The researchers developed a thin lens with a continuously adjustable focal length. The new lens could one day make eyestrain from augmented and virtual reality (AR/VR) devices a thing of the past.

“Many 3D displays used in today’s AR/VR devices cause discomfort after long-term use due to vergence-accommodation conflict,” said research team leader Yan Li from the Shanghai Jiao Tong University in China. “Our lens, known as the Alvarez lens, can be used to mitigate this issue. It could provide a more comfortable and realistic 3D experience that would enable more widespread use of AR/VR headsets.”

The researchers describe their new goal in Express Optics. It is composed of two flat or planar liquid crystal elements that can be moved relative to each other to continuously change the focal length of the lens. To demonstrate the new Alvarez lens, they integrated it into an AR display system that displayed virtual images on a real-world view at different depths.

“This lens has a continuous and wide adjustment range, a slim form factor, is lightweight, and can be made using a simple and inexpensive manufacturing process,” said Li, who collaborated with the lab. of Shin-Tson Wu at the University of Central Florida College of Optics and Photonics. “In addition to AR/VR devices, this type of compact tunable lens could be useful for microscopic imaging, machine vision, laser treatment and ophthalmology.”

Improve virtual experience

In AR/VR devices, the vergence-accommodation conflict occurs because the left and right eyes receive two slightly different images that the brain stitches together to form a virtual 3D image. To see the image clearly, each eye focuses on the fixed 2D plane where the image is displayed. This causes an inconsistency between the merged 3D image and the focus of a single eye on the 2D plane, which leads to dizziness and eyestrain.

It is possible to mitigate the vergence-accommodation conflict with a varifocal display, which dynamically changes the depth of single-plane virtual objects so that the virtual objects appear to exist at different depths at different times. Another option is a multifocal display system, which simultaneously renders multiple 2D cross-sections of a virtual object at multiple depths to reconstruct a 3D volume. In either case, the VAC problem is removed because the human eye can focus on the correct depths of virtual objects.

Varifocal or multifocal display systems require an adjustable lens that can change focus continuously over a wide range while being compact and light enough to be useful in head-mounted AR/VR devices. Li has been working on fatigue-free AR displays and liquid crystal devices for about 10 years and has recently developed a way to fabricate a liquid crystal-based diffractive optical component known as the Pancharatnam-Berry (PB) optical element that can be used to create a tunable lens that meets these requirements.

“Our method enables Pancharatnam-Berry optical elements with the complicated and irregular phase profiles needed to create an Alvarez lens with unprecedented high precision, low cost and convenience,” Li said. “We wanted to see if this Alvarez ultra-compact tunable lens could offer a solution to the long-standing problem of vergence-accommodation conflict in VR and AR displays.”

AR display demo

The researchers used their new approach to create a tunable Alvarez lens composed of two planar Pancharatnam-Berry liquid crystal elements. In each element, an ultrathin layer of polymetric liquid crystal a few hundred nanometers thick is deposited on a 1 mm thick glass substrate. They incorporated this Alvarez lens into an AR display system built using off-the-shelf optics on an optical table. By laterally moving the two elements of the Alvarez lens, they were able to continuously adjust the depth of the virtual image from near distances to far distances.

“No matter how deep, the virtual image exhibited the same focus and blur effect as real 3D objects in the real world,” Li said. “This meant that the human eye could always properly focus on the depth of the virtual 3D image, thus overcoming the problem of vergence-accommodation conflict.”

The Alvarez lens shown in this work has been optimized for monochrome operation at 532 nm, but researchers are working on ways to use it for color display. They also want to adopt an electronic method of controlling the lateral displacement between the optical elements, which was carried out manually in this research.