The success of 3D movies has been accompanied by complaints from some viewers of headaches and eyestrain. And with 3D TVs, Blu-Ray players, and games coming to the home this year, some experts are calling for more research into the possibility of eyestrain associated with 3D viewing, particularly on smaller screens that are closer to the viewer.

The eyestrain issue "has come up very recently, anecdotally, with people having symptoms of headaches after Avatar," says Michael Rosenberg, MD, and associate professor at Northwestern University's Feinberg School of Medicine. "It was the first time there was a 3D movie that attracted the volume of people it did and had the kind of advanced technology it did."

3D technology tricks the brain by showing the left eye one image and the right eye another. The brain layers these images together to produce a 3D image. For the latest 3D movies, polarizing glasses filter different images for each eye. In 3D TV sets, battery-powered active-shutter glasses open and shut many times a second in sync with the TV image to show each eye a different picture.

To look at a three-dimensional object in real life, a set of eyes must do two things. Firstly they must "verge"--rotate slightly inward or outward so that the projection of an image is always in the center of both retinas. Secondly, the eyes must "accommodate"--change the shape of each lens to focus the image on the retinas. "Without appropriate vergence, you would see double, and without appropriate accommodation, you'd see blurry," says Martin Banks, a professor of optometry at the University of California at Berkeley who is researching the effects of 3D on the visual system.

Artificial 3D causes "vergence-accommodation conflict," according to Banks, because viewers must focus at one distance (where light is emitting from the screen) but verge at another distance (wherever the 3D object appears to be in space). This difference in distance in 3D viewing may be the source of headaches and other discomforts, he says. "In 3D, the natural linkage between vergence and accommodation is broken."