The Truth About Holography
Episode 3: May The Focus Be With You
Holo again and welcome to our third episode! In episode two, we discussed modern headset displays and the problems they present, despite their claims of being “holographic” or “3D”. In this episode, we will dive into the science and explore a crucial physics concept behind the side-effects experienced within these headsets.
Let’s start by defining the various types of immersive technologies where these side-effects exist: AR, MR & VR. Augmented Reality (AR) and Mixed Reality (MR) are two methods of presenting virtual information within headsets, yet these two terms can sometimes be quite difficult to distinguish.
- Virtual Reality (VR): your visual view is fully immersed into a virtual surrounding (examples of VR headsets include Sony’s PSVR, Oculus Rift and HTC Vive).
- Augmented Reality (AR): virtual objects/information is overlaid onto your view of reality (examples include the Google Glass: a simple pair of glasses with an information screen held within your view of reality).
- Mixed Reality (MR): virtual objects/information interact with your view of reality (for example: allowing a virtual object to appear on a real table).
These technologies currently have one thing in common: they display virtual objects in-focus simultaneously, which is the primary cause behind unwanted headaches and eye-strains during usage. The theory behind these symptoms is known as the Vergence-Accommodation Conflict (VAC). Let me explain.
Our eyes naturally focus onto what we look at through special muscles that pull and relax the soft lens within our eyeballs. This is known as the accommodation reflex and it is an important depth cue to focus light rays onto our retina (depth cues are explained in Episode 1 here).
Our eyes can also move independently, thus when you are looking at an object, they converge on it. This is another depth cue.
When observing a natural scene, vergence and accommodation are always in sync. Issues arise when we are presented virtual information that is all-in-focus. For example, a virtual world experienced through a VR headset display is usually around 3 metres away. The actual screen is of course much closer, but there are optics in the headset that make it appear as if it were at that distance. Your eyes remain focused on the screen all the time, therefore accommodating the near-distance screen while converging on the virtual world 3 metres away. Our eyes can work against their instincts to make sense of what we view, but after a while, it just gets tiring. Headaches, nausea and fatigue ensue.
In the image (left), Diagram A is a representation of our eyes converging and focusing (accommodating) on a real world point, whilst diagram C presents this concept in-action. In diagram C, the central dot is in-focus, whilst the surrounding grid is out-of-focus. This is what is naturally happening in our everyday view of the real world.
In this image (left), Diagram B illustrates the VAC issue, whilst diagram D shows it in action. Despite focusing on the central dot in Diagram D, every detail on the mesh is still in focus, which is unnatural and results in conflicting depth cues. Naturally, we want objects presented at different depths to come into focus when our eyes converge on them, whilst the objects at other depths remain out-of-focus.
“3D” display technologies like 3D cinema or 3D TVs cause VAC-related symptoms, albeit to a slightly lesser degree. This is because these displays don’t completely block out the real world (compared to VR) and are usually further away from the viewer. Holographic projections, on the other hand, avoid the VAC completely as they recreate the light reflected from a real-world scene, which ensures all depth cues are in-sync. Images created “holographically” are incredibly realistic as they possess visual depth cues that change naturally with the angle and distance of the observer, just like when looking at a physical object.
In summary, the Vergence-Accommodation Conflict is a major issue in current AR/VR technologies that needs to be solved, and we have described what true holography needs to have: natural depth perception. Stay tuned for our final episode where we discuss the struggles of achieving real-time holographic projections and how VividQ has been able to solve this.