Introduction

I had the opportunity to do a teardown of both a Nreal development kit and the Nreal LG product sold in Korea. I completed the Nreal teardown before I got to see Lumus’s Maximus that I reported on last time. The Maximus size, brightness, efficiency, and image quality were spectacular; I decided to report on the Maximus before finishing writing this Nreal teardown.

This article is not about whether Nreal’s design is good or bad per se, but rather to use it to help describe some of the design trade-offs and limitations with a very common optical design. I first wrote about Nreal after CES 2019.   

This is looking like it will be a 2 or 3 part article. Part 1 will discuss some basic issues of the very common Birdbath Design. In the next part(s), I will share some high resolution through the optics pictures and give details from the teardown. I want to say the image quality of the Nreal is very good compared to any other AR headset, but it also has a lot of drawbacks.

Lumus Maximus Appears to be ~30x more Display Light Efficiency than Nreal

Before I delve into the Nreal design, I want to point out that the Lumus Maximus has more than an order of magnitude better power-to-brightness efficiency over Nreal. The subject of efficiency is so important that it deserves its own article, but I want to touch on the subject here.

While the casual observer thinks of battery life, the bigger issue is managing heat dissipation. In a small headset, the heat from the display is usually bottled up into a small case/frame near the user’s face, and there is very little area for heat to radiate. If the display will be bright enough to be used in most environments, it also has to be efficient.

In the last article on the Lumus Maximus, I covered that the numbers indicate that Maximus is ~10x more efficient than WaveOptic’s diffractive waveguide. I missed mentioning that Lumus believes they will get >4,500 nits from only 1 Watt of LED power (up from >4,000 nits/Watt in their SPIE AR/VR/MR 2021 presentation below). Based on measurements, Nreal gets about 110 nits from about 0.85 Watts of OLED power (just the 10V power supply that “lights” the OLEDs). This suggests that the Maximus will be >30x brighter for the same power to light the display with a similar 50-degree FOV.

“Attack of the Clones”

On The AR Show, I discussed “The Attack of the Clones,” Since Nreal made a big splash at CES2019, many very similar birdbath designs were released by Lenovo (ThinReality A3), Pacific Futures (AM Glass), Qualcomm’s AR Reference Design, 0Glasses, among many others.

The Nreal’s design, in turn, is very similar to Osterhout Design Group (ODG) R-6, R-8, and R-9 that I wrote about back in 2017.  Also, at least two former ODG people are now working at Lenovo on a similar A3 headset.

While the designs share the same birdbath configuration, there are still many design choices for various components. While some of the differences are impossible to tell from the pictures, some are obvious. If we look at the AM Glass design (lower left), a “light shield” is on the bottom. Qualcomm’s AR Smart Viewer (upper right) has a smaller “cup” around just the optics.

The AM Glass version shows a “light shield” below the optics. As I noted in my 2019 article on Nreal, a significant amount of light is reflected up (see lower right picture above). This is because even coated glass will reflect light when hit at ~45 degrees. Note in the two figures below on both the Nreal (left) and ODG R9 (right) that you can see the CES Media badge I was wearing.

The Nreal, ODG, and Lenovo designs don’t show light-blocking below the birdbath that prevents light from being reflected from below being redirected to the eye. Nreal announced an “Enterprise” version earlier this year that looks a bit like slimmed-down Hololens 2. Notice the light-blocking “cups” in the newer version, as indicated on the frame below from Nreal’s youtube video.

Nreal Type of Birdbath

A “birdbath” is the common name given to an optical structure with a curved mirror with a beam splitter used to make the light strike the curved mirror (nearly) on-axis (perpendicular to the curvature). There are two forms of birdbath optics, one where the curved mirror is only partially reflective so the user can look through it, and one where the curved mirror is fully reflective, and you look through just the beam splitter.

I have included a discussion of an alternate (non-Nreal) type birdbath in the appendix. I have also discussed various birdbath optics structures and techniques in many articles, including Samsung AR Design Patent – What’s Inside, near-eye Bird Bath Optics Pros and Cons – And IMMY’s Different Approach Disney-Lenovo AR Headset – (Part 1 Optics).

