People sometimes find that when taking a group photo, the more the portrait is distorted to the edge, this is because the camera lens has distortion. To mitigate this negative result, and perform well in group shots of large numbers of people, use wide-angle imaging that helps capture a lot of information.
In addition, wide-angle imaging can optimize the overall performance of typical machine vision applications, such as displaying obstacles that cannot be seen when the vehicle is in reverse in autonomous driving.
It is understood that the bulk refractive lens used in traditional wide-angle imaging is too heavy and bulky, and there are many limitations in application. In recent years, researchers have tried to replace bulk refractive lenses with metalens with extremely thin dimensions, and have further increased the size of metalens through schemes such as “double-layer metasurface structure and secondary phase metasurface structure”. range of viewing angles.
However, the comprehensive performance of these schemes is not ideal at present, and a highly compact and practical wide-angle imaging system has not yet been realized.
MIWC wide-angle imaging principle and device architecture diagram
Recently, the team of Professor Li Tao from the School of Modern Engineering and Applied Science of Nanjing University has developed a new type of flat wide-angle camera based on the metal lens array—a single-layer metal lens array wide-angle camera. The camera is only 0.3 cm thick, but has more than 120 degrees. It can better realize wide-angle imaging without distortion, and can be used in portable imaging equipment such as vehicle/airborne monitoring, remote sensing and detection.
Li Tao told the media, “Due to the flexible design of the metasurface, the focusing and imaging performance of each metalens in this camera can be independently optimized to achieve high-quality final wide-angle images after stitching processing. In addition, its metalens The arrays are all made in a single-layer structure, which facilitates fabrication and helps reduce costs.”
On April 14, 2022, a related paper was published under the title “Flat Wide-Angle Imaging Camera Supported by Metal Lens Arrays”, and Li Tao is the corresponding author of the paper.
Phase distribution and nanostructure of metalens
It is understood that metalens were proposed in 2016. Compared with traditional lenses, they have the advantages of being smaller, thinner, and flexible in phase design, and can be optimized according to specific application scenarios. In addition to wide-angle imaging, this lens can also be used in microscopic scenes such as microscopes, taking into account the imaging details and the full view of the field of view.
In this study, the team built a single-layer metal lens array integrated wide-angle camera without sacrificing imaging performance. It first used multiple carefully designed metal lenses to clearly capture parts of a wide-angle image from different angles. Then, the high-definition image parts obtained by each sub-lens are stitched together through an algorithm to obtain a high-quality wide-angle image of the entire scene.
Li Tao said, “Considering that a single lens cannot obtain a relatively large viewing angle, we chose a different approach. Instead of stacking vertically, a series of metalens are spread horizontally to form a single-layer metalens array, which gives full play to the lens. The advantages of ultra-thin and light.” In the
study, the team also made a comprehensive analysis of the imaging performance of its development of an integrated wide-angle camera and a single metalens, and calculated their focusing efficiency and modulation of light under different illumination angles. Transfer function, etc., to verify the advantages of the integrated wide-angle camera in the performance of large-angle light imaging. The results show that “each metalens in the array can focus a different angular range of illumination angles and cover a 120-degree viewing angle range.”
Next, the researchers designed metalens arrays based on nanofabrication techniques and directly Integrated onto a complementary metal-oxide-semiconductor image sensor, a miniaturized flat wide-angle camera with a size of 1 cm × 1 cm × 0.3 cm was finally developed.
Focusing performance of MIWC
Finally, the team used a wide-angle camera based on a single traditional metal lens and an array metal lens to compare images of the words “NANJING UNIVERSITY” that had been projected on the screen. The study found that the plane camera composed of the array metal lens can not only display all letters in high-definition, but also greatly expand the viewing angle range without reducing the resolution, which has reached more than 120 degrees, exceeding a single metal lens. more than 3 times.
In fact, this is not the first time that the team has designed new devices with excellent performance through metalens arrays. They have also previously developed achromatic metalight-field cameras and highly integrated metamicroscopes using metalens arrays.
Experimental wide-angle imaging results of MIWC
In this regard, the researchers said, “Meta-lens arrays can fully demonstrate the advantages of highly integrated and flexible design of metasurface devices, and have broad application prospects in imaging detection, communication positioning and other fields.”
In addition, they said that this time The new flat-screen wide-angle camera developed is a verification of a new principle. The diameter of the single metal lens used is 0.3 mm, which belongs to the category of microlenses, so the light collection and imaging quality of the lens are not high enough. Next, they plan to increase the diameter of these metalens by another 1 to 5 millimeters. In this way, the optimized large-sized metal lens array can be mass-produced at a lower cost, and the image quality will be higher.
Li Tao said, “The metal lens imaging technology we are doing is not completely aimed at replacing applications, but is expected to develop a new application field, making the previous impossible possible, and making imagination a reality.”