A 3D Camera for Advanced Biomedical Imaging

In this new method, objects at different distances from the lens will come into focus at different points inside the camera. (Source: S. Alvey, U. Michigan)

Researchers at the University of Michigan have proven the viability of a 3D camera that can provide high quality three-dimen­sional imaging while determining how far away objects are from the lens. This information is critical for 3-D biological imaging, robotics, and autonomous driving. Instead of using opaque photo­detectors tradi­tionally used in cameras, the proposed camera uses a stack of transparent photo­detectors made from graphene to simul­taneously capture and focus in on objects that are different distances from the camera lens.

The system works because of the unique traits of graphene, which is only one atomic layer thick and absorbs only about 2.3% of the light. A pair of graphene layers can be used to construct a photo­detector that can effi­ciently detect light, even though less than 5% of the light is absorbed. When placed on a transparent substrate, instead of a silicon chip for example, the detectors can be stacked, with each one in a different focal plane.

“When you have a camera, you have to have a focusing adjustment on your lens so that when you’re focusing on a particular object like a person’s face, the rays of light that are coming from that person’s face are focused onto that single plane on your detector chip. Items in front or behind the object are out of focus. But if it were possible to stack different detector arrays each in different focal planes, then they could each image accu­rately a different place in the object space simul­taneously. What’s more, if you can detect multiple focal planes of data all at the same time, you can use algorithms to reconstruct the object in three dimensions. That is called a light field image. We have demons­trated how to use transparent focal stacks to do light field image and image recon­struction”, said Ted Norris.

In addition to basic object identi­fication, the current paper shows how their device can detect how far away something is – making it suitable for appli­cations in autonomous driving and robotics. It is also ideal for biological imaging in cases where it is important to image three-dimen­sional volume. For its ultimate success, the project required comple­mentary expertise in three areas. Zhaohui Zhong’s team developed the graphene devices; Norris’ group worked on the design features of the optical instrument and demons­trated the devices in the lab; and Jeff Fessler’s group, which developed the image recon­struction algorithm.

Fessler echoed the other faculty in stating the group of nine researchers consisting of faculty, postdocs and students “coalesced as a great team, all learning from each other and contri­buting different aspects of the final paper.” Inspi­ration for the camera came from previous research of Zhong and Norris on highly sensitive graphene photo­detectors. The current trans­parent graphene sensors fabricated so far are too low-reso­lution to depict images, but the initial experiments showed that the lens focused light from a different distance on each of the two sensors. (Source: U. Michigan)

Reference: M.-B. Lien et al.: Ranging and light field imaging with transparent photodetectors, Nat. Phot., online 20 January 2020; DOI: 10.1038/s41566-019-0567-3

Link: Dept. of Electrical Engineering, University of Michigan, Ann Arbor, USA

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