Big Brother's Skewed Perception
If it seems as if security cameras are everywhere these days, that's because they are. The city of Chicago, for example, is installing 2,000 more.
Nanophotonics' wide angle camera system is a low-cost alternative to rectilinear lenses. The initialsystem lensescosts about $300: $110 for the catropic lens, $160 for the bullet camera, and $30 for a mounting bracket.
Many surveillance cameras use wide-angle, or “fisheye,” lenses, which have a field of view of 180 degrees or more, depending on their design. By comparison, the human eye has a field of view of about 46 degrees. Wide-angle lenses are particularly attractive for some applications, such as to monitor an entire room when only a single camera can be used for budget or aesthetic reasons.
The downside is that fisheye lenses give a distorted, funhouse-style view of the world. That can be an issue if the distortion impairs the application, such as the ability to identify facial characteristics in a security video. One solution is to use rectilinear lenses, which can minimize distortion but at a steep price, as much as $1,400 each depending on the model.
Researchers at Honam University in South Korea say they've developed an alternative lens that costs only about $100. The affordable price and generous field of view make the technology a viable option for high-ceiling environments such as factories, hotels, theaters, resorts, and auditoriums, says Gyeongil Kweon, a researcher in Honam University's Department of Optoelectronics.
The Spoon Effect
One way to understand how wide-angle lenses work is to look up at a metal spoon held over your head. If the spoon is shiny enough, it's possible to see much of the room around you. But that view is distorted. Your forehead probably looks big enough to sell ad space on it, while large objects off to your sides look tiny in comparison.
This “barrel” type of distortion stretches lines outward. If the view were of a grid, for example, the squares toward the center would be largest, while those toward the periphery would be smaller and stretched. (In an undistorted image, all of the squares would be of equal size.) Barrel distortion occurs because the closer an object is to the optical axis, the more it's magnified.
Rectilinear lenses minimize this distortion, but at the expense of the field of view. Horizontal coverage is limited to about 100 degrees, a drawback for applications where 180 degrees or more is needed.Cone-Shaped Mirrors
To get around the limits of fisheye and rectilinear lenses, Kweon and his colleagues used a cone-shaped mirror, called a “catoptric” lens, encased in a clear, polycarbonate dome that can be mounted to a ceiling or a wall. They then hung a bullet camera off the bottom of the dome.
“An optical ray from an object — a frog, for example — is first reflected at the catoptric lens,” says Kweon. “The reflected ray passes through the dioptric lens and is finally captured by the pixel in the image sensor of the camera.”
The cone-shaped mirror provides a relatively accurate picture of the distance between each ray, thereby keeping distortion to a minimum throughout the field of view.
“Horizontal intervals between the neighboring rays are constant for our rectilinear mirror and, therefore, the scale is preserved, and distortion is negligible,” says Kweon. “Distortion of our lens computed using professional lens design software is under 1 percent.”
The entire system is known as a “catadioptric” lens, because it combines the catoptric lens inside the dome with the “dioptric” lens hanging outside. The system also can be categorized as a rectilinear lens because of the way it addresses distortion.
Kweon and his colleagues didn't invent the catadioptric lens. But their breakthrough has improved the lens' efficiency and price tag to the point that it's become practical for more than just science experiments.
Their initial system costs about $110 for the catoptric lens, $160 for the bullet camera, and $30 for a mounting bracket.
Kweon and his colleagues' initial catadioptric lens has a field of view of 151 degrees but can be expanded to 160 degrees. The mirror in the catoptric lens also can be engineered to work with other wavelengths, such as infrared. That would let the system target other applications, such as finding people inside a building during search and rescue.
One design drawback is “central obscuration,” where the camera appears as a large dot in the center of the field of view.
To get around the limitations of fisheye and rectillinear lenses, Nanophotonics catadioptric camera system pairs a cone-shaped mirror, called a "catropic" lens with a dioptric bullet camera. The company says its system features a 151-degree field of view with minimal distortion.
One of Kweon's research partners, lens-design expert Milton Laikin, developed an alternative to the catoptric lens that is said to eliminate the dot but reduces the field of view to about 120 degrees.
Kweon says he envisions the catadioptric lens being paired with a pan-tilt zoom. The catadioptric lens system is designed primarily for indoor use because of its size and fragility, but Kweon says that it could be re-engineered for outdoor use.Finding a Market
Although Kweon's catadioptric lens system doesn't use off-the-shelf components, it keeps costs low by using widely available raw materials. For example, the dome is made of Acrylonitrile butadiene styrene, a material that's used in products from contact lenses to car windows. A kilogram sells for less than $10.
Customers also have some flexibility that could keep their costs below $300. For example, the lens thread for the bullet camera uses a common size, so customers could shop around for the best deal on a bullet camera.
Kweon founded Nanophotonics to commercialize the catadioptric lens system, but so far sales are quite slow. One reason might be the camera's quality.
“It is expensive, but it is good,” says Kweon. “Since our lens captures such a wide area, everything is captured as a tiny image, and the screen is very crowded. Therefore, if the camera or the monitor do not have enough pixels, the image lacks detail.”
The size of Nanophotonics is another limiting factor, but Kweon's willingness to license his technology could help it tap a wider market.
“Mine is a one-man company with six shareholders,” says Kweon. “I design the lens and write papers and patents. All the production is out-sourced. For this reason, it is difficult to lower the production cost.”FEEDBACK
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Tim Kridel is a freelance writer and analyst in Kansas City, Kan., who covers telecom and technology. He can be reached at firstname.lastname@example.org.