19 May 2016

 Researchers Turn an iPad Mini into an Invisibility Cloak

New optical cloaking approach uses commercial technology to make objects disappear from multiple viewing angles
WASHINGTON — Imagine you could see through blind spots in your car, or that a surgeon’s hand could be invisible during a delicate procedure. Both could be possible one day with a new type of digital invisibility cloak. 
Although scientists have been working to develop invisibility cloaks for years, they haven’t yet achieved a cloak that hides an object from all viewing angles and that works for all colors of light. In The Optical Society's journal for high impact research, Optica, researchers Joseph S. Choi and John C. Howell, University of Rochester, New York, USA, report a new method for digital invisibility cloaking that can do both.
    To make an object disappear, the digital cloak collects the location and directional information for each ray of light for the background scene (solid line on left), computes where all of those rays would end up in the future (dotted lines), and then displays those new rays (solid lines on right). Because the observer always sees the light rays as coming from the background, an object between the detectors on the left and the display on the right appears to disappear. Image Credit: Michael Osadciw, University of Rochester

“We used currently available commercial digital technology, which is only going to improve over time, to make a good approximation to what you would expect from a cloak like Harry Potter’s,” said Choi. “It is a simple realization of how to make something invisible.”
Making an object disappear
To make an object seem invisible you need to preserve the appearance that light coming from a distant object has traveled in a straight line, even as the viewer moves. At every depth and every viewing angle, the background must change correctly or the observer will detect that something is wrong.
To accomplish this, Choi and Howell collect the location and directional information for each ray of light from the background scene, compute where all of those rays should end up in the future, and then display those new rays on an Apple iPad mini placed in front of the object that they want to make disappear. No matter the distance or angle of viewing, the observer always sees the light rays as if they are coming from the background. 
The approach is based on a fundamentally different principle than the invisibility cloaks that have been pursued by other researchers. “Many other approaches to invisibility cloaking try to guide light around an object,” said Howell. “We collect light rays at one position and display them at another position, making everything in between invisible.”
Demonstrating the cloak
To demonstrate their new approach to invisibility cloaking, the researchers used a consumer-grade digital camera to collect the light rays from the background. They placed the camera on a mechanical slider that moves horizontally to collect light rays from a range of positions. Video acquired by the camera as it moved was sent to a laptop for processing. The processed background was then displayed on an iPad mini that the researchers had covered with an array of cylindrical lenses to provide a 3D effect.
“This is cloaking because there’s an object between the output screen and the input camera, and a gap in distance between where we capture the input rays from the digital camera to where we project it onto the screen,” said Howell. “We have to calculate where the input rays came from, what directions they were traveling in, and then know where they should go on the display, and in what direction. The computer program calculates this and then puts the right pixel onto the display with the correct color and angle at the right position.”
For the demonstration, the researchers achieved a spatial resolution of 20 pixels per inch with 51 possible viewing angles. A higher-density lens array and a higher-resolution display would increase both the number of viewing angles and the spatial resolution, greatly improving the quality of the cloak.
“Because the views are relatively close to each other it looks like it's continuous,” said Choi. “The views change every half a degree, but in such a small way that it is hard to tell that it changed.”
Although the researchers used the flat screen of an iPad mini, bendable displays that are already on the market could be shaped around an object to make a cloak that hides an object on all sides. With some additional engineering, the digital invisibility cloak could also be scaled to hide large objects by using large displays or stitching multiple smaller displays together.
The scanning and processing methods used in the demonstration do create a lag time of around a minute for displaying a changing background, but the researchers say that a real-time version of the cloak could be achieved by replacing the scanning camera with an array of detectors, automating the data processing and improving the hardware interfaces.
Invisible hands and walls
“We usually think about cloaking in terms of spies, eavesdropping and Harry Potter’s cloak,” said Howell. “However, there are many situations where we don’t necessarily want to prevent someone from seeing something. Maybe an observer simply doesn't like how something looks or needs to look through it.  We’re not tricking them into thinking it’s not there, just enabling them to not see it.”
The researchers say that their digital invisibility cloak could have a wide variety of uses for hiding objects that the viewer knows are present. For example, they could imagine making a special glove that makes a surgeon’s hand invisible. The glove would collect light rays from one side and then project them from the other side, rendering the hand invisible so the medical team could have a clearer view of what’s being operated on.
Similarly, putting a cloaking device over a blind spot in the car could allow a driver to see through that part of the car, or placing one on a wall could turn an area into a “window” that shows what’s outside. But, of course, these applications will require a speed up of the recording and display processes.
The researchers would also like to further develop their cloak so that instead of being rigid, it could be worn on the body. “Since we already know what pixel needs to go where even if the detector and display deform or change in some way, making a wearable invisibility cloak is, in theory, possible based on our digital cloaking method,” said Howell. “It would, however, require heavy engineering and a lot of computational power to always know the position and orientation of the detector and display.”
Paper: J.S. Choi and J.C. Howell, "Digital integral cloaking," Optica, 3, 5, 536 (2016).
DOI: 10.1364/optica.3.000536.
About Optica
Optica is an open-access, online-only journal dedicated to the rapid dissemination of high-impact peer-reviewed research across the entire spectrum of optics and photonics. Published monthly by The Optical Society (OSA), Optica provides a forum for pioneering research to be swiftly accessed by the international community, whether that research is theoretical or experimental, fundamental or applied. Optica maintains a distinguished editorial board of more than 30 associate editors from around the world and is overseen by Editor-in-Chief Alex Gaeta, Columbia University, USA. For more information, visit Optica.
About The Optical Society
Founded in 1916, The Optical Society (OSA) is the leading professional organization for scientists, engineers, students and entrepreneurs who fuel discoveries, shape real-life applications and accelerate achievements in the science of light. Through world-renowned publications, meetings and membership initiatives, OSA provides quality research, inspired interactions and dedicated resources for its extensive global network of optics and photonics experts. For more information, visit osa.org/100.

Media Contacts: