The Future of AR/MR Device Optics: Metalenses
Ever since the release of the first Augmented Reality (AR) smart glasses product back in 2011 (Google Glass), multiple companies have launched versions of enterprise and consumer-oriented smart glasses and headgear. But while industry excitement remains high for the capabilities and potential of these devices, mass-market consumer adoption has been slow. Many experts and analysts agree that a big deterrent has been the size, bulk, and appearance of these devices.
Examples of AR/MR headsets and smart glasses. Clockwise from upper left: Microsoft HoloLens 2 (Image © Microsoft), Vuzix Next Gen Smart Glasses (Image © Vuzix), XIAOMI Smart Glasses (Image: Source), and Lenovo ThinkReality A3 Smart Glasses (Image © Lenovo).
AR and MR (Merged or Mixed Reallity) headsets and smart glasses have been selling well for enterprise use cases. For example, headset adoption is strong for applications such as manufacturing, training, medical, and military use. But consumers have not responded so far—in fact 70% of U.S. consumers1 are unsure of what augmented reality is (even if they may have used AR/MR photo filters for fun on their mobile devices or played Pokémon Go). Clearly the AR/MR industry has some consumer education and marketing work ahead of it. Another barrier is that the market hasn’t yet seen a “killer app” that could create a product breakthrough.
An additional factor in slow consumer adoption of AR/MR is the devices themselves. “The widespread use of AR/VR devices is hindered by many technical issues, especially the bottlenecks imposed by optical architectures. For example, the form factors of current AR headset devices are limited by bulky refractive optics and the distance between eyepieces and displays. Chromatic aberration of a refractive lens is another issue that results in poor imaging quality and causes display images of different colors to appear with different magnifications. What is more, complicated optical systems such as projection compound lenses are commonly seen in light engines.”2
Recent developments are finally pointing the way towards solving these challenges. Advanced waveguide technologies, microdisplays, and metalenses now offer the potential of much smaller and lighter AR/MR smart glasses and headsets that will appeal to consumer users. This blog post takes a look at the emerging nanotechnology of metalenses.
Metasurfaces and Metalenses
Metalenses are flat lens technology made from nano-scale layers of metasurfaces that can focus and scatter light. Metasurfaces consist of tiny pillars—also called antennae or nanofins—arranged in different formations. The arrangement of these 3D nanostructures changes the polarization, intensity, phase, and direction of incoming light waves. The waves can be focused by the metasurface structures into almost any form. Metalenses are much thinner than traditional glass lenses, opening up many new possibilities for making a wide range of optical devices that are smaller and lighter than ever before.
Illustration of how a metalens refracts light. (Image credit: Giuseppe Strangi & Federico Capasso)
Metalenses in AR/MR
The last 18 months has brought new developments and breakthroughs in metalens technology for AR/VR/MR applications (collectively XR), including:
Metasurfaces and Holograms. A team at Seoul National University in Korea have been researching metasurface holograms and metalenses. They have developed a new metasurface that “allows light control over the entire space. Utilizing this platform, independent hologram images and beam deflections for transmission and reflection are demonstrated.”3
They also developed “a device that reproduces different holograms depending on the angle of incidence”4 and a metalens doublet where “one side corrects chromatic aberration and monochromatic aberrations, and the other side performs focusing and filtering of the three primary colors in the visible spectrum. This doublet metalens corrects the aberrations of the targeted colors while having a high numerical aperture (NA). Finally, the metalens eyepiece with a high numerical aperture can realize a compact system to combine a real scene and a virtual image. In addition, our metalens shows a wide field-of-view, which can overcome the flaws of existing AR devices.”5 Learn more…
Achromatic, Aberration-free RGB Metalenses. A Harvard University team has developed an achromatic metalenses that can focus RGB (red, green, blue) colors without aberrations. At two millimeters in diameter, this metalens is the centerpiece of a miniaturized display for virtual and augmented reality applications. “The lab previously invented achromatic metalenses that work across the entire visible spectrum, which had previously only been achieved by stacking multiple lenses.”6
This invention “allows that technology to work at a scale relevant to AR/VR systems. This new optical system solves a common issue of chromatic aberration in traditional lenses. Additionally, the use of flat optics enables a system that is lightweight and scalable to manufacture.”7
This new display, “inspired by fiber-scanning-based endoscopic bioimaging techniques, uses an optical fiber through a piezoelectric tube. When a voltage is applied onto the tube, the fiber tip scans left and right and up and down to display patterns, forming a miniaturized display.”8 It provides high resolution, high brightness, high dynamic range, and wide color gamut. Learn more…
An illustration of the Harvard AR/VR metalens display in a VR or AR platform. The metalens would sit directly in front of the eye, and the display would sit within the focal plane of the metalens. The patterns scanned by the display are focused onto the retina, where the virtual image forms, with the help of the metalens. (Image Source)
Bringing Metalens XR Devices to Market
These recent advances in metasurfaces promise a future of ultra-thin, lightweight, flat metalenses for XR headsets. In AR, metalenses sit in front of the eye with the display in the metalens' focal plane. Light from the display is focused onto the retina by the metalens forming the virtual image, which is overlaid onto the real-world scene viewed through the glasses.
