A prototype developed by engineers in South Korea can accommodate virtual reality experiences on a device resembling a pair of sunglasses, introducing a new optical system that drastically reduces the physical size of headsets and bringing VR technology closer to adoption.

While VR has long held the potential to be used for more than just entertainment, the uncomfortable and bulky design of most commercial VR headsets has kept the technology from full market penetration. In their paper published March 23 in IEEE Transactions on Visualization and Computer Graphics, researchers from Seoul National University investigated the potential for a camera-based pass-through VR experience using a device smaller than the average VR headset. Their proposed prototype is less than one-sixth of that size, using just 3.3 millimeters of physical space between the glasses' display panel and the lens.

Kiseung Bang, the first author of the paper, told The Academic Times that this research is a first step in bringing smaller VR devices to market, because pass-through VR technology is a relatively new concept. Bang explained that this builds off existing "see-through" augmented reality glasses, in which 3D images, animations and videos are overlaid to the wearer's real-world setting. Those AR glasses resemble a normal pair of glasses that can easily be removed.

Pass-through VR does the same thing, incorporating virtual reality with a real-time view of the user's surroundings. Bang has been studying "see-through" AR glasses for several years, but he was inspired to expand his research after being introduced to pass-through VR.

"All desired properties for AR already existed in the pass-through VR, such as wide [field of view], eye-box and the occlusion effect," Bang said. "And I realized that the only reason I cannot use this in a public place right now is the weird look of wearing a headset. After that, I couldn't stop dreaming of using a glasses-like VR device."

Most current VR headsets are large and uncomfortable to wear, and their presence in everyday life may be unwelcome to people unfamiliar with or suspicious of wearable technology. Right now, they are typically only used in one location at a time because they are often hooked up to a computer with wires. 

But what's inside the headset is mostly empty space. The physical distance between the display panel and the lens is required by the conventional VR optical design, which uses a single floating lens for each eye, Bang explained. 

"This primitive optical design has been used without drastic changes since the early days of VR," the authors said in the paper. "The slow development of optical design has been a bottleneck for the explosive expansion of VR. In order to widen this bottleneck and open a new page of the VR market, next-level VR optics is required."

The researchers' proposed optical system is designed to minimize empty space in the headset. It successfully reduced the required space thickness to 3.3 millimeters, a fraction of the standard headset shape that usually has a thickness of about 5 centimeters and a width of about 15 centimeters. The system also achieves a 102-degree field of view, which refers to the extent of the observable VR environment. This is comparable to current state-of-the-art VR products.

Because conventional VR optics use a single lens per eye, the distance from the center of the single lens to the focal points of the lens, known as the focal length, cannot be shorter than a certain practical limitation, according to the paper. Existing VR headsets are thick because the single lens is performing two different functions at the same time — namely, floating the display panel at a far distance and collecting the light from the display panel to the eye-box, the area in which the full, high-quality image can be seen.

"We try to solve the problem by breaking the link between these two functions. Instead of a single lens, our proposed system consists of a lenslet array and a collecting lens," the authors said in the paper. "The lenslet array is in charge of the first function, and the collecting lens is in charge of the second function."

The new system inserts an additional lenslet array on the lens, which circumvents the limitation so that the focal length can be shorter than one-half of what it is in a conventional VR lens. A lenslet array consists of a set of lenslets, which are small lenses, in the same plane.

In their design, the researchers also incorporated an optical path folding technique called a pancake lens, in which light bounces back and forth between the surfaces with polarization-dependent coating, making the required physical distance shorter than the system's optical path length. 

This technique is meant to reduce the overall size of the VR optical system and reduce the required thickness by one-third. As a result, the researchers' proposed VR optical system has a thickness of less than one-sixth of conventional VR, Bang said.

Altogether, the proposed VR display design is a combination of a Fresnel lenslet array, a Fresnel lens, and the polarization-based optical folding technique. A Fresnel lens is a type of composite compact lens made up of a succession of concentric rings that was originally developed for use in lighthouses.

In addition to achieving a wide field of view of 102 degrees, the system has a wide eye-box of 8.8 millimeters and an ergonomic eye-relief of 20 millimeters. Eye-relief refers to the distance between the lens of the head-mounted display and the user's cornea, for a full viewing angle. All the lenslets in the system can be designed identically, and off-the-shelf Fresnel optics can be used, the authors said, which helps lower production costs. 

Additionally, Bang and his co-researchers built a sunglasses-sized VR prototype. Its total thickness measures 8.8 millimeters, including all necessary optical components and the LCD display.

Previous research regarding lenslet array displays has shown that thin VR devices are a possibility, the authors said in the paper, but they weren't able to achieve a sufficient optical performance for VR. 

"Many researchers have proposed a compact near-eye display using a lenslet array. However, many of the studies focused more on 3D display capability rather than display performance, and their systems did not have a sufficient [field of view], resolution and a proper eye-relief," the authors said in the paper.

"The key point of our research is that we successfully improved the performance of the lenslet array structure by optimization with the fundamental analysis of VR optics," Bang said.

More work needs to be done before the system is ready for commercial application, and its biggest current drawback is the resolution of the display. But Bang said that this can be improved by optimizing the Fresnel lenses, and as the technology advances, he believes that VR at this scale will eventually be a part of everyday life. 

The study, "Lenslet VR: Thin, Flat and Wide-FOV Virtual Reality Display Using Fresnel Lens and Lenslet Array," published March 23 in the IEEE Transactions on Visualization and Computer Graphics journal, was authored by Kiseung Bang, Youngjin Jo, Minseok Chae and Byoungho Lee, Seoul National University.