Image credit: Invisibility Shield Co.
Wearing an invisible cloak that allows itself to disappear without a trace, he is a frequent visitor in fantasy works. People have also been experimenting with all kinds of stealth means. However, if the standard of invisibility is lowered a little, invisibility is actually not difficult.
Written by | Wang Yu
Reviewer | Bai Defan
People have been trying all kinds of invisible products. Recently, a British startup unveiled its invisible shield product. The shield blends the person standing behind and the background, just as all the scenery behind the shield is covered with a filter of horizontal blur.
Familiar principles, unfamiliar uses
In fact, the principle of this invisible shield is the same as the principle of the light grid sticker you saw on the street when you were a child. This kind of raster sticker has many names, raster painting, stereoscopic painting, tilt card, etc., all of which use Lenticular printing technology.
The surface of the grating print is a cylindrical lens array. It can refract different parts of a pattern on the other side of the plane to a specific direction.
Image source: wikipedia
In this way, looking at the raster painting from different directions, we can see two (or several) different pictures. The editorial department also has a small editor to paste this raster picture on the computer camera, in addition to physical voyeurism, you can also remind yourself of the activity of the cervical spine from time to time - after all, who does not like the grating sticker made of sand sculpture emojis?
Image source: Xiaobian's smart computer
If you replace each frame of the image in the emoji with the pattern of the same object at different angles of view, and then make some adjustments to the image distribution and lens array, so that the picture is more detailed with the change of the viewing angle, you can see the lifelike 3D effect.
On this invisible shield, the engineer precisely adjusted the orientation of the lens, so that most of the light from behind the lens was scattered to the sides of the observer. The lenses on this invisible shield are arranged vertically, which allows the shield to refract more background light from other directions into the viewer's eyes. Because large areas of background tend to be brighter and more proportionate than hidden subjects, observers can often only see backgrounds that are blurred horizontally, while objects hidden directly behind shields are almost gone.
Schematic diagram of the principle of the invisible shield. Image credit: Invisibility Shield Co.
The company's ideas are so popular that they have made their plans public on the crowdfunding site. The crowdfunding target is £5,000 and now they have received £380,000. Obviously, everyone is still very interested in this gadget. Engineer Ian Charnas was already a little impatient, and even published a tutorial on the making of this invisible shield directly online.
After all, the technical principles of raster printing are not complicated. As early as 1915, The Nobel laureate in physiology, Walter Rudolf Hess, patented the structure. After 100 years, we can now easily buy the corresponding raw materials to make this transparent shield on the shopping website.
Invisibility is not difficult
Of course, such a system is not perfect. Obviously, although this shield makes it difficult for the observer to distinguish whether there is a person hiding behind the shield, the observer must be able to easily realize the existence of the shield. It is as if in the opening picture, although the model's lower body is hidden, the floor tiles under his feet are also blurred.
If you just want to achieve not-so-perfect optical invisibility, you don't actually need any very tall and powerful "black technology". In addition, there are many ways to achieve optical invisibility with common objects in life. For example, in 2014, researchers at the Rochester Optical Institute achieved optical invisibility with four lenses.
Image credit: University of Rochester
In this optical system, light travels to one of the points. In this focal plane, except near the focal point, the rest of the ring area of view of the lens becomes invisible. They also compiled the study and published it in the Optics Letters.
Optical path diagram of the system. Image credit: photo by J. Adam Fenster / University of Rochester
The imaging effect of this system is indeed much better than that of the invisible shield, at least the whole picture does not look unnatural lateral blur. However, the price is that the invisible range is much smaller, because the manufacturing cost of large diameter lenses is very high, and the ability to concentrate light is too strong, which may lead to the risk of burns.
Better results can be achieved by using achromatic lenses. Image credit: University of Rochester
The Rochester Optical Institute has also generously published a tutorial on the production of such optical systems. You only need 4 convex lenses, two with a focal length of 200 mm (f) and two lenses with a focal length of 75 mm (f). And arrange it as shown below. where t = f + f = 275 mm, t = 2f (f + f )/(f -f ). Try to adjust all the lenses to a straight line to create an invisible area between lenses 1 and 2.
True invisibility is not simple
If more ambitious, true optical invisibility requires photons to completely pass through or bypass objects. Experiments have been reported in the journal Science to achieve quantum stealth using the phenomenon of Pauli blocking of gases at very low temperatures. In short, at very low temperatures, all the low-energy modes of the Fermid subsystem are already occupied. Originally, the fermions in the system can scatter photons, but in this case, all the near vibration modes of the fermions are occupied, and the vibration mode cannot be easily changed, and it can no longer interact with the photons, so the light can be transmitted through the system without scattering. This enables true quantum stealth.
The relationship between material waves and vibration patterns can be likened to the relationship between the audience and the seat. When the temperature of the fermion subsystem is high (as shown on the left), the material waves will randomly vibrate in various modes, and the vibration mode of its neighbor may be vacant; when the system temperature is very low (as shown in the right figure), all the material waves will vibrate in the mode with the lowest energy as possible, and the vibration mode of its neighbor is most likely occupied. (Image source: MIT)
This kind of stealth sounds perfect, but if you want to achieve stealth in this way, you have to transform yourself into a "degeneracy fermian gas" and cool it to a very low temperature and compress it to a very high density. Even if you don't understand what "Degenerate Fermi gas" is, you can roughly guess that if you really become this state, with the modern technology of human beings, I am afraid that you will not be able to save your life.
After all, life is always full of compromises. If it weren't for the imposition of perfect quantum invisibility, perhaps the first two simpler ways could have helped you achieve invisibility.
Reference Links:
https://www.kickstarter.com/projects/invisibility-shield/invisibility-shield
https://en.wikipedia.org/wiki/Lenticular_printing
https://www.sciencealert.com/these-people-made-a-functioning-invisibility-shield-all-thanks-to-physics
https://www.instructables.com/Invisibility-Shield/
https://www.rochester.edu/newscenter/watch-rochester-cloak-uses-ordinary-lenses-to-hide-objects-across-continuous-range-of-angles-70592/
https://opg.optica.org/oe/abstract.cfm?URI=oe-22-24-29465%20
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
If you need to reprint, please contact the original public account
Source: Global Science
Edit: Cloud open leaves fall