Examples of various metalens nanostructure patterns. (Image Source)
While metalenses have design advantages, they also have lower optical efficiency than traditional lenses. This is observed as reduced brightness, contrast, and clarity. Not to mention how difficult it is to place nanoscale structures on such small components. Imprecise metalens fabrication can cause chromatic and wave aberration,. This is observed as poor intensity distribution, color uniformity and low clarity. To successfully commercialize metalenses for XR displays, devices will require careful design and testing.
Ensuring Optical Quality of Metalens Optics: MTF Testing
Testing MTF (modulation transfer function) can tell us how optical components are performing in terms of transmission and reflection of light and how this affects the perceived sharpness of the final image. Three MTF test methods we apply at Radiant are:
- Slanted Edge Contrast. This measures the ratio between black and white areas of a slanted edge.
- Line Pair Method. This measures similar values between closely spaced vertical and horizontal lines.
- Directly measuring the Line Spread Function, or LSF.
An MTF measurement can be used to analyze the imaging quality of metalenses (how well they produce an intended image). A microscope can be useful for testing MTF, especially since metalenses are incredibly small. To isolate measurements to the metalens, the component can be illuminated using an LED light source. An imaging system with standard or microscope lens can then capture the output image for MTF analysis.
Sample measurement results of a see-through metalens for AR, showing intensity and MTF of different wavelengths of light. (Image source: Nature.com)
The sample data above shows how the intensity and MTF of different wavelengths are impacted by metalens properties. The chart below shows the lowest MTF for the red wavelength. This means virtual images in red will be less sharp. Measuring the MTF with quantifiable data helps guide developers as they work to improve device design. Within Radiant’s library of XR visual inspection tools, unique MTF analyses have been specially developed to characterize qualities of light produced by AR/MR displays and give precise data points for understanding the impact of metalens properties on visual elements.
Graph of MTF measurements indicates that red images will not appear as sharp, blue images will be the sharpest.
XR Device Measurement Solutions
Radiant’s new XRE Lens offers a flexible solution for measuring the complete ecosystem of XR optical elements—including metalenses—as well as various optical configurations of XR devices. Our patent-pending lens design incorporates a unique internal-focus mechanism with electronic control to instantly adjust the focus of the connected imaging system to multiple focal planes. Used with a ProMetric® imaging colorimeter or photometer and the TT-ARVR™ module of our TrueTest™ software, the XRE Lens can be used to measure XR displays across a broad range of devices and visual performance criteria.
Radiant’s portfolio of visual inspection and measurement solutions provides effective solutions for all optical components in AR/VR/MR devices—from microLED displays to waveguides to metalenses to IR sensors for eye and gesture tracking—all the way to final assembly where we can evaluate all visible elements inside the headset exactly as they will be perceived by a user.
Radiant’s suite of measurement solutions for XR includes ProMetric® Imaging Photometers and Colorimeters, TrueTest™ Software, our Microscope Lens, NIR Intensity Lens with TT-NIRI™ Software, and AR/VR Lens or XRE Lens with TT-ARVR™ Software.
To learn more about design parameters, quality considerations, and measurement solutions for XR devices—those with metalenses or a wide variety of other optical designs—watch the webinar “Replicating Human Vision for XR Display Testing: A Flexible Optical Solution for In-Headset Measurement” presented by Radiant’s Optics Development Manager.
- "Key Augmented Reality (AR) Statistics You Should Know", Finances Online. Sources: IB Times, GearBrain, Inception, BPC Consulting. (Accessed July 7, 2022)
- Z. Li et al., "A Metalens-Based Virtual Reality (VR) / Augmented Reality (AR) System", 2020 Conference on Lasers and Electro-Optics (CLEO), 2020, pp. 1-2, IEEE Xplore.
- B. Lee, et al. "Metasurface for imaging and AR/VR devices", Proc. SPIE 12019, AI and Optical Data Sciences III, 1201906, March 2022. DOI: 10.1117/12.2610400
- Z. Li et al., “Meta-optics achieves RGB-achromatic focusing for virtual reality,” Science Advances, Vol 7(5), Janu 27, 2021. DOI: 10.1126/sciadv.abe4458
- L Burrows, “A metalens for virtual and augmented reality”, Harvard University John A Paulson School of Engineering and Applied Sciences, News & Events. January 27, 2021.