Varifocal display smooths user experience of VR

Researchers at Japan's Shibaura Institute of Technology have developed a custom varifocal display that could improve user interactions in virtual and augmented reality.

Published in Virtual Reality, the findings may lead to VR and AR systems where information throughput and user movement speed are improved, although the project noted that the benefits varied between different users in the study.

In both AR and VR environments, interaction with virtual objects at different apparent depths can be challenging when displays do not reproduce how human eyes focus, commented the project. Looking at different distances requires the eyes to converge on an object and adjust focus together.

Most AR and VR headsets show images at a fixed focal distance, even when virtual objects appear at different depths. This mismatch, called the vergence-accommodation conflict (VAC), may affect comfort and interaction. The Shibaura Institute solution is a display that adjusts focus to match an object's depth.

Previous work on varifocal stereo displays has concentrated on reducing the mismatch associated with VAC as a route to improving the visual perception of the virtual environment, rather than as a route to better and faster user interactions with that environment, noted the project.

Natural focusing for faster performance

The Shibaura platform is based on an optical see-through configuration, using a beam-splitter to combine real and virtual imagery to form a stereo AR display. Two focus-tunable lenses and suitable spherical mirrors are incorporated in the design, allowing the system to achieve an optical power of 0D to 5D, equivalent to focusing from infinity to 20 centimeters.

Subjects wearing the display used a pointing device to select virtual targets presented at different focal distances, while cameras tracked their motion. Researchers compared fixed-focal and varifocal viewing during lateral movements and movements in depth, measuring movement time, error rate and throughput, a measure of task efficiency. 

The results showed that varifocal viewing could significantly reduce movement time and increase throughput, wrote the project. Error rates did not significantly change, indicating that a faster performance did not come at the cost of accuracy; and benefits appeared greater for depth movements, although the difference by direction was only borderline significant for throughput.

This indicates that varifocal displays could help AR and VR headsets provide more natural focusing cues when people interact with objects at different distances. At the same time, however, the data revealed substantial variability in how different users responded to the varifocal viewing, with the magnitude and direction of the performance changes varying across participants. 

 "Our results show that adjusting focal distance can improve how people interact with virtual objects, beyond its effects on visual perception," commented Shibaura's Xiaodan Hu.

"But users do not all benefit equally, making individual differences an important question for research. Future studies can examine which user characteristics predict greater benefits and how displays might adapt to individual needs."