Wearing an invisible cloak that can make you disappear without a trace is a frequent visitor in fantasy works. People have been trying all kinds of stealth methods. However, if the standard of invisibility is slightly lowered, invisibility is not difficult.

Author Wang Yu

People have been trying all kinds of invisible products. Recently, invisibility shield, a British start-up, announced their stealth shield products. This shield can integrate the person standing behind with the background, just as all the scenes behind this shield are added with a layer of horizontal blur filter.

Familiar principles, unfamiliar uses

In fact, the principle of this invisible shield is the same as that of the grating sticker you saw on the street when you were a child. This kind of raster sticker has many names, such as raster painting, stereo painting, tilt card and so on. They all use the technology of raster printing.

The surface of grating printed products is a layer of cylindrical lens array. It can refract different parts of the pattern on the other plane to a specific direction.

In this way, when we look at the raster painting from different directions, we can see two (or several) different pictures. Some editors in the editorial department also pasted this raster painting on the computer camera. In addition to physical peeping prevention, they can also remind themselves to move their cervical spine from time to time - after all, who doesn’t like the raster sticker made of sand sculpture expression bag?

If each frame of the image in the expression package is replaced with the pattern of the same object in different angles of view, and some adjustments are made to the image distribution and lens array to make the picture more detailed with the change of observation angle, we can see the lifelike 3D effect.

Most of the light from the invisible lens is accurately adjusted from the back of the invisible lens. The lenses on this invisible shield are arranged vertically, which will allow the shield to refract more background light from other directions into the observer’s eyes. Because the large-scale background is often brighter and accounts for more than the hidden subject, the observer can only see the horizontally blurred background, while the hidden object directly behind the shield almost disappears.

Invisibility shield’s ideas are very popular, and they disclosed their plans on KickStarter, a crowdfunding website. The crowdfunding target is £ 5000, and now they have received £ 380000. Obviously, everyone is still very interested in this gadget. Engineer Ian charnas can’t wait. He even published the production tutorial of this invisible shield directly on the Internet.

After all, the technical principle of raster printing is not complex. As early as 1915, Walter Rudolf Hess, the winner of the Nobel Prize in physiology, applied for a patent for this structure. After 100 years, now we can easily buy the corresponding raw materials on the shopping website to make this transparent shield.

Invisibility is not difficult

Of course, such a system is not perfect. Obviously, although this shield can make it difficult for the observer to distinguish whether there is a person hidden behind the shield, the observer must be easily aware of the existence of the shield. As in the opening picture, although the lower body of the model is hidden, the floor tiles under his feet are also blurred.

If you just want to achieve less perfect optical stealth, in fact, you don’t need any very tall and powerless “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 Institute of optics realized optical stealth with four lenses.

In this optical system, light will focus on one point during its propagation. On this focal plane, except near the focus, the other annular areas in the lens field of view become invisible. They also published the study in the optical express.

Optical path diagram of the system. Photo by J. Adam Fenster / University of Rochester

The imaging effect of this system is really much better than that of the invisible shield. At least the whole picture doesn’t look unnatural horizontal 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 condensing ability is too strong, which may lead to the risk of burns.

The Rochester Institute of optics also generously released a tutorial on the production of this optical system. You only need four convex lenses, two with a focal length of 200 mm (f ₁) and two with a focal length of 75 mm (f ₁). And arranged according to the following figure. Where t ₁ = f ₁ + F ₁ = 275 mm, t ₁ = 2F ₁ (f ₁ + F ₁) / (f ₁ - F ₁). Try to adjust all the lenses to a straight line, and you can create an invisible area between lenses 1 and 2.

True invisibility is not easy

If more pursued, true optical invisibility requires photons to pass completely through or around the object. At the end of last year, we reported on the experiment of quantum invisibility using the Pauli blocking phenomenon of gas at very low temperature in the journal Science. In short, at very low temperatures, all the low-energy modes of the Fermi subsystem have been occupied. Fermions in the original system can scatter photons, but in this case, all the adjacent vibration modes of fermions are occupied. If the vibration mode cannot be easily changed, they can no longer interact with photons, so light can pass through the system without scattering. This realizes real quantum stealth.

The relationship between material wave and vibration mode can be compared to the relationship between audience and seat. When the temperature of Fermi subsystem is high (as shown in the left figure), the material wave will vibrate in various modes randomly, and the vibration mode of its nearest neighbor may be vacant; When the system temperature is very low (as shown in the right figure), all material waves will vibrate in the mode with the lowest energy as far as possible, and the probability of its nearest neighbor vibration mode is occupied. (image source: MIT)

This kind of invisibility sounds perfect, but if you want to use this method to achieve invisibility, please