Nreal, ODG R6/R8/R9, Lenovo A3, and Qualcomm reference design use the partially reflective mirror type birdbath.

Nreal Birdbath Overview

Below is a diagram of the Nreal based on the teardown. Light from the OLED is polarized (1) and goes through a lens (2) to enlarge and start the change in focus of the image. The polarizing beam splitter reflects the image light (3) toward the curved partial mirror (60% passing and 40% reflecting). Light going to the curved mirror goes through a Quarter Wave Plate (QWP) on the way to (4) and from (6) the mirror (5). Passing through the QWP twice causes the polarized light to be rotated 90 degrees and then passes through the polarizing beamsplitter (7 and 8). Based on my measurements, the nits to the eye are only ~15% of the nits from the display.

Real-world light has to pass through the front polarizer, a QWP, the 60% transparent curved mirror, and a second QWP. The action of the front polarizer and the two QWPs will cause the real-world light to be polarized so that they will pass through the polarizing beam splitter, so there is only nominally one polarization loss. The net is that ~26% of the real-world light will make it through to the eye.

Nreal also has an additional quarter waveplate and polarizing film used to block about 95% of the front projecting polarized image light. Note that light from the real world must also pass through the front polarizer, QWPs, the partial curved mirror, and the polarizing beam splitter to reach the eye.

The assembly is reasonably light as the only glass is in the lens and the thin glass plate supporting the polarizing beam splitter. All the other components are plastic with air between them.

I will go into more detail on the materials and efficiency numbers of the Nreal design, along with some observations in the next article on Nreal.

Thickness and FOV without Pupil Replication

Ruediger Sprengard (from Schott) presentation at SPIE AR/VR/MR 2021 (behind SPIE paywall) discussed the thickness of the conventional optics using birdbaths to combine and turn light toward the eye versus waveguides (content from Slide #5 below). Sprengard shows a curve (below right) for how the beam splitter thickness must grow with FOV. IN the case of the Nreal-type birdbath, you also have to add the front mirror curvature.

The total for the Nreal ends up being about 25mm thick versus just a few millimeters for typical waveguides. In addition to the thickness, most of the Nreal’s weight of the display and optics ends up forward of the nose bridge, making the “glasses” front heavy and making the nose bear the most weight.

Blocking Front Projection

The pink path on the Nreal Optics Diagram above shows light from the display that could project forward. If there were no polarize (A), 60% of the polarized light from the OLED not reflected by the partial mirror would be projected forward. The front polarizing film acts to block about 95% forward projected light. For the picture below, I removed the polarizer from one side (below left) and retained the polarizer on the other side (below right) to see the difference. I should add that due to how the human visual system works and its dynamic range, they look less different to the eye than they do to the camera. When someone is wearing Nreal glasses, you can clearly see what they are watching, even with the polarizer.

Another function of the front polarizer is to darken the mirror reflection. The picture below with the front polarizers removed shows how the partial mirror acts as a wide-angle mirror reflecting everything in the room.

Can’t See the Eyes

Going back to last week’s article on the Lumus Maximus, notice how (below) with the Maximus you can clearly see the user’s eyes. Because the light that illuminates the eyes comes through the glasses, the light has to pass through the glasses two times (in and out). Even with ~40% transmission, as with the Hololens 2, the eyes get very dark. With birdbaths transmitting ~25% combined with the mirror reflection, the wearer’s eyes can’t be seen at all.

Lenovo ThinkReality A3 Prototypes Mirror Reflections

Lenovo’s CES 2021 ThinkReality A3 video shows a birdbath design without the front polarizer and has wide-angle mirror reflections. Even without the polarizer, you can’t see the user’s eyes. Also, it looks to me in these videos the display not active (either off or a dummy unit) as you don’t see headsets display.

I also noticed in one of Lenovo’s pictures A3 that it looks like you can see through it without any mirroring (right). At first, I thought this might be a dummy unit, but I think it is an optical effect of having more light coming from behind the unit rather than from the front as partial mirrors work as on-way glass. You won’t see this when wearing the glasses as the person would block the light.

