Birdbath optics
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- See also: Optical combiner and Waveguide display
Birdbath optics is an optical combiner architecture used in augmented reality (AR) smart glasses and other near-eye displays. A birdbath module combines two elements: a flat beam splitter and a curved, partially reflective mirror. The name comes from the bowl shape of the curved mirror, which resembles a garden birdbath.[1][2] As of October 2026 it is still used in tethered "display glasses" that mirror a phone, console or computer onto a large virtual screen, including the Xreal One and Xreal 1S from Xreal and the Viture Luma Ultra.[3][4][5]
The design takes light from a small display, usually a Micro-OLED microdisplay, reflects it off the beam splitter toward the curved mirror, and then sends it back through the beam splitter into the eye. Because the curved mirror is partially transparent, the wearer also sees the real world through the optic, which makes the architecture suitable for optical see-through AR.[1][6]
Birdbath optics became a common approach in low-cost consumer AR glasses of the early 2020s, including the Xreal Air series and several Rokid products, because the parts are inexpensive and deliver good image quality.[1][2] The main drawbacks are low light efficiency and a bulky form factor.[1] In Road to VR's description, birdbath optics are typically cheaper and easier to manufacture than the slimmer waveguides used in some AR glasses and can reach higher brightness, at the expense of more bulk and weight.[5]
How it works
A birdbath combiner combines reflection off mirrors with refraction through lenses to form an image. The two core components are a flat beam splitter, conventionally set at about 45 degrees, and a curved, concave mirror that doubles as the see-through combiner.[2][7] The beam splitter lets light strike the curved mirror nearly on-axis. Guttag notes that using the spherical combiner off-axis would distort the image and vary the magnification across it, and that, depending on the mirror's curvature and distances, extra lenses may not be needed at all.[2][1]
In a typical see-through birdbath, light from the display first passes through a lens that begins to magnify the image and set its focus. It then reaches the beam splitter, which reflects the light toward the curved mirror. The mirror reflects part of that light back through the beam splitter, and this second, transmitted pass reaches the eye as a magnified virtual image. Because the mirror is only partially reflective, the wearer simultaneously sees ambient light from the scene passing through both the mirror and the beam splitter.[1] In their 2021 review of AR and VR displays, Jianghao Xiong and colleagues describe the extra optics on the display side as space for aberration correction, and the beam splitter's folded optical path as a way to reduce the form factor "to some extent" compared with a design that uses a single freeform half-mirror.[8]
Many implementations use a polarizing beam splitter together with a quarter-wave plate to recover light that a simple 50/50 splitter would waste. The display light is polarized, reflected by the polarizing beam splitter, then passed twice through a quarter-wave plate placed at the curved mirror. The double pass rotates the polarization by 90 degrees, so on its return the light is in the orthogonal polarization state and transmits through the polarizing beam splitter instead of being reflected back toward the display. Optics analyst Karl Guttag describes this quarter-wave "trick" as one commonly used with polarized light.[1][2] In the Nreal design Guttag examined, the curved partial mirror passes 60 percent and reflects 40 percent of the light, and additional polarizing layers on the front of the glasses block about 95 percent of the image light that would otherwise project forward.[1]
Guttag distinguishes two variants of the architecture. In the first, the curved mirror is only partially reflective, so the wearer looks through both the mirror and the beam splitter at the real world. In the second, the curved mirror is fully reflective and the wearer looks at the real world through the beam splitter alone, which lets more real-world light reach the eye and makes more of the display light usable.[1] In 2017 he noted that the birdbath was normally used in designs that are not see-through, where the see-through losses do not apply.[2] Guttag's 2021 analysis puts Nreal, Osterhout Design Group's R-6, R-8 and ODG R-9, the Lenovo ThinkReality A3 and the Qualcomm AR reference design in the first, partially reflective group. He places Google Glass in the second group: its optics are embedded in solid glass, with a polarizing beam splitter, a quarter-wave film and an LCoS display lit by LEDs.[1][2]
Light efficiency
Birdbath optics are inefficient because the display light must pass through the beam splitter twice and reflect off a partially transmissive mirror. With an ideal 50/50 beam splitter, the throughput is limited to about 48 percent on the reflective pass multiplied by 48 percent on the transmissive pass, or roughly 23 percent at best, before the further loss at the curved mirror is counted. An 80/20 splitter gives about 14 percent.[2]
In practice the efficiency is lower. Guttag estimated that the Nreal Light delivers only about 15 percent of the display's nits to the eye, and later wrote that optical see-through birdbaths deliver about 10 percent of the display light, or about 20 percent when a fully reflective front mirror is used for VR.[1] In a 2017 analysis of a Disney-Lenovo AR headset that used a phone as its display, he estimated that less than 10 percent of the phone's light reached the eye.[9]
Peer-reviewed estimates put geometric (free-space) combiners as a class well ahead of diffractive waveguides on this measure. Xiong and colleagues estimate a combiner efficiency of about 4,200 nit/lm for geometric designs (about 4,000 nit/lm in their summary table), assuming 50 percent half-mirror transmittance and 50 percent efficiency in the other optics, against a typical 50-200 nit/lm for diffractive waveguide combiners.[8] This larger light budget is one reason birdbath glasses can pair with moderate-brightness Micro-OLED panels. Guttag notes that the design suits Lambertian emissive displays such as OLEDs and is much more efficient with them than waveguides are.[1] In 2017 he gave typical OLED microdisplay brightness as about 200 to 300 nits, far below the tens of thousands of nits that LED-lit LCoS and DLP panels can reach.[2]
A consequence of the partial mirror is that the optic also blocks much of the incoming real-world light. Guttag measured about 26 percent of real-world light reaching the eye through the Nreal Light, which blocks about 75 percent, and describes the design as inherently limited to about 25 to 30 percent see-through or worse. Consumer birdbath glasses therefore look and behave like dark sunglasses.[1] Devices that try to be more transparent pay with a much dimmer virtual image: in Guttag's worked example, a birdbath tuned for 81 percent see-through would deliver only about 0.3 percent of the display light to the eye.[2] His camera-based estimates for the ODG R-9, whose combiner had to be highly reflective with little transmission to reach its target image brightness, came to only about 4 to 5 percent see-through.[2]
Field of view and form factor
Birdbath optics can produce a wider field of view than diffractive waveguides because the display can sit farther from the eye and the curved mirror provides strong magnification. Guttag writes that the architecture fundamentally supports a very wide field of view, which is difficult for waveguides.[2] Xiong and colleagues list a diagonal field of view of 50 to 120 degrees for geometric combiners as a class and about 50 degrees for diffractive waveguides.[8] Shipping consumer birdbath glasses are narrower: manufacturers list 46 degrees for the Xreal Air 2, 50 degrees for the Rokid Max and the Xreal One, and 52 degrees for the Nreal Light.[10][11][3][12]
The trade-off is thickness. Because the beam splitter and curved mirror need physical space and an air gap between them, a birdbath module is far bulkier than a waveguide. Guttag measured the Nreal birdbath at roughly 25 mm thick, of which the beam splitter accounts for about 18 mm, against just a few millimeters for a typical waveguide plate. He also notes that the beam splitter's thickness has to grow with the field of view, and that the top-mounted display creates a thick brow above the image. This bulk pushes the optics forward on the face and makes birdbath glasses front-heavy compared with waveguide designs.[1][7] Zhang, Zhou and Fang, in a 2022 review chapter, likewise note that a birdbath's size and weight grow with its eye box and field of view.[6]
Image quality, by contrast, is a strength. Guttag describes the result as essentially a magnified view of the display, and the parts are mostly thin, inexpensive plastic; the air-filled version is also relatively light.[1] Because a birdbath forms its image with ordinary refractive and reflective surfaces rather than diffraction gratings, it does not produce the grating-induced rainbow artifacts that Xiong and colleagues list among the problems of diffractive waveguides.[8][2]
Image artifacts and other limitations
Guttag lists several drawbacks besides brightness and thickness. The two faces of the beam splitter can create double or ghost images; he counted at least four ghost images in a photograph taken through the ODG R-9.[2] Some of the image light also escapes forward: even with a front polarizer, about 5 percent of the light headed to the eye projects outward in the Nreal design, a form of eye glow. Birdbath glasses hide the wearer's eyes from other people and generally have very short eye relief.[1]
Reviewers still report reflections in current products. In its January 2026 review of the Xreal 1S, Gaming Nexus noted that the birdbath optics show minor reflections when the wearer has bright clothing, an issue the reviewer said Xreal's prism-style optics had mostly solved.[4]
Comparison with waveguides
The main alternative for see-through AR is the waveguide display, which couples light into a thin flat plate of glass or plastic and routes it to the eye by total internal reflection, releasing it through diffractive or reflective structures. The two architectures trade off along several axes.[8][1]
| Property | Birdbath | Diffractive waveguide |
|---|---|---|
| Thickness | About 25 mm in the Nreal Light[1] | A few millimeters[1] |
| Combiner efficiency (Xiong et al. estimates) | About 4,000 nit/lm for geometric combiners as a class[8] | About 50-200 nit/lm[8] |
| Display light reaching the eye | About 10 to 15 percent in consumer see-through designs[1] | Not compared in the cited sources |
| Real-world transmission | About 25 to 30 percent at most[1] | Not compared in the cited sources |
| Field of view | 50 to 120 degrees for geometric combiners; 46 to 52 degrees in shipping consumer birdbath glasses[8][10][12] | About 50 degrees[8] |
| Eye box | About 8 mm for geometric combiners[8] | About 15 mm[8] |
| Image quality | Essentially a magnified view of the display[1] | Subject to light leakage, see-through ghosts and rainbow artifacts[8] |
| Cost and manufacturing | Relatively inexpensive; typically cheaper and easier to manufacture[1][5] | Costlier and harder to manufacture, in Road to VR's comparison[5] |
Because birdbath glasses are bright and inexpensive but bulky and dark, they have found their main market in tethered display glasses aimed at watching video and mirroring a phone or computer screen, where a bright, high-quality image matters more than a slim, fully clear lens.[1][5]
History
An early consumer example of the beam-splitter-plus-combiner approach was Google Glass, which used a small projector and a beam splitter prism to send a virtual image to the eye while letting the wearer see through to the real world.[6][2] In 2017 Guttag analyzed the birdbath optics of the Osterhout Design Group R-9 and concluded from videos that a Disney-Lenovo AR headset used a classical birdbath design, noting that ODG's R-8 and R-9 also used a birdbath optical path. The Disney-Lenovo unit used the wearer's phone as the display, a roughly 50/50 semi-mirror beam splitter and a spherical semi-mirror combiner, and by his rough estimate blocked about 80 percent of real-world light.[2][9]
The architecture became widespread once bright Micro-OLED panels became available and made the efficiency penalty tolerable. Nreal (later renamed Xreal) drew wide attention at CES 2019, and its Nreal Light, whose design Guttag found very similar to ODG's, went on sale in South Korea on 21 August 2020 through the carrier LG Uplus.[1][12] A wave of similar glasses followed: Guttag's 2021 teardown names Lenovo's ThinkReality A3 (shown at CES 2021), Pacific Futures' AM Glass, the Qualcomm AR reference design and 0Glasses "among many others".[1]
Notable uses
Birdbath optics are used in many low-cost consumer AR display glasses of the early and mid 2020s.
- Xreal (formerly Nreal) used birdbath optics in the Nreal Light and the entire Xreal Air line; Guttag wrote in 2025 that all prior Xreal and Nreal devices used a birdbath architecture.[7] The Xreal Air 2 pairs dual 0.55-inch Sony Micro-OLED panels at 1920x1080 per eye with a birdbath combiner, giving a 46-degree field of view and up to 500 nits of brightness.[10][13] In their December 2024 review of the Air 2 Pro, Anshel Sag and Patrick Moorhead wrote that the glasses' field of view is "limited by the birdbath optics that help keep them affordable".[13]
- Rokid used a new BirdBath module in the Rokid Max, announced on 21 March 2023, which has 1920x1080 Sony Micro OLED displays, a 50-degree field of view, up to 600 nits at the eye, a front section 18.5 mm thick at its thickest point, and a claimed 90 percent reduction in forward light leakage.[11]
- The Xreal One, announced on 4 December 2024 at US$499, uses what Xreal calls a "triangular birdbath" design with a 50-degree field of view and 600 nits of peak brightness.[3] The Xreal 1S, which went on sale in the United States on 4 January 2026 at US$449, also uses birdbath optics; Gaming Nexus gave its field of view as 52 degrees.[4]
- VITURE's Luma Ultra, launched in September 2025 with Sony Micro-OLED panels, uses a birdbath system with a curved, semi-transparent mirror.[5]
- Earlier enterprise and developer headsets that used birdbath-style combiners include Osterhout Design Group's R-series (such as the R-9), the Lenovo ThinkReality A3, and Qualcomm AR reference designs.[1][2]
| Product | Maker | Optic (as described by maker or source) | Field of view (manufacturer) | Brightness (manufacturer) |
|---|---|---|---|---|
| Nreal Light | Nreal | Birdbath[1] | 52 degrees[12] | Not in cited sources |
| Rokid Max | Rokid | BirdBath[11] | 50 degrees[11] | 600 nits at the eye[11] |
| Xreal Air 2 | Xreal | Birdbath[7] | 46 degrees[10] | Up to 500 nits[10] |
| Xreal One | Xreal | Triangular birdbath[3] | 50 degrees[3] | 600 nits[3] |
| Xreal One Pro | Xreal | Flat prism, 11 mm[3] | 57 degrees[3] | 700 nits[3] |
| Xreal 1S | Xreal | Birdbath[4] | 52 degrees (per Gaming Nexus)[4] | Not in cited sources |
| Viture Luma Ultra | VITURE | Birdbath[5] | Not in cited sources | Not in cited sources |
Flat-prism successors
Starting with the Xreal One Pro, announced in December 2024, Xreal moved its flagship glasses away from birdbath optics. The One Pro introduced a "flat-prism lens design", which Xreal markets as X-Prism and says is an 11 mm thin plane and 40.9 percent thinner than traditional birdbath optics, and which widens the field of view to 57 degrees while still delivering 1080p per eye.[3][14][15] Guttag found that four diagrams Xreal published for the One Pro were identical to diagrams in a 2023 presentation by Ant Reality, which called that design a "Mixed Waveguide", and noted reports that Google acquired Ant Reality in December 2023. In the One Pro, a total internal reflection bounce inside solid plastic optics lets the polarizing beam splitter sit at less than 45 degrees, with an air gap between the beam-splitter prisms and the curved mirror and a quarter-wave plate in front of the mirror as in a conventional birdbath. The design lets the eye sit closer to the optics; Guttag concluded that "it is a slightly more complex version of a birdbath, where the optics are encased in plastic". In its most transparent dimming state, he wrote, the One Pro lets through only about 22 percent of real-world light.[7] The standard Xreal One, sold alongside the One Pro, kept the birdbath optic.[3][14]
Xreal also uses a flat prism lens in Xreal Aura, its tethered Android XR glasses first shown as Project Aura in 2025; the company said in June 2025 that the new lens enables a 70-degree field of view, the largest it had made.[16]
Research
Academic work treats the birdbath as one of several geometric (free-space) combiner layouts. In Xiong and colleagues' 2021 review it sits alongside the single freeform half-mirror, the freeform total-internal-reflection prism and relay-optics designs; the review also shows a pin-mirror variant in which an array of tiny mirrors replaces the beam splitter, shrinking the system and giving a large depth of field, at the cost of possible etendue and diffraction limits.[8]
The layout has also been adapted for variable or multiple focal planes, a way of addressing the vergence-accommodation conflict. A patent assigned to E-Vision Smart Optics (inventor Anthony Van Heugten; priority December 2015, granted September 2020) describes a "dynamic focusing head mounted display": a birdbath with a tunable lens placed between the beam splitter and the concave mirror, so the focus of the virtual image can change; the patent says it can be implemented without moving parts.[17] A 2025 paper by He Wang, Dewen Cheng, Da Wang, Ximeng Wang and Yongtian Wang in Optics and Lasers in Engineering applies the birdbath structure to a large-aperture biocular virtual reality head-up display with spatial multiplexing and dual focal planes.[18]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 Guttag, Karl (2021-06-01). "Nreal Teardown: Part 1, Clones and Birdbath Basics". KGOnTech. https://kguttag.com/2021/06/01/nreal-teardown-part-1-clones-and-birdbath-basics/. Retrieved 2026-06-15.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 Guttag, Karl (2017-03-03). "Near-Eye Bird Bath Optics Pros and Cons - And IMMY's Different Approach". KGOnTech. https://kguttag.com/2017/03/03/near-eye-bird-bath-optics-pros-and-cons-and-immys-different-approach/. Retrieved 2026-06-15.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 "Introducing XREAL One Series: World's First Cinematic AR Glasses with X1 Independent Spatial Computing Chip for Complete Spatial Screen Control". PR Newswire. XREAL. 2024-12-04. https://www.prnewswire.com/news-releases/introducing-xreal-one-series-worlds-first-cinematic-ar-glasses-with-x1-independent-spatial-computing-chip-for-complete-spatial-screen-control-302322181.html. Retrieved 2026-10-14.
- ↑ 4.0 4.1 4.2 4.3 4.4 John Yan (2026-01-04). "XREAL 1S". Gaming Nexus. https://www.gamingnexus.com/Article/16340/XREAL-1S/. Retrieved 2026-10-14.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Scott Hayden (2025-09-17). "VITURE Launches 'Luma Ultra' AR Glasses with Sony Micro-OLED Panels". Road to VR. https://www.roadtovr.com/viture-luma-ultra-ar-glasses-release-oled/. Retrieved 2026-10-14.
- ↑ 6.0 6.1 6.2 Jufan Zhang, Yao Zhou, Fengzhou Fang (2022-10-22). "Advancements in Optical See-through Near-Eye Display". Modern Development and Challenges in Virtual Reality (IntechOpen). https://www.intechopen.com/chapters/84367. Retrieved 2026-06-15.
- ↑ 7.0 7.1 7.2 7.3 7.4 Guttag, Karl (2025-02-24). "Xreal One Pro Optics and Its Connections to Ant-Reality and Google". KGOnTech. https://kguttag.com/2025/02/24/xreal-one-pro-optics-and-its-connections-to-ant-reality-and-google/. Retrieved 2026-06-15.
- ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 8.11 8.12 Jianghao Xiong, En-Lin Hsiang, Ziqian He, Tao Zhan, Shin-Tson Wu (2021). "Augmented reality and virtual reality displays: emerging technologies and future perspectives". Light: Science & Applications, vol. 10, art. 216. https://doi.org/10.1038/s41377-021-00658-8. Retrieved 2026-10-14.
- ↑ 9.0 9.1 Guttag, Karl (2017-07-18). "Disney-Lenovo AR Headset - (Part 1 Optics)". KGOnTech. https://kguttag.com/2017/07/18/disney-lenovo-ar-headset-part-1/. Retrieved 2026-10-14.
- ↑ 10.0 10.1 10.2 10.3 10.4 "XREAL Air 2". Xreal. 2023. https://us.shop.xreal.com/products/xreal-air-2. Retrieved 2026-06-15.
- ↑ 11.0 11.1 11.2 11.3 11.4 "Rokid Max AR glasses launch (Rokid Open Day 2023)". IT Home. 2023-03-21. https://www.ithome.com/0/681/321.htm. Retrieved 2026-10-14.
- ↑ 12.0 12.1 12.2 12.3 Jeremy Horwitz (2020-08-10). "Nreal Light AR glasses add VR lenses, launch August 21 in South Korea". VentureBeat. https://venturebeat.com/business/nreal-light-ar-glasses-add-vr-lenses-launch-august-21-in-south-korea/. Retrieved 2026-10-14.
- ↑ 13.0 13.1 "Xreal Beam Pro And Air 2 Pro Glasses Review". Moor Insights & Strategy. 2024. https://moorinsightsstrategy.com/xreal-beam-pro-and-air-2-pro-glasses-review-the-best-entry-level-ar/. Retrieved 2026-06-15.
- ↑ 14.0 14.1 "XREAL One Pro". Xreal. https://us.shop.xreal.com/products/xreal-one-pro. Retrieved 2026-10-14.
- ↑ "Hands-on: Xreal One Series AR glasses announced with custom X1 spatial chip, more customization, and wider FOV". Tom's Hardware. 2024-12-04. https://www.tomshardware.com/peripherals/wearable-tech/hands-on-xreal-one-series-ar-glasses-announced-with-custom-x1-spatial-chip-more-customization-and-wider-fov. Retrieved 2026-10-14.
- ↑ Ben Schoon (2025-06-10). "XREAL says its Android XR glasses only work wired, teases 'largest screen,' 2026 launch". 9to5Google. https://9to5google.com/2025/06/10/xreal-android-xr-project-aura-glasses-wired-screen-launch-date/. Retrieved 2026-10-14.
- ↑ Anthony Van Heugten (2020-09-22). "US10782526B2 - Dynamic focusing head mounted display". Google Patents. E-Vision Smart Optics. https://patents.google.com/patent/US10782526B2/en. Retrieved 2026-10-14.
- ↑ He Wang, Dewen Cheng, Da Wang, Ximeng Wang, Yongtian Wang (2025). "A large-aperture biocular virtual reality head-up display system with spatial multiplexing and dual focal planes using birdbath optical structure". Optics and Lasers in Engineering, vol. 192, art. 108980. https://doi.org/10.1016/j.optlaseng.2025.108980. Retrieved 2026-10-14.