Motion Sickness and the Physics of VR

Article By : Haxhi Pantina

We cannot get around the problem with simple computational or technical tricks. We need to treat this problem optically.

The technology of virtual reality has seen a rapid improvement in recent years as its applicability expands into more areas and big players are getting involved. And yet VR technology has still a few flaws that are immensely hard to address. Overcoming those issues is essential in making wider application possible. As many research groups try to take different approaches toward these technical issues, an essential part of it is certainly a better understanding of the physics of human vision.

The problem with VR

Sickness motion, dizziness, nausea, headache, etc. are very common among VR users.  A survey from Austrian VR company Junge Römer showed that over 75% of 991 survey participants had experienced at least one of those symptoms. The survey also revealed that these negative effects were particularly strong for first-time users. Moreover, there was an almost linear relationship between the persistence of these symptoms and the time of use.

Apart from providing a comfortable VR experience for entertaining purposes, the greatest motivation for scientists to invest themselves in solving these problems is to enable VR applicability in areas like medicine or education.

So the question is how can physics help us improve the VR experience?

How do VR headsets work?

To be able to appreciate the role physics plays in VR, it’s helpful to review how VR headsets and the human eye work in the first place.

Basically what’s happening when you’re wearing these fancy VR goggles is that you’re looking at a display that is very close to your eyes, occupying your eyes’ entire field of view. However, your eye cannot focus on objects that are very close. To overcome this issue we use complex optical systems, mainly consisting of lenses (read more on this below) for our eyes to be able to focus those objects.

The amount of light from the display will go through this optical system and then hit your eye. Your eye lenses will then bend that light to focus it on the retina at the rear part of your eye. In the next step, certain vision receptors will convert that amount of electromagnetic radiation to electric pulses and transfer it to your brain.

In physics, we represent this situation by making use of the so-called ray model (see picture, right).

Typically, a simple VR optical system consists of a set of very sensitive lenses (commonly Fresnel lenses) sandwiched between your eye and some display.

Those lenses are extremely important in these devices. Why? Remember, when using a VR headset, you’re actually watching a display that is very close to the eye and not a real object. This poses a challenge for your naked eye. You can easily demonstrate this for yourself if you start with your phone screen at a certain distance and pull it closer; eventually it becomes blurry. Moreover, if you hold it for enough time that close, you will surely feel eye strain and headache. This is because you’re pushing your eye to do something that is uncomfortable for it.

With VR headsets, how do we keep that screen that close to the eye, but still enable a clear image of it? We plant complex optical elements, consisting of particular of lenses, to make the necessary corrections.

However, our eyes are very sensitive and the optical elements used are far from being perfect. Unfortunately, just a tiny degree of mismatch makes a huge difference in the immersive power of VR. Actually, due to these tiny imperfections, we still have people reporting experiences like sickness motion, eye stray, dizziness, headache, etc.

The vergence-accommodation problem

A big part of the solution will be solving one of the most fundamental optical challenges: the so-called vergence-accommodation problem. Most VR developers consider it to be among the three hardest challenges along with improving displays’ resolution and widening the field of view.

Put a finger in front of your face and look at it. Two things will happen with your eyes. First, in a very short time, your eyes will focus on your finger (this is accommodation), and then your eyes have to verge which means they must point toward your finger (this is vergence). In the real world, our eyes accommodate and verge toward the same point.

However, this is not the case with VR headsets. When you’re wearing VR goggles your eyes are always focusing on the VR screen that is close to your eye. On the other hand, the vergence will be pointing to the virtual object whose distance and location changes over time. This makes the whole thing very uncomfortable for your eyes and is one of the reasons you experience eye strain and nausea.

Addressing the problem

VR companies are putting a lot of effort to solve the vergence-accommodation problem. What every endeavor in this direction has taught us so far is that we need to treat this problem optically. We cannot get around it with simple computational or technical tricks. This means we have to integrate optical systems in future VR headsets that mimic the behavior of light in the real world.

One of the first ideas to handle this issue was to create VR devices that contain multiple displays rather than just one. Each of those displays would have a different focal length and as such would show different parts of the virtual environment. However, this comes at a high price because multiple displays will make the contrast low.

Later on, VR developers turned their attention to adaptive optics. The idea was to replace traditional VR components that have a single focal length with lenses that are flexible in that regard, i.e., they are able to change between different focal lengths on-demand in less than a millisecond.

By mounting these lenses in between the human eye and VR display scientists were able to create a much better virtual experience and less negative effects for the human subject. Despite that adaptive optics places strict requirements on head positions which make it very unpractical.

company in Singapore seems to have taken this work a step further. Instead of throwing away the idea involving adaptive optics they build on it. In 2018 they developed a software that can determine the optimal location for the focus in every virtual scene. Moreover, they introduced an infrared eye-tracker to check where the user is looking at and feed this information to mechanical actuators which can then easily adjust their position.

A third popular approach to this problem is the so-called light-field technology. The idea behind this approach is to project multiple views of the virtual object to the single VR display via two or more light rays emerging from incremental areas of that virtual object.  Those rays will then be projected onto the pixels of the display. The disadvantage of it is that the more light rays we need the higher is the number of pixels required.

A Final Note

VR technology has experienced a massive and rapid improvement in recent years. However, VR devices are still far from being perfect. People still report problems like nausea, motion sickness, dizziness, eye strain, etc. after having carrier VR headsets for some time. No matter how much subtle these issues might look, the importance of addressing them is obviously huge if we want to make this technology applicable in areas like science, medicine, education, etc. Our work so far has convinced us that an essential part of accomplishing this goal is to understand the physics of the human visual system and develop VR optical systems in full compliance with that. However, in view of the efforts and investments companies are putting in this technology we have every reason to hope for nearly perfect VR headsets.

This article was originally published on EE Times.

Haxhi Pantina is a physicist and journalist.

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