Quick Note On Photonics West Next Week
I’m going to Photonics West next week in San Francisco. I plan on attending the AR/VR sessions on Feb. 3rd and 4th followed by the main show on the 5th and 6th. If you would like to meet, please email me at firstname.lastname@example.org.
Update On Nreal’s Transparancy January, 31, 2019
I have been contacted by Nreal that they don’t agree with the transparency measurement I have made. I measured roughly 10% linearly with my camera’s photograph of white paper. In the past, I have found this to be a reasonably accurate way to estimate transparency in comparisons between my camera and light meters. Nreal used a light meter (which I forgot to bring to the show flow) and they get between 25% and 30%. This number percentage is certainly possible to obtain with the optics they are using. So I am adding this note and will check in more detail the next time I have a chance.
2019-June: I met with Nreal at AWE was able to take measurements to confirm that the Nreal optics are about 30% transparent.
Introduction – Picking Up From the Summary Article
As I wrote last time, Nreal caused a lot of buzz both in the AR hall in news articles. They have a very simple “birdbath optical” design with a Sony 1080p Micro-OLED display. With a former Magic Leap person founding the company and a similar feature set and even system configuration, the comparisons are inevitable. waveguide based solutions, other than Magic Leap, have a major advantage over most simpler AR optics.
Nreal – Blowing away Magic Leap on Image Quality
Nreal has 1080p resolution and delivers about 3X the horizontal and 2X the effective vertical resolution of Magic Leap with much better contrast and far fewer artifacts. In a way, Nreal demonstrates the poorness of Magic Leap’s optics with a direct comparison. As I discussed in my comparison of Magic Leap to Hololens and Lumus, Magic Leap optics significantly blur/soften the native display resolution and only deliver about 640 by 480 resolution to the eye. And in an earlier article, I showed how the Magic Leap One has a lot of other optical issues that are not present in the Nreal design.
Nreal was founded by a former Magic Leap engineer, Chi Xu, and has a similar system configuration with a separate computer pack with battery and a separate controller. Like Magic Leap and Hololens, it supports 6 degrees of freedom tracking and SLAM (simultaneous localization and mapping).
Just the headset is only 85 grams compared to the Magic Leap One which tips the scales at about 335 grams. The Nreal has a much smaller (about 3 x3 inches) compute and battery pack which they call “Toast,” and a disked shaped controller which they call “Oreo.” I suspect they have less processing power and less capable SLAM mapping, but I have not evaluated these aspects of the overall design. Nreal did say that they are still working on improving the SLAM.
The cable is wired into the headset on one side and has a standard USB-C connection on the other side. I very much liked the idea of using a standard USB-C connection, but they should have used it on both the Compute pack and the headset side. By having the USB-C on both ends, there would be a “breakaway” feature to the headset if the cable gets snagged as well as support the use of different length USB-C cables. This would enable using a very short USB-C cable to put the compute pack on the back of a headband virtually eliminating the cable snag hazard.
Keep It Simple Stupid (KISS) Optical Design
A classic case of Keep It Simple Stupid (KISS), there is absolutely nothing remarkable about the optical design. Some form of birdbath (beam splitter and spherical mirror) optics very commonly used in near-eye displays (see the figure from an ODG patent). I should emphasize that many companies have used a similar birdbath design. The reason the birdbath is so popular is that it gives relatively good image quality, particularly with respect to chroma aberrations, at a very low cost.
Issues With The Birdbath Optical Design
Update (January 31, 2019) – I have been contacted by Nreal and they claim that when measured by a meter, their optics let through between 25% and 30% of the light which translates to blocking 75% to 70% of the light. This new value would be in the expected range for the type of birdbath optics Nreal is using. As I only made a quick measurement of a demo system on the floor and with Nreal providing a measurement, I’m adding this note and striking through the text below. Assuming the Nreal information is correct, then they would be more transmissive than Magic Leap.
