Focus-Free Camera with a New Flat Lens

Using a single lens that is about one-thousandth of an inch thick, researchers have created a camera that does not require focusing. The tech­nology offers consi­derable benefits over traditional cameras such as the ones in most smar­tphones, which require multiple lenses to form high-quality, in-focus images. “Our flat lenses can drastically reduce the weight, complexity and cost of cameras and other imaging systems, while increasing their func­tionality,” said research team leader Rajesh Menon from the University of Utah. “Such optics could enable thinner smartphone cameras, improved and smaller cameras for biomedical imaging such as endoscopy, and more compact cameras for auto­mobiles.”

Using a single lens that is about one-thousandth of an inch thick, researchers have created a camera that does not require focusing. (Source: R. Menon, U. Utah)

Menon and colleagues show that their new flat lens can maintain focus for objects that are about 6 meters apart from each other. Flat lenses use nano­structures patterned on a flat surface rather than bulky glass or plastic to achieve the important optical properties that control the way light travels. “This new lens could have many interesting appli­cations outside photo­graphy such as creating highly efficient illu­mination for LIDAR that is critical for many auto­nomous systems, including self-driving cars,” said Menon. The researchers say the design approach they used could be expanded to create optical components with any number of pro­perties such as extreme bandwidth, easier manu­facturability or lower cost.

Conventional cameras, whether used in smartphones or for micro­scopy, require focusing to ensure that the details of an object are sharp. If there are multiple objects at different distances from the camera, each object must be focused separately. “The new lens eli­minates the need for focusing and allows any camera to keep all the objects in focus simul­taneously,” said Menon. “Conven­tional cameras also use multiple lenses to keep different colors of light in focus simultaneously. Since our design is very general, we can also use it to create a single flat lens that focuses all colors of light, drastically simplifying cameras even further.”

To focus light, tradi­tional lenses transform parallel light waves into spherical waves that converge into a focal spot. In an important breakthrough, the researchers realized that waves with other shapes could produce a similar effect, vastly increasing the number of possible lens designs. “In stark contrast to what is taught in optics textbooks, our research has shown that there is more than one way that light trans­mission is affected by an ideal lens – a concept known as pupil function,” said Menon. “This opened essen­tially infinite possi­bilities for the lens pupil function, and we searched through these possi­bilities for one that achieved an extreme depth of focus.”

After choosing the best lens design for depth of focus, the researchers used nano­fabrication techniques to make a proto­type lens. Experiments confirmed that the new lens performed as expected and achieved a depth of focus several orders of magni­tude larger than that of an equivalent conven­tional lens.

The researchers demons­trated the new lens using infrared light and relatively low numerical aperture. They plan to extend the lens to larger numerical apertures and to use it with the full visible light spectrum. Work to ensure that the lenses could be mass manu­factured is also needed before they could be commer­cialized. “This research is a good example of how abandoning traditional notions can enable devices previously considered impossible,” said Menon. “It serves as a good reminder to question dictates from the past.” (Source: OSA)

Reference: S. Banerji et al.: Extreme-depth-of-focus imaging with a flat lens, Optica 7, 214 (2020); DOI: 10.1364/OPTICA.384164

Link: Dept. of Electrical and Computer Engineering, University of Utah, Salt Lake City, USA

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