TAIPEI, TAIWAN — Researchers have designed a new camera module that uses a curved hexagonal microlens array and all spherical surfaces. Traditionally camera designs are difficult to assemble due to tight tolerance, because they consist of many separated lenses. If there is only one lens element in the camera, tolerance buildups can be avoided. The way to achieve a one-lens camera is through combining many lenses.
Researchers led by Prof. Guo-Dung J. Su from the Micro Optics Device Laboratory of the Graduate Institute of Photonics and Optoelectronics at National Taiwan University, Taiwan, experimentally demonstrated a one-lens camera design using a biologically inspired artificial compound eye with multiple focal lengths to avoid tolerance buildups. The artificial compound eye is a curved hexagonal microlens array arranged across a hemispherical photopolymer dome, wherein each microlens collects light with a small angular acceptance.
Fig. 1. SEM images of the one-lens camera module system using a curved hexagonal microlens array attached to a hemispherical lens. The oblique view after cutting with a dicing saw is used to confirm the fabricated curved microlens array is spherical and its center of curvature locates at the down surface of the glass substrate. Image taken by Wei-Lun Liang, National Taiwan University.
“This structure is integrated by the principles of both the human eye as well as an insect’s compound eye. It helps us to achieve a compact and wide field-of-view camera module.” said Wei-Lun Liang of the Micro Optics Device Laboratory, who was instrumental in designing the one-lens camera. “Most camera sensors are flat, due to the lithographic fabrication process. Our research showed that the field curvature aberration can be reduced by focusing on the best way to project a sharp image onto a flat surface.”
There are several benefits of using a hemispherical lens for a camera system, such as avoidance of coma aberration, wide field-of-view collection, and reduction of astigmatism aberration. However, a common optical problem of a hemispherical lens is known as field curvature aberration, and its image plane can be referred to as a curved Petzval surface. If the microlens array is arranged on the hemispherical surface, it can help with solving the problem.
To create the one-lens camera, the researchers used inkjet printing technology with the hydrophilic confinement effect to establish microlens shapes with different profiles on a planar substrate. The replication process can convert the planar array into a curved shape. Next, liquid photopolymer is filled into a deformed elastomer membrane and then cure when exposed to ultraviolet light. In the end, the spherical configuration of the hexagonal array of multiple focal lengths is accomplished by applying the template architecture to a reconfigurable surface shape.
To verify that the fabricated lens has hemispherical shape, the researchers observe scanning electron microscope (SEM) images of the lens sectioned by a dicing saw. “The sample is fed into a high-precision dicing saw, and the cutting blade accurately passes through the centers of the microlenses,” said Liang. “From the SEM images, we can check that the curved array is spherical and its center locates on the planar side of the lens.”
After an image is captured by the experimental setup of the camera module system which is built up of the one lens component, the researchers stitch every partial image formed by each microlens within its segmented channel into a combined image. “Tracing the path of a light beam emitted from a point source toward the image plane, the rays of the light beam pass through neighboring microlenses and produce several convergent beams of light.” said Liang. “This phenomenon leads to duplicate regions in neighboring sub-images, and these duplicate regions become a benefit for the subsequent image stitching.”
The researchers are now managing to convert their design into a commercial product. The funding support is from Taiwan Ministry of Science and Technology (MOST).
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