Image: Goal
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Multiple planes of focus are a major feature of upcoming virtual reality displays. They allow us to see virtual worlds similar to reality. The videos show how the complex display technology works.
It’s a problem as old as VR headsets themselves: after a long time in VR, your eyes ache and your skull hums. The cause is a phenomenon also known as vergence accommodation conflict (HOLIDAYS).
When you look at an object in the real world, two things happen. First, their eyeballs adjust to the object by gently rotating to provide optimal stereoscopic vision. This phenomenon is called vergence. Second, your lenses are shaped by muscle contraction so that the object appears sharp. This phenomenon is called accommodation.
These mechanisms of the human eye are normally coupled and in constant interaction. When you put on a VR headset, these two reflexes conflict, which can lead to eye strain, headaches, and even nausea. This makes it difficult to be in VR for long periods of time. That’s why VR headsets with progressive lenses would be a big plus.
How is the conflict between vergence and accommodation produced?
The reason for the VAC lies in the design of today’s VR headsets. The interaction of the screen and the lenses creates a single, fixed focal plane at a distance of about two meters.
Anything beyond this range is infinitely distant and uniformly sharp to the eye. Since natural environments consist of an infinite number of continuous focal planes, this is an optical anomaly for the eye.
In VR, if you’re looking at an object more than two meters away, this isn’t a serious problem because convergence and accommodation work together more or less unperturbed under these conditions.
These three comparison images illustrate the vergence accommodation conflict. Above: View of a natural environment. Medium: View in a distant virtual reality environment. Bottom: View of a nearby VR object. | Image: Douglas Lanman/Meta
A the conflict between these mechanisms arises only at a short distance: The eyeballs converge to bring the near object into the field of view, while the lenses assume that the object is infinitely far away due to erroneous depth information and consequently cannot focus correctly. The result is a blurred image of the entire virtual environment.
The fact that convergence and accommodation act on objects at different distances irritates the brain, which can lead to the unpleasant symptoms mentioned above if the object remains in focus for a longer period of time.
Half-Dome: Varifocal Display Prototypes Metas
The industry has been working for many years on screens that allow varifocal vision in VR, that is, the natural approach of virtual objects at any distance. Varifocal displays and light field displays are seen as particularly promising.
In 2015, Meta developed a bulky varifocal prototype with precision-machined parts. In the years that followed, Meta miniaturized and simplified the technology. The result is a prototype called half dome 3, which is based on fixed liquid crystal lenses. They generate up to 64 different focal planes through different electrical charges, providing the eye with the depth information it needs to properly focus on nearby objects.
Half-Dome 3 offers varifocal vision despite a narrow form factor. | Image: Goal
One drawback to varifocal displays is that they require fast, accurate eye tracking: VR headsets must determine what the eye is looking at and activate the closest focal plane. Rendered artificial blur provides more depth information that supports accommodation.
The video below shows the varifocal image of Meta’s first varifocal prototype, which already works surprisingly well: you can see the focal point of the eye and how the system switches between different focal planes.
The headset user can switch between foreground and background focus by accommodation, such as when viewing a natural environment, with one plane in focus and the other out of focus. This is particularly evident in the pattern of the fence.
Creal Light Field Display
Light field displays are a second promising technology that could resolve the vergence accommodation conflict. One of the companies betting on this solution is the Swiss startup Creal (pronounced “Kriil”), which has been working on light field technology for five years and has since presented several virtual reality and augmented reality prototypes.
Creal’s screen mimics the behavior of reflected light from the natural environment. This means that the planes of focus are already included in the image information and do not need to be specifically simulated: the eye can focus naturally, eye tracking is not even necessary. The following video illustrates how the technology works.
Creal has posted videos showing the light field visualization in practice. The latest video is from June and shows the virtual reality prototype. It excellently shows the change in focus between foreground and background, depending on what the camera is focused on: once the hand and fingers, once the screen behind it.
The disadvantages of the current prototype are that the the field of light only reaches a field of view of 30 degrees and that the device is still very bulky. Creal wants to reduce the size of headphones to the form factor of ski goggles in the next few years.
If you want to learn more about Creal’s VR and AR light field displays, you can view a co-founder and CEO Thomas Sluka presentation on Youtube.
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