Getting the Wide View While Keeping Lines Straight
In applications such as surveillance, security and robot navigation, it sometimes helps to get the wide picture. Unfortunately, fish-eye lenses, the traditional method for wide-angle viewing, produce barrel distortion. If the field of view can be less than 180°, a rectilinear lens can be used to make straight lines appear correct. A disadvantage is that rectilinear lenses do not offer as wide a field of view as is often necessary. In addition, they, as well as fish-eye lenses, are heavy and expensive.
Now a research team from Honam University in Gwangju and the Korea Basic Science Institute in Daejeon, both in South Korea, has come up with a solution. Combining a specially designed convex mirror and a conventional refractive lens, the engineers created a catadioptric lens that not only yields a wide-angle view, but that also is lighter, smaller and cheaper than commercially available alternatives.
A catadioptric lens designed to capture an ultrawide field of view (left) was used to image a university bookstore (right). Image courtesy of Optics Express.
The approach involves careful attention to design of the mirror, which collects light and sends it through a refractive lens to an image sensor. According to the university’s Gyeongil Kweon, the design mimics the distortionless imaging of a simple pinhole camera but without its long exposure time. “The ideal rectilinear lens captures images that are practically identical to that of pinhole, but it captures images fast enough to obtain a live video image.”
With their baseball-size catadioptric lens, the researchers achieved a field of view of 151° with <1 percent image distortion. They describe the lens design in the Dec. 1, 2006, issue of Applied Optics.
For a test of the lens, they used a 1/3-in. color CCD chip inside a long and thin, or bullet, camera made by Visionhitech Co. Ltd. of Bucheon, South Korea, with a resolution of 768 × 494 pixels. They mounted the lens atop the camera on a pole and then captured an entire 4 × 6-m room, with square tiles appearing true in the resulting image. They also used the setup inside a bookstore and surveyed the whole area (see figure).
Their results indicated that the lens could be useful for indoor surveillance. Kweon, who has started a company to sell the lens, noted that the technique has attracted attention, with reactions falling into several categories. “Many people think it [is] interesting, and some people think it is a hoax,” he said.
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