Washington, Dec 3 -- South Korean researchers claim to have designed and built a new inexpensive optical lens that collects light from a large area and produces a virtually distortion-free wide angle image.
In contrast to commonly known fisheye lenses, which produce significant amounts of visual distortion, low-distortion wide-angle lenses can potentially improve image-based applications such as security-camera systems and robot navigation.
The new wide-angle lens is lighter, smaller and more affordable than commercially available "rectilinear" lenses, which produce low-distortion views.
Lead author Gyeong-il Kweon of Homan University, South Korea, says the new wide-angle lens has been designed specifically to improve indoor security.
"For spacious places with high ceilings such as factories, hotels, theatres, resorts, and auditoriums, the lens can capture the entire floor and this will help security personnel to easily monitor those places. In this scenario, the lens would be attached to inexpensive, commercially available bullet cameras. The lens is made of inexpensive components and available for little more than 100 dollars," says Kweon.
As Kweon explains, a wide-angle lens produces image much like the reflection as seen on the convex surface of a highly polished silver spoon.
"Think about holding an immaculate silver spoon above your head and looking up. Then you will notice that the entire room can be seen from the reflections on the spoon. But there's a problem. The reflected image from the spoon is severely distorted. For example, straight lines become curved, and the distances between objects become skewed. The challenge is to design a lens that collects light from a wide area (i.e., from the entire room) and yields an image that is "perspectively correct," in that it accurately depicts the shapes and relative dimensions of imaged objects," he says.
"The most creative part of our work was the discovery of the right shape of the spoon which gives a perspectively correct image of the room," he adds.
Hailed as an elegant piece of optics technology, the new lens looks like a snow globe in the shape of the US Capitol dome.
Light from a large area enters the dome of the lens and encounters a v-shaped mirror. This reflective lens then redirects the light rays to a second lens that resembles the slender statue atop the Capitol dome.
This "refractive" lens produces a sharp image of the large area at the exact location of the image sensor within the bullet camera.
The v-shaped lens is called a catoptric (reflective) lens and the second lens is known as a dioptric (refractive) lens, so the combined design is called a "catadioptric" lens, Kweon adds.
"Ingenious catadioptric lenses having similar characteristics have been designed by other researchers. However, those lenses were optically inefficient and were mostly of academic interest. In contrast, this new design delivers straightforward, practical wide-angle images, producing a "field of view" (FOV) of 151 degrees. The FOV from this technology can be increased to 160 degrees by adding a little more complexity," Kweon says.
"A FOV of 180 degrees would mean capturing everything that you see in front of you, as well as on your left and right sides. Mathematically, this is the upper limit of what is possible with rectilinear imaging, the kind of imaging that renders straight lines as straight rather than being curved and distorted. By comparison, the human eye has a field of view of approximately 46 degrees.
"Some fish-eye lenses have a FOV that exceeds 180 degrees. However, they suffer from "barrel distortion," in which lines are stretched outward. In a fish-eye picture of a jail cell, for example, the metal bars would appear stretched outward, as if a cartoon character had pulled them apart," he adds.
Kweon is hopeful his invention will find use in intelligent security systems and robot navigational aid.
"One possible application is to use the lens as an ingredient of intelligent security systems. In this scenario, the new catadioptric lens would capture a large swath of space, and a camera with "pan-tilt" ability would zoom in on the region of interest (ROI), such as the location of an intruder. This can be more effective than a multitude of cameras watching their respective ROIs," Kweon said.
"When this lens is installed on a ceiling, the room is captured in a perspectively correct manner. In other words, the captured image is a scaled version of the room. Therefore it is easier to estimate distances and object sizes, and it can help home robots to effectively navigate the room," he added.
The findings appear in the December issue of Applied Optics, a publication of the Optical Society of America.
Published by HT Media Ltd. with permission from Asian News International.
