example, heat the vehicle sheet metal at the rear, which can be seen as a bright spot on the thermal image. However, this only works if the person has been lying in the trunk for a long enough time to heat up the vehicle panel, and if there are no objects with a high heat capacity (bottles with cold water or the like) between the person and the trunk wall. And even when a tarpaulin is between the person and the WBK, it takes some time for a slightly warmer area to form on the tarpaulin, which can be seen on the thermal image. If the tent wall is cooled by rain or strong wind, or if the person is some distance behind the tent wall or is constantly moving back and forth, the WBK is useless for recognizing this person.
X-rays are known to be able to penetrate many materials in principle. It is more likely to be scattered than reflected by objects. The scattered radiation can be used with the help of available devices to detect people or objects in trunk and containers. In the case of thick-walled buildings or containers, however, the penetrability of X-rays is not sufficient to get through the wall and back again to a sufficient extent.
X-rays are routinely used in medicine, both as classic fluoroscopy and as much more powerful computed tomography (CRT).
Radar-based devices can penetrate a variety of wall materials relatively easily, even if the walls are comparatively thick. Such devices are already available today, e.g. for police, military and probably intelligence purposes. Electrically non-conductive walls are usually easily penetrated, whereas richly reinforced reinforced concrete, interior cladding made of metal foil or metal nets (by esoteric people occasionally installed in their houses to shield from so-called "earth rays") or rows of steel cabinets for radar rays form an almost impenetrable barrier represent.
By and large, the same statements apply to them as to radar radiation
They are difficult to shield and can therefore penetrate almost any conventional wall. The course of their field lines is mainly influenced by ferromagnetic substances, while other materials only have a minor influence on a magnetic field. Although the author has no information on the current state of the art in question, it can be assumed that "looking through" walls with the aid of magnetic fields requires very efficient data processing.
This easily penetrates many materials, especially hard ones such as concrete and brick walls, and interacts adequately with most materials. Composite walls made of materials with different sound speeds, in which the sound waves are broken and scattered in an unforeseen way, can be problematic.
Nowadays, ultrasound devices are used routinely to look inside patients in medical operations.
Neutrons penetrate normal building walls made of concrete, masonry and metal almost unhindered, but are strongly absorbed by organic tissue (living beings, water, plastics). In theory, this would make them ideal for recognizing people through walls. However, they are hardly or not at all reflected, so that a large-area neutron source behind the building and a large-area detector in front of the building would be required to "x-ray" a building with the help of neutrons, which would be very difficult to implement. In addition, radiation with neutrons, to put it mildly, can certainly lead to health problems for those affected.
With the help of neutrons, however, e.g. in certain research projects, it is possible to "look into" running apparatus, namely in fuel cells.
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