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A new cloaking device uses inexpensive, readily available materials to hide objects from view.

“There’ve been many high tech approaches to cloaking and the basic idea behind these is to take light and have it pass around something as if it isn’t there, often using high-tech or exotic materials,” says John Howell, a professor of physics at the University of Rochester.

Forgoing the specialized components, Howell and graduate student Joseph Choi developed a combination of four standard lenses that keeps the object hidden as the viewer moves up to several degrees away from the optimal viewing position.

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“This is the first device that we know of that can do three-dimensional, continuously multidirectional cloaking, which works for transmitting rays in the visible spectrum,” says Choi, a PhD student at the University of Rochester Institute of Optics.

Many cloaking designs work fine when you look at an object straight on, but if you move your viewpoint even a little, the object becomes visible, explains Howell.

Choi adds that previous cloaking devices can also cause the background to shift drastically, which makes it obvious that the cloaking device is present.

In order to both cloak an object and leave the background undisturbed, the researchers determined the lens type and power needed, as well as the precise distance to separate the four lenses.

Broadband cloaking

To test their device, they placed the cloaked object in front of a grid background. As they looked through the lenses and changed their viewing angle by moving from side to side, the grid shifted accordingly as if the cloaking device was not there.

There was no discontinuity in the grid lines behind the cloaked object, compared to the background, and the grid sizes (magnification) matched.

The new cloak can be scaled up as large as the size of the lenses, allowing fairly large objects to be cloaked. And, unlike some other devices, it’s broadband so it works for the whole visible spectrum of light, rather than only for specific frequencies.

Their simple configuration improves on other cloaking devices, but it’s not perfect. “This cloak bends light and sends it through the center of the device, so the on-axis region cannot be blocked or cloaked,” says Choi.

This means that the cloaked region is shaped like a doughnut. He adds that they have slightly more complicated designs that solve the problem. Also, the cloak has edge effects, but these can be reduced when sufficiently large lenses are used.

In a new paper submitted to the journal Optics Express and available on arXiv.org, Howell and Choi provide a mathematical formalism for this type of cloaking that can work for angles up to 15 degrees, or more. They use a technique called ABCD matrices that describes how light bends when going through lenses, mirrors, or other optical elements.

Howell has some thoughts about potential applications, including using cloaking to effectively let a surgeon “look through his hands to what he is actually operating on,” he says.

The same principles could be applied to a truck to allow drivers to see through blind spots on their vehicles.

How to build your own

Purchase 2 sets of 2 lenses with different focal lengths f 1 and f 2 (4 lenses total, 2 with f 1 focal length, and 2 with f 2 focal length) Separate the first 2 lenses by the sum of their focal lengths (So f 1 lens is the first lens, f 2 is the second lens, and they are separated by t 1 = f 1 + f 2 ). Do the same in Step 2 for the other 2 lenses. Separate the 2 sets by t 2 =2 f 2 (f 1 + f 2 ) / (f 1 — f 2 ) apart, so that the two f 2 lenses are t 2 apart.

Additional notes:

Achromatic lenses provide best image quality.

Fresnel lenses can be used to reduce the total length (2t1+t2)

Smaller total length should reduce edge effects and increase the range of angles.

For an easier, but less ideal, cloak, you can try the 3 lens cloak in the paper.

Source: University of Rochester