How to build a low-cost ‘cloaking’ device using ordinary lenses

“Rochester Cloak” can hide objects across range of angles and wavelengths
September 30, 2014

A multidirectional “perfect paraxial” cloak using four lenses. From a continuous range of viewing angles, the hand remains cloaked, and the grids seen through the device match the background on the wall (about 2 m away), in color, spacing, shifts, and magnification. (Credit: J. Adam Fenster / University of Rochester)

University of Rochester scientists have developed a cloaking (as in Harry Potter) method that uses four standard lenses that keeps the object hidden as the viewer moves up to several degrees away from the optimal viewing position.

Previous cloaking devices have used “high-tech or exotic materials,” said John Howell, a professor of physics at the University of Rochester.

“This is the first device that we know of that can do three-dimensional, continuously multi-directional cloaking which works for transmitting rays in the visible spectrum,” said Joseph Choi, a PhD student at Rochester’s 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. That is, the background shifts drastically, making it obvious that the cloaking device is present.

How the ‘Rochester Cloak’ works

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. 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 Rochester 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 wavelengths (such as blue light).

Setup of the multidirectional “perfect paraxial” cloak as seen from the side. Laser shows the paths that light rays travel through the system, showing regions that can be used for cloaking an object. (Credit: J. Adam Fenster / University of Rochester)

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,” said Choi. This means that the cloaked region is shaped like a doughnut. He added 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 (open access), 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.

Potential applications include letting a surgeon “look through his hands to what he is actually operating on,” and could be applied to a truck to allow drivers to see through blind spots on their vehicles, Howell said.


UniversityRochester | The Rochester Cloak

* To build your own Rochester Cloak, follow these simple steps:

For their demonstration cloak, the researchers used 50mm achromatic doublets with focal lengths f1 = 200mm and f2 = 75mm

  1. Purchase 2 sets of 2 lenses with different focal lengths f1 and f2 (4 lenses total, 2 with f1 focal length, and 2 with f2 focal length)
  2. Separate the first 2 lenses by the sum of their focal lengths (So f1 lens is the first lens, f2 is the 2nd lens, and they are separated by t1=f1+ f2).
  3. Do the same in Step 2 for the other two lenses.
  4. Separate the two sets by t2=2 f2 (f1+ f2) / (f1— f2) apart, so that the two f2 lenses are tapart.

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.

Abstract of Paraxial Ray Optics Cloaking

Despite much interest and progress in optical spatial cloaking, a three-dimensional (3D), transmitting, continuously multidirectional cloak in the visible regime has not yet been demonstrated. Here we experimentally demonstrate such a cloak using ray optics, albeit with some edge effects. Our device requires no new materials, uses isotropic off-the-shelf optics, scales easily to cloak arbitrarily large objects, and is as broadband as the choice of optical material, all of which have been challenges for current cloaking schemes. In addition, we provide a concise formalism that quantifies and produces perfect optical cloaks in the small-angle (‘paraxial’) limit, and must be satisfied by any good cloaks.