Lenovo A3 Doesn’t Seem to Have Enough Support

While on the subject of Lenovo’s CES 2021 ThinkReality A3 video, something else caught my eye and ear. They say that they have an interchangeable ergonomic fit kit which appears to be from the image to amount temples that wrap around the back of the head somewhat but don’t connect. They also spec. The A3 is 130 grams, whereas the Nreal is at 85 grams or about 1.5x less.

The Nreal “consumer edition” is too heavy to wear very long, like glasses at 85 grams, and it is made worse by being front-heavy. The birdbath design inherently requires most of the weight to be beyond the nose bridge (see the side view of Nreal (right). They may look like glasses from the front, but they projector out considerably. I don’t see how the A3 can be 1.5x heavier than Nreal and expect a bit of grip at the back of the neck to make up the difference. I think the Nreal “Enterprise Design” (shown earlier) makes a lot more sense.

Why Birdbaths Are Common

• Relatively Inexpensive – most components are thin plastic. The only expensive component is the display device (OLED microdisplays are usually relatively expensive).
• Relatively light, particularly the first type and air-filled
• Very good image quality – basically just a magnified view of the display
• Works well with emissive Lambertian displays such as OLEDs
  • Vastly more efficient with Lambertian displays than waveguides
  • Very good image quality – close to a magnified mirror image of the display

Downsides to Birdbaths AR

• Thick – The beam splitter must be at least as thick as the image (FOV including Eye Box is tall. In the case of Nreal, the beam splitter is about 18mm thick, plus the curved mirror results in a total thickness of about 25mm (~1-inch).
• Blocks a significant amount of real-world light. In the case of Nreal, it blocks ~75% of the real-world light. Pretty much, it is like wearing moderately dark sunglasses. As I will discuss in the next part of this series, the light-blocking can’t be reduced much.
• You can’t see the eyes of the wearer. You get either a mirror reflection or dark sunglasses look plus some amount of front projection.
• Requires a “top shooter,” resulting in a thick brow above the image. There is almost no view upward as the projector severely blocks it. The user can only see straight out or down.
• Generally very short eye relief. To give more eye relief would require a much bigger and even thicker design.
• Most due to size and weight use OLEDs which have limited brightness. Typically the optics only allow ~15% of the light from the display to reach the eye. Thus an 800 nit OLED would only output
• Even with the front polarizer, I found that about 5% of the amount of light that goes to the eye projects forward.

Conclusion and Next Time

The Nreal seems to be the epitome of a reasonably good birdbath design. It has many of the same advantages, and disadvantages as any other good birdbath design will have. This is not to pick on Nreal. Similar birdbaths such as Lenovo A3, AM Glass, and the Qualcomm Reference designs will have the same issues. I’m using Nreal as an example because I could get my hands on it.

Next time, I will discuss the “design box” that limits the birdbath design, limiting the transparency to the real world and limiting the brightness. I also plan on sharing pictures from the Nreal.

Acknowledgment

I want to thank David Bonelli, the CEO of Pulsar, an engineering consulting company (and a consultant to Ravn), for reviewing my analysis and giving feedback. From 2015 to 2018, David works at the Osterhout Design Group. His work included working on the ODG R8 and R9, similar in basic structure to Nreal’s design.

Appendix: Second Type of Birdbath (Less Common)

The second type of birdbath only has the eyes looking through the beam splitter with a completely reflecting curved mirror. Google Glass most famously used this type of birdbath. But others such as Samsung and Raontech have used it as well.

When used as the main AR optic, the entire birdbath is usually has a solid (usually glass) prism forming the beam splitter with the curved mirror also being on a reflective coating on a solid. The second type of birdbath has the advantage that the user only has to look through the beam splitter resulting in more real-world light reaching the eye. Because the mirror is fully reflective, it is also more efficient for the display.

With the very small FOV of Google Glass, the display/projector was on the side. This becomes impractical as the FOV becomes wider. AR headsets using birdbaths with larger FOVs put the display and projector above the optics and project downward, what I call a “down shooter,” as is the case with Nreal.

The downside of the second type is that usually, most of the optics are a solid mass rather than air which is both more expensive and significantly heavier. Most of the headsets seem to be adopting the first (Nreal) type birdbath.