There is literally a “dark side” to the birdbath in that it blocks a lot of light
. In the case of the Nreal headset, I found that it blocks about 90% of the real-world light which is more than most dark sunglasses and more than Magic Leaps blocking of 85%. Additionally, controlling double images due to reflections is a well know problem with birdbath designs. I discussed the issues with the birdbath design back March 2017.
The birdbath is “thick” due to the beam splitter at 45 degrees. The Nreal optical design is about 1-inch (25mm) front to back. While they may look like ordinary glasses from the front, they are much thicker. While the weight is “only” 85 grams which is light for a headset, that is very heavy for glasses. Also due to the 1-inch thickness, the weight is very far forward on the nose.
It is worth point out that while waveguide-based optics start out as relatively thin glasses, by the time they are encased in protective shields and with SLAM cameras and other features, they are often thicker than the Nreal optics. For example, Magic Leap Ones are about the same thickness as Nreal and Hololens are much thicker.
An obvious problem with the Nreal design is that the 45-degree beam splitter can direct light from below it into the eye. I have shown the light path with a red arrow in the picture above, and there is a reflection of my badge which was below the glasses in the through the optics picture below. This reflection could be easily fixed by adding some form of the eyecup. This might also be an area where something like eye tracking could be added. In the picture below, I also added some arrows pointing to some double images I see in the image. The number of reflections does not seem as bad as they were in the similar ODG R9 design, but I have not had a chance to do a rigorous evaluation.
This problem could be solved with some form of structure to block light from below. This structure would have to be compatible with their vision inserts.
The headset gets too hot against the forehead. It was not that noticeable in the quick demos that shut down when a person was not viewing them but was a common complaint with users in the demo area. Nreal is aware of this problem and said that these are just early prototypes. Still, it would be nice if they had thought of better thermal management from the start. I expect that fixing this problem will add some weight to the headset.
Vision Correction Support
As with all other “glasses like” AR designs such as Magic Leap and ODG, vision correction is supported only via custom lens inserts. Then there is North Focals where you have to go to their stores to get custom lenses made at their stores.
There is then as the obvious drawback of needing custom inserts made. Many people do not have vision that can be corrected by simple diopter problems and need fully custom lenses.
There is not an alternative to inserts for a small form factor. If they make the glasses big enough to accommodate even relatively small “normal” frames for glasses, the AR glasses become much larger. Additionally, accommodating existing frame would cause the optics have to be further from the eye, which in turn would result in larger, heavier, and likely more expensive optics to keep the field of view and eyebox the same.
Thus, you see a gap between the size of headsets like Hololens that support wearing most vision correction and Magic Leap and Nreal that use inserts. It is a major design trade-off between ease of use and size.
Conclusions and Suggestions
Nreal had a very simple optical design and coupled it with a Sony high contrast 1080p OLED which resulted in a very good image. To a large degree, the Nreal demonstrates the sacrifices made in image quality by other AR/MR headset such as Magic Leap and Hololens.
As I wrote above, I would highly recommend they would add a USB-C socket on the headset rather than wiring it in. This would allow flexibility in the length of cable and even support a self contained headband or cap.
The image quality and simplicity of the optics comes at the expense of poor real-world light throughput and display inefficiency. Nreal or others competing with them could tweak the reflection percentage in the birdbath design to improve the light blocking of the real world at the expense of a dimmer image or need to drive the display harder.
They certainly need to work on their heat management in the design and this, in turn, should have a negative effect on size and weight. While this may seem simple, it may proved to be a significant problem.
They are doing in-system computing, 6 DOF tracking and some level of SLAM, but I did not evaluate their capabilities relative to the likes of Hololens and Magic Leap. This blog has enough to do just covering the optical designs.I would highly recommend using USB-C on both the glasses side as well as the Computer side of the glasses cable. In this way, the user could easily choose a shorter cable. Then with a short USB-C cable, they could have a “cap,” visor, or headband option to mount the “Toast” (Computer and Battery) on the back of the user’s head. I have found the cable on Magic Leap to be a serious snag and dragging hazard that can cause the headset to be broken.