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According to The American Heritage Science Dictionary, "catadioptric" is an adjective that relates to reflection and refraction of light, especially by a combination of mirrors, prisms and lenses. Traditionally, catadioptric optics are found in calibration equipment, Fresnel lenses, some telescopes and a few very long focal-length photographic lenses. Since mirrors and prisms are not subject to the same distortion as lenses, they are used in catadioptric optical systems to help minimize off-axis comatic aberration and astigmatism, both of which are common to wide-angle, fisheye-type lenses.
Now, researchers at Nanophotonics Company Limited (Gwangju, South Korea) have built an inexpensive lens that collects light across a large dimensional area and produces a panorama that is virtually free of distortion. Designed specifically for indoor security applications, this lens is lighter, smaller and less costly than commercial rectilinear optics.
A rectilinear lens reproduces straight lines as opposed to a fisheye's inherently skewed perspective. Additionally, the mathematical nature of rectilinear projection restricts the field-of-view to 180°; although 160° is considered to be the practical limit. By comparison, the human eye's field-of-view is approximately 46°.
The novel catadioptric lens, which Nanophotonics' Founder and CEO Gyeong-il Kweon affirms is cheaper and weighs less than currently available fisheyes, resembles a semispherical globe. Light enters the dome, encounters a V-shaped mirror and is reflected to a secondary lens which, in turn, generates the image. The mirror is termed catoptric (reflective) and the other major element is dioptric (refractive); so, the combined design is called catadioptric.
Kweon is not the first to imagine this methodology: academicians at Columbia University's Computer Vision Laboratory (New York), among others, investigated catadioptric imaging systems with unusually large fields-of-view. Nanophotonics was, however, one of the foremost to develop a viable product. Similarly, prototypes developed at Columbia also have been turned into marketable systems by RemoteReality Corporation (Westborough, Mass.).
RemoteReality's OmniAlert360S is a durable, high resolution camera for surveillance of physical assets and locations. Unlike rotating pan-tilt-zoom units, OmniAlert360S captures 360° in every video frame. By leveraging the company's single-viewpoint catadioptric optics, OmniAlert360S and OmniAlert360M allow users to secure virtual areas well beyond their physical boundaries. Both cameras feature high frame rates, low latency and afford all-around situational awareness.
Nanophotonics' lens does not have quite the field-of-view of some fisheyes. Still, there is no shortage of potential applications. One possibility, says Kweon—who also holds a faculty appointment at Honam University's Business Incubator (Gwangju-city, South Korea) —is an intelligent security system. The new catadioptric lens could capture a large swath of space and—based upon image content such as the location of an intruder—direct a secondary pan-tilt-zoom camera to the region-of-interest. This approach can be more effective than a multitude of surveillance cameras, each surveying its respective territory.
Other indoor applications are possible. "For spacious places with high ceilings such as factories, hotels, theaters, resorts and auditoriums, the lens can capture the entire floor and this will help security personnel to easily monitor those places," says Kweon.
With the promise of strengthening panoramic imaging capabilities, engineers are turning to one important area for improvement. The camera itself unavoidably shows up as a circle in the middle of the image: a phenomenon known as "central obscuration." That artifact can be avoided by removing the catoptric element, but the resulting lens system has a narrower field-of-view at approximately 120°. Furthermore, due to its small size and relative fragility, the lens only can be used indoors. According to Kweon, however, "...it is easy to create a larger version for outdoor commercial needs." The lens already is fitted with an M12-standard thread for mounting on a one-third-inch CCD video camera. And, because it primarily contains a mirror, translating the overall design to other regions of the spectrum would be rather straightforward. At infrared wavelengths, for example, wide-angle lenses can bolster wildfire detection and search-and-rescue efforts. "An outdoor version... [of the] lens could...monitor traffic violations and pedestrians," Kweon adds.
Contributing editor and industry analyst, Lee J. Nelson, is at the forefront of emerging as well as evolving technologies for compute-intensive electronic imaging applications. Contact him at: 1-703-893-0744, lnelson@rcn.com or http://www.garlic.com/biz
author: By Lee Nelson