Wide-angle lens promises distortion-free images

15 Dec 2006

Gyeong-il Kweon of Honam University in South Korea tells optics.org about his plans to market a new wide-angle lens for security applications.

Optical researchers in South Korea have developed an inexpensive wide-angle lens that rivals the field-of-view (FoV) of conventional fish-eye lenses, and does not add any distortion. Designed by a team at Honam University, the lens costs a little over $100, and can improve the quality of indoor security cameras and CCTV systems (Appl. Opt. 45 8659).

Speaking to optics.org, Prof. Gyeong-il Kweon of Honam University said: "We have successfully designed a wide angle lens that can provide a FoV of over 150 degrees with less than 1% distortion, and are very excited about its potential in the security arena."

Dubbed as a "catadioptric" wide-angle lens, it is made up of a mirror that reflects the light from a wide area (catoptric), and lenses that focus this light on the sensor of a small camera (dioptric).

The setup consists of cone-shaped mirror fixed inside a hemispherical glass dome. At the top of the dome are a series of lenses leading up to a slot for connecting a small camera. Light entering from the dome strikes the mirror and is directed toward the lens. Here, it is focused to form a sharp image at the exact location of the camera's sensor.

Like conventional wide-angle lenses, this design produces images that exaggerate nearby objects. For example, an image of a person standing in a low-ceiling room would make the head appear larger and out-of-proportion with the rest of the body. This is not a real distortion, but is actually an illusion created by extreme wide-angle images on the human eye.

The FoV of the new lens is 151.8 °, and it can be pushed up to 160 ° with some enhancements. Unlike fish-eye lenses, which render straight lines that don't run through the center of the frame as curved, the new lens accurately depicts shapes and relative dimensions of imaged objects. "The most challenging part of the project was to design the conical mirror with very high precision so as to prevent any distortion in the final image," says Kweon.

A possible application for this lens is in intelligent security systems. In such a system, the catadioptric lens would capture light from a large area in space, and a PTZ (Pan-tilt-zoom) camera could then be used to focus on a particular region of interest. Such an arrangement would be more cost effective than using a large number of cameras to cover a wide area.

The lens also has several other potential applications. Infrared imaging is a possibility, and the lens could also be used for wild-fire monitoring and industrial fire-detection schemes. A big advantage of the lens for these applications is that the distances are accurately captured, which means that it can be programmed to alert authorities to the affected areas automatically. An outdoor version is also being developed that could be used to keep watch at traffic junctions and parking lots.

Looking at some of the sample images one can't help but notice a small black spot at the centre of every picture. This phenomenon, called central obscuration, is actually a reflection of the camera appearing on the mirror. Kweon and his research partner Milton Laikin are looking for ways to overcome this problem. Currently, they have designed a purely dioptric lens that doesn't suffer from this problem and has a FoV of 120 °.

"We are looking forward to licensing this design to interested companies, and reaching out to potential customers in Europe and America," says Kweon, who has already established a company called Nanophotonics to produce and market these lenses.