SAN FRANCISCO – Whether through a helicopter ride over the Antarctic or a futuristic battle in space, immersion defines the ultimate goal of virtual and augmented reality (VR and AR). But at the interface of human eyesight and virtual immersion lies a (literally) uncomfortable challenge called the vergence–accommodation conflict. The conflict arises from the disparity between your eyes focusing on the single plane of a headset display, while converging and diverging to look at the virtual distances of objects projected as stereo images.
Some have tried to overcome this stereoscopic issue, and the side effects it can cause, by using a relatively simple optical trick called monovision. New work, however, has now quantitatively concluded that despite its attractive simplicity, monovision is not a viable solution.
A Stanford University research team of Robert Konrad and Nitish Padmanaban, will present their work investigating monovision and its virtual reality applications at The Optical Society’s Imaging and Applied Optics Congress, to be held 26–29 June 2017 in San Francisco, California.
“For the last year or two, we’ve been looking into different ways to reduce discomfort in virtual reality and augmented reality,” Padmanaban said. “The idea with monovision was literally just to put one screen at a far optical distance and another much closer, so that when a person looking at something in virtual reality were to verge at a nearby object, they would accommodate whichever eye had a sharp image of the nearby object, and vice versa for a far object.”
By using monovision, originally developed as a way of treating presbyopia, these ocular responses were thought to potentially alleviate the cumulative effects of the vergence–accommodation conflict that can leave viewers feeling eye discomfort, and even headaches, after less than an hour of use. In practice, monovision is achieved in VR by small displacements or focal differences between the left and right lenses.
“The easiest way to do monovision with something like a Vive or (Google) cardboard, is with two lenses,” Padmanaban said. “All you’d have to do is put another lens in front of one of the original lenses, or take it out and replace it with a lens with a different focal power. That’s all it takes to make monovision happen.”
Given how complex the inner workings of VR and AR headsets are, especially today, such a simple solution would also offer minimal need for change to a device’s overall design. “The problem is,” said Padmanaban, “though it’s easy, it doesn’t work.”
After an initial, qualitative investigation of the issue that produced mixed results, Padmanaban and his colleagues designed a quantitative study to measure the accommodation effect in real time and objectively test monovision’s effectiveness.
They developed their own model of a VR headset display that mimics conventional display modes, but also includes an autorefractor to measure each eye’s focusing power in real-time. Their prototype also featured a focus-tunable lens to adjust the focal differences between left and right that create the monovision effect. The group conducted two studies on groups of volunteers, comparing their ocular responses without monovision to those for a range of focal depth differences creating the monovision effect.
“We measure from 10 meters to 25 centimeters, which more or less covers the full range of things people care about,” Padmanaban said. “Once we actually got the accommodation measurements, we found that it sometimes works for some people. For the most part, it doesn’t really.”
For some users, they could quantitatively measure the benefits of the differing focal depths. Overall, however, the benefits weren’t consistent enough to support monovision as a general solution.
“When we look at the plots individually we see that for some people there’s an effect, but it’s highly inconsistent, and the fact that it only helps some people, coupled with the qualitative fact that a lot of people don’t like it anyway, made it for us just a bad solution going forward,” Padmanaban said. “I think the overall takeaway was that since there was a good contingent of people that don’t like it, and it doesn’t always work as expected or intended, it might not be a great solution despite how simple it is to implement.”
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