In the last few years, single-pixel cameras have begun to revolutionize the field of imaging. These counterintuitive devices produce high-resolution images using a single pixel to detect light. They do not need lenses, the images of have none of the distortions that lenses produce, and the entire picture is always in focus. Physicists have used them to make movies and even to create 3-D images.
And that raises an interesting question: how much more can these devices do?
Today we get an answer of sorts thanks to the work of Bin Bai and co at Xi'an Jiaotong University in China, who have built a single pixel camera that can see around corners. Their new device can photograph objects even when they are not in direct view.
The technique is similar to that used with other single pixel cameras. The trick is to first randomize the light that the pixel detects, record the resulting light intensity, and then repeat this process thousands of times.
It’s easy to think that this randomization makes the task of creating an image even harder, but the reverse is true.
The randomization process changes the intensity of light each time the pixel records it. These differences in intensity are not random but instead correlated with the scene in front of the pixel. So producing an image is simply a question of mining this data to find the correlation. And the more data that is collected, the better the image becomes.
So by recording the intensity of light many times, it is possible to create a high-resolution picture with a single pixel.
And that’s how Bin and co have used their camera to peer round corners. What they mean by this is that they have produced a picture using light from an object that is scattered off a wall. The pixel records the scattered light but cannot see the object directly.
The setup is simple. The team begins by illuminating a toy airplane with light from a beamer that projects a random pattern of illuminated squares. This random pattern changes with each exposure.
The airplane is next to a white diffuse wall that scatters light toward a single pixel that cannot otherwise “see” the plane. This pixel records the light intensity from the wall. The team repeats this imaging process some 50,000 times. Finally, the team uses a data-mining algorithm to crunch the resulting data set and to create the image of the plane shown here.
The results are impressive. “All results show that the single-pixel camera can image the diffuse object when the camera cannot look at object directly,” say Bin and co.
The airplane is clearly visible in the image and the team says further improvements are possible by, for example, optimizing the algorithm and by reducing the size of the squares in the random pattern of projected light. “When the size of speckle patterns becomes smaller, the resolution of the image becomes higher,” say Bin and co.
An interesting question is how quickly it is possible to take 50,000 images, since this places an important limit on how the camera can image moving objects.
The Chinese team doesn’t suggest potential applications for the device, but it’s not hard to think of ways it might be used. Single-pixel cameras are tiny, simple to make, and cheap to build.
One way or another, single-pixel cameras are set to have significant impact in the not too distant future.
Ref: arxiv.org/abs/1612.07120: Imaging Around Corners with Single-Pixel Detector by Computational Ghost Imaging