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.”

“There is an inherent mismatch between 3D cues, says Alan Sullivan, a senior research scientist at the Mitsubishi Electric Research Laboratories in Cambridge, MA, who has worked on 3D technologies. “Your eyes must remain focused on the display, but must converge at the depth–either in front or behind the display–where the image from each eye overlaps correctly.”

Many people can view 3D without experiencing any strain at all, but Rosenberg believes that minor eye problems might be amplified slightly by watching 3D. If children experience eye strain while watching 3D, he says, “it may be an indication that they should see an ophthalmologist to see if they have an underlying problem.”

The vergence-accommodation conflict may be worse on the small screen. “As you go farther away [from the screen], the consequences of the conflict are going to be less,” says Banks. I’m not worried about the cinema situation because the distance is long.” But he recently conducted a survey suggesting that users are more likely to experience problems when they are less than a meter from a screen.

Banks’s group has developed a device designed to avoid vergence-accommodation conflict when viewing 3D. A lens in front of each eye focuses the images for the viewer to resolve the discrepancy. “Because the user’s position is known in the setup, the device can optically correct the focus accurately. In tests on 17 volunteers, users reported significantly fewer symptoms related to eye tiredness, vision clarity, and headaches than when not using the device to view 3D images. While it the setup is too cumbersome and expensive to be used for consumer products, Banks suggests that it could be used for specialized 3D applications in surgery or computer design.

Sullivan says the unconscious mental effort of reconstructing artificial 3D objects isn’t the only thing that can produce headaches and other negative symptoms. He says such problems can also be caused by mistakes in content–even small differences in sizes between the left and right images or any other kind of misalignment. “These things are easy to control with CG movies but quite hard with real scenes like football,” which has to be turned into 3D rapidly, says Sullivan. “I predict these problems will start to appear when we see 3D infomercials and low-quality content produced with less attention to detail.”