Augmented and virtual reality devices are often thought of as simply ‘leisure’ or ‘gaming’ devices, but they are experiencing significantly increased adoption in industrial, educational and medical settings. For all of these use cases, a device that is lightweight, comfortable and can be worn for long periods of time is crucial.
In this blog we look at how to achieve this by solving one of the most important visual comfort problems in AR and VR – Vergence-Accommodation Conflict (VAC).
Device types and VAC
Augmented reality devices can broadly be separated into two types, based on how they display images for the user: See-through and passthrough.
See-through AR devices allow the user to see the real world directly – they typically have transparent lenses/displays much like conventional glasses.
Passthrough AR devices do not, and rely on a video feed of the real world from externally-mounted cameras, presenting this to the user on a display – a method sometimes referred to as ‘Mixed’ Reality.
Virtual reality devices work in a similar way to passthrough AR, but do not provide a view of the real world – everything presented to the user is entirely virtual. However, it is common for VR devices to have passthrough cameras and capabilities, usually employed for user safety purposes (such as Meta’s Guardian feature).
As adoption, and in particular, wear-time increases, the main challenge is to provide a comfortable and natural viewing experience during prolonged use. One comfort factor, and arguably the most important to solve, is the issue of VAC.
What is VAC and why does it matter?
VAC is a visual phenomenon that occurs when the brain receives mismatching cues between vergence and accommodation of the eye. Vergence is the inward or outward rotation of the eyes to focus on an object at a certain distance, while Accommodation is the adjustment of the lens of the eye to produce a sharp image on the retina. These two mechanisms work in tandem to create a clear and coherent image.
However, in both AR and VR, vergence and accommodation cues are often mismatched. This is because the virtual images are displayed on a fixed 2D plane, while the real world has varying depths. For example, if a device renders an object that appears to be close to the user, the eyes need to converge to see it in focus as a single object, but they also need to accommodate to the fixed display plane. This creates a conflict between the two cues, making it difficult to focus correctly. VAC can cause eye strain, visual fatigue, headaches, and reduce the feeling of immersion.
One way to resolve VAC is to use tunable lenses. These are lenses that can change focal length dynamically according to the virtual scene. By doing so, tunable lenses can reduce or eliminate the mismatch between vergence and accommodation distances, and provide a more natural and comfortable viewing experience.
Tunable liquid crystal lenses – a flexible, lightweight solution
FlexEnable’s tunable lenses are made of liquid crystal (LC) materials that can change their refractive index when an electric field is applied. By controlling the electric field across different regions of the lens, the focal length can be adjusted accordingly.
Our LC lenses provide multiple dioptres of lens power (a measure of their ability to bend light), large aperture (the size of the lens opening), low power consumption and can focus unpolarised as well as polarised light.
LC lenses can be integrated into AR and VR devices in various ways. For example, stacked together for additional power, configured as a pair of dynamic push-pull lenses in AR, or combined with other LC cells for ambient dimming and other features. They can also be biaxially curved (curvature in more than one direction) to integrate with the complex design of existing optical components. By combining these lenses with eye-tracking technology they can also provide adaptive focus based on where the user is looking.
FlexEnable’s tunable LC lenses improve both AR and VR performance, by providing a more natural and comfortable viewing experience for users, and by enabling new features and form factors for product designers and OEMs, whilst helping to make headsets smaller and lighter to further increase comfort.
*VAC image from https://commons.wikimedia.org/w/index.php?curid=123242579 (Rosedaler—own work, CC BY-SA 4.0)