Magic Leap – Fiber Scanning Display Follow UP

Some Newer Information On Fiber Scanning

Through some discussions and further searching I found some more information about Fiber Scanning Displays (FSD) that I wanted to share. If anything, this material further supports the contention that Magic Leap (ML) is not going to have a high resolution FSD anytime soon.

Most of the images available is about fiber scanning for use as a endoscope camera and not as a display device. The images are of things like body parts they really don’t show resolution or the amount of distortion in the image. Furthermore most of the images are from 2008 or older which gives quite a bit of time for improvement. I have found some information that was generated in the 2014 to 2015 time frame that I would like to share.

Ivan Yeoh’s 2015 PhD dissertation

2015-yeoh-laser-projection

In terms of more recent fiber scanning technology, Ivan Yeoh’s name seems to be a common link. Show at left is a laser projected image and the source test pattern from Ivan Yeoh’s 2015 PhD dissertation “Online Self-Calibrating Precision Scanning Fiber Technology with Piezoelectric Self-Sensing“at the University of Washington. It is the best quality image of a test pattern or known image that I have found of a FSD anywhere. The dissertation is about how to use feedback to control the piezoelectric drive of the fiber. While his paper is about the endoscope calibration, he nicely included this laser projected image.

The drive resulted in 180 spirals which would nominally be 360 pixels across at the equator of the image with a 50Hz frame rate. But based on the resolution chart, the effective resolution is about 1/8th of that or only ~40 pixels, but about half of this “loss” is due to resampling a rectilinear image onto the spiral. You should also note that there is considerably more distortion in the center of the image where the fiber will be moving more slowly.

2015-yeoh-endoscope-manual-calibrationYeoh also included some good images at right showing how had previously used a calibration setup to manually calibrate the endoscope before use as it would go out of calibration with various factors including temperature. These are camera images and based on the test charts they are able to resolve about 130 pixels across which is pretty close to the Nyquist sampling rate from a 360 samples across spiral. As expected the center of the image where the fiber is moving the slowest is the most distorted.

While a 360 pixel camera is still very low resolution by today’s standards, it is still 4 to 8 times better than the resolution of the laser projected image. Unfortunately Yeoh was concerned with distortion and does not really address resolution issues in his dissertation. My resolution comments are based on measurements I could make from the images he published and copied above.

Washington Patent Application Filed in 2014

uow-2016-fsd-applicationYeoh is also the lead inventor on the University of Washington patent application US 2016/0324403 filed in 2014 and published in June 2016. At left is Fig. 26 from that application. It is supposed to be of a checkerboard pattern which you may be able to make out. The figure is described as using a “spiral in and spiral out” process where the rather than having a retrace time, they just reverse the process. This applications appears to be related to Yeoh’s dissertation work. Yeoh is shown as living in Fort Lauderdale, FL on the application, near Magic Leap headquarters.   Yeoh is also listed as an inventor on the Magic Leap application US 2016/0328884 “VIRTUAL/AUGMENTED REALITY SYSTEM HAVING DYNAMIC REGION RESOLUTION” that I discuss in my last article. It would appear that Yeoh is or has worked for Magic Leap.

2008 YouTube Video

ideal-versus-actually-spiral-scan

Additionally, I would like to include some images from a 2008 YouTube Video that kmanmx from the Reddit Magic Leap subreddit alerted me to. White this is old, it has a nice picture of the fiber scanning process both as a whole and with close-up image near the start of the spiral process.

For reference on the closeup image I have added the size of a “pixel” for a 250 spiral / 500 pixel image (red square) and what a 1080p pixel (green square) would be if you cropped the circle to a 16:9 aspect ratio. As you hopefully can see the spacing and jitter variations-error in the scan process are several 1080p pixels in size. While this information is from 2008, the more recent evidence above does not show a tremendous improvement in resolution.

Other Issues

So far I have mostly concentrated on the issue of resolution, but there are other serious issues that have to be overcome. What is interesting in the Magic Leap and University of Washington patent literature is the lack of patent activity to address the other issues associated with generating a fiber scanned image. If Magic Leap were serious and had solved these issues with FSD, one would expect to see patent activity in making FSD work at high resolution.

One major issue that may not be apparent to the casual observer is the the controlling/driving the lasers over an extremely large dynamic range. In addition to support the typical 256 (8-bits) per color and supporting overall brightness adjustment based on the ambient light, the speed of the scan varies by a large amount an they must compensate for this or end up with a very bright center where the scan is moving more slowly. When you combine it all together they would seem to need to control the lasers over a greater than 2000:1 dynamic range from a dim pixel at the center to a brightest pixel at the periphery.

Conclusion

Looking at all the evidence there is just nothing there to convince me that Magic Leap is anywhere close to having perfected a FSD to the point that it could be competitive with a conventional display device like LCOS, DLP or Micro-OLED, not less the 50 megapixel resolutions they talk about. Overall, there is reasons to doubt that a electromechanical scan process is going to in the long run compete with an all electronic method.

It very well could be that Magic Leap had hoped that FSD would work and/or it was just a good way to convince investors that they had a technology that would lead to super high resolution in the future. But there is zero evidence that have seriously improved on what the University of Washington has done. They may still be pursuing it as an R&D effort but there is no reason to believe that they will have it in a product anytime soon.

All roads point to ML using either LCOS (per Business Insider of October 2016) or a DLP based what I have heard is in some prototypes. This would mean they will likely have either 720p or 1080p resolution display, or the same as others such as Hololens (which will likely have a 1080p version soon).

The whole FSD is about trying to break through the physical pixel barrier of conventional technologies.  There are various physics (diffraction is becoming a serