Text Remains Clear Even in Sunlight... A Step Closer to Commercializing Everyday AR Glasses

POSTECH and Samsung Research develop ultra-thin 1.5 mm optical system
Reduces light loss to display text outdoors... Research published in "Nature"

An optical technology that could accelerate the commercialization of augmented reality (AR) glasses, which display navigation directions and text information before the wearer’s eyes even outdoors in strong sunlight, has been developed. Korean researchers applied a new design technique to reduce light loss and successfully displayed virtual text outdoors in bright conditions using an ultra-thin optical system just 1.5 mm thick. By addressing a technical limitation of existing AR glasses, which have struggled to combine a thin optical system with a bright display, the technology opens up new possibilities for developing smart glasses for everyday use.

The structure of a metasurface waveguide and an AR glasses prototype. The operating principle of AR glasses using a metasurface waveguide (top), the actual prototype (bottom left), and a cross-sectional view of the waveguide (bottom right). By combining two layers that transmit red and green light and blue and green light, the system produces full-color images using optics just 1.5 mm thick. Courtesy of the research team.

The Ministry of Science and ICT announced on November 12 that a research team led by Professor No Junseok of Pohang University of Science and Technology (POSTECH), in collaboration with Samsung Research at Samsung Electronics, had developed “metasurface waveguide” technology that reduces light loss and produces bright images even when the wearer’s eye position changes. The research findings were published in the international journal Nature on the same day.

AR glasses are devices that overlay digital information, such as navigation directions, text, and video, on the real-world view. To be used in everyday life like regular glasses, they must be thin and lightweight, and their displays must remain visible outdoors in strong sunlight. It is also important for images to remain stable when the wearer moves their eyes or the glasses slip slightly.

Existing AR glasses transmit light from a miniature display through a thin waveguide, then use microstructures to direct the light toward the eye. A waveguide is an optical structure that guides light along a designated path. However, as the light changes direction multiple times, some of it escapes in other directions, making the image appear dim. Light loss becomes greater in particular as the range in which the image remains visible when the wearer moves their eyes is expanded. Another challenge has been that red, green, and blue light have different transmission efficiencies, causing certain colors to appear excessively bright or dim.

Nanostructure design reduces light loss, enabling a 1.5 mm optical system

Rather than focusing solely on the conventional approach of improving the light transmission efficiency of individual microstructures, the research team precisely designed how light travels throughout the waveguide. They placed nanostructures smaller than the wavelength of light on the waveguide surface and varied their shapes and spacing according to their position and the angle at which light enters.

This allowed more of the light that would previously have been lost to be used to form the image. The key was to optimize both the light transmission paths throughout the waveguide and the optical properties of each region by combining ray tracing, which calculates the paths light travels, with electromagnetic analysis.

AR footage captured outdoors and indoors. White text displayed in bright outdoor conditions (upper left and center), and a color image rendered indoors (upper right). The bottom shows the results of evaluating color reproduction, contrast, and resolution in a dark environment. Provided by the research team.

The team also combined two waveguide layers to reduce differences in light transmission efficiency among colors. One layer was designed primarily to transmit blue and green light, and the other red and green light. Each layer used a silicon nitride (SiNx)-based metasurface, with different nanostructures placed in each of 29 regions to control the direction and distribution of light.

Based on this design, the team fabricated an optical system with a total thickness of 1.5 mm and incorporated it into an AR glasses prototype. One waveguide layer weighs 5.2 g, and the waveguides used in both lenses weigh 20.8 g in total. The diagonal field of view was 30 degrees. When the distance between the eye and the waveguide was 15 mm, images could be viewed across an area measuring 8 mm horizontally and 8 mm vertically. The space in which an image can be viewed even when the eye position changes is called the “eyebox.”

Performance measurements showed that, across multiple eye positions within the eyebox, the average white-light luminous efficiency at the center of the image was 1,938 nits/lm, with a maximum of 4,225 nits/lm. This metric indicates the brightness of the image delivered to the eye relative to the amount of light entering the optical system. The average luminous efficiencies for red, green, and blue were also measured at 1,897, 1,977, and 2,170 nits/lm, respectively, indicating relatively balanced light transmission across the colors.

Outdoor text and indoor color images demonstrated, but commercialization challenges remain

Tests using the glasses prototype successfully displayed white text over a bright outdoor scene. Indoors, the team produced color images even with the display output reduced to 30%. The results show that reducing light loss can enable virtual images to be displayed under a range of ambient lighting conditions, even with a thin optical system.

The technology is significant in that it eases the trade-offs among optical-system thickness, display brightness, and the range of eye movement that everyday AR glasses must accommodate. It could be used in smart glasses that display navigation directions or work-related information before the wearer’s eyes. By improving light transmission efficiency, it is also expected to help reduce display power consumption.

However, further technological development will be needed before the technology can be commercialized as a product. The research team set a goal of increasing average luminous efficiency from the current level of about 2,000 nits/lm to more than 3,000 nits/lm, while ensuring a certain level of brightness throughout the eyebox, the entire area in which images remain visible as the eye position changes. Currently, brightness drops at some edges of the eyebox, and light can spread into rainbow-like streaks when the wearer looks directly at the sun. The team must also address color differences depending on eye position and image area, as well as the difficulty of reading small text outdoors.

Photo of the research team. Courtesy of the Ministry of Science and ICT.

The research team plans to investigate the possibility of mass production by pursuing follow-up research to reduce the two waveguide layers to one and applying a nanoimprint process that precisely replicates nanostructures over large areas. It is also considering combining the technology with artificial intelligence (AI)-based image processing to automatically adjust image color and contrast according to ambient brightness.

“This research shows that by precisely controlling the path of light inside a lens, it is possible to produce bright, clear images outdoors even with a thin lens,” Professor No said. “Going forward, we will work to reduce rainbow-like light streaks and establish mass-production technology, while combining the system with AI-based personalized image processing to develop AR glasses that are comfortable for everyday use.”

Yoon Jeonggeun, a researcher at Samsung Research, and Kang Hyunjung, a doctoral student at Pohang University of Science and Technology, participated in the study as co-first authors.

This content was produced with the assistance of AI translation services.