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Watching 3D films such as Avatar could give some viewers headaches, experts have warned.

The success of James Cameron's science-fiction blockbuster, which has already taken more than $1 billion (￡600 million) at the box office, is fuelling a surge in popularity for three-dimensional movies and new 3D television sets.

But prolonged viewing may result in an aching head, according to Dr Michael Rosenberg, an ophthalmology professor at Northwestern University Feinberg School of Medicine in Chicago.

He 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."

Watching a 3D movie confronts viewers with an entirely new sensory experience. "That translates into greater mental effort, making it easier to get a headache," he said.

Dr Deborah Friedman, a professor of ophthalmology and neurology at the University of Rochester Medical Center in New York, said: "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."

Experts say there are no studies tracking how common it is to get a headache after watching a 3D film.

Rick Heineman, a spokesman for RealD, which provides 3D equipment to cinemas, said headaches and nausea were the main reasons 3D technology never took off before.

But he said new digital technology addresses many of the problems that had previously caused sore heads.

(Read by Lee Hannon. Lee Hannon is a multimedia journalist at the China Daily Web site.)

(Agencies)