HICAGO - Movie buffs and sports fans looking to 3D televisions for the ultimate home theater experience may want to get their eyes checked first -- or risk a 3D headache, U.S. eye experts said on Saturday.
　　The growing popularity of three-dimensional movies such as James Cameron’s "Avatar" -- now a $1 billion box office hit -- has inspired a crop of 3D TV sets, unveiled this week at the International Consumer Electronics Show in Las Vegas.
　　And while new digital 3D technology has made the experience more comfortable for many, for some people with eye problems, a prolonged 3D session may result in an aching head, they said.
　　"There are a lot of people walking around with very minor eye problems, for example a minor muscle imbalance, which under normal circumstances, the brain deals with naturally," said Dr Michael Rosenberg, an ophthalmology professor at Northwestern University Feinberg School of Medicine in Chicago.
　　He said in a 3D movie, these people are confronted with an entirely new sensory experience.
　　"That translates into greater mental effort, making it easier to get a headache," Rosenberg said in a telephone interview.
　　In normal vision, each eye sees things at a slightly different angle.
　　"When that gets processed in the brain, that creates the perception of depth," Dr Deborah Friedman, a professor of ophthalmology and neurology at the University of Rochester Medical Center in New York.
　　"The illusions that you see in three dimensions in the movies is not exactly calibrated the same way that your eyes and your brain are. If your eyes are a little off to begin with, then it’s really throwing a whole degree of effort that your brain now needs to exert.
　　"This disparity for some people will give them a headache," she said.
　　DIGITAL 3D TECHNOLOGY
　　Dr John Hagan, ophthalmologist in Kansas City, Missouri, and a fellow with the American Academy of Ophthalmology, said some people who do not have normal depth perception cannot see in 3D at all.
　　He said people with eye muscle problems, in which the eyes are not pointed at the same object, have trouble processing 3D images.
　　Experts say there are no studies tracking how common it is to get a headache after watching a 3D movie, but Rick Heineman, a spokesman for RealD, a provider of 3D equipment to theaters, said headaches and nausea were the chief reasons 3D technology never took off.
　　The company, which provides 3D equipment to 90 percent of U.S. movie theaters with 3D capability and has cut deals with Sony Corp, Panasonic, JVC, Toshiba Corp and with Direct TV, said its newer digital technology addresses many of the problems that typically caused 3D moviegoers discomfort.
　　Heineman said older 3D technology involved the use of two film projectors, one that projected a left eye image and one that projected a right eye image. Three-D glasses would allow viewers to see a different image in each eye.
　　"People often complained of headaches and it was really because the projectors weren’t lined up," Heineman said.
　　Heineman’s company uses a single digital projector, which switches between the left and the right eye image 144 times a second, to help overcome some of the old problems.
　　"By going to a single digital projector, those issues were solved," he said.
　　Friedman said she thinks most people will do fine with 3D movies and with 3D TVs, but Rosenberg said people may quickly tire of the novelty.
　　"I think it will be a gimmick. I suspect there will be a lot of people who say it’s sort of neat, but it’s not really comfortable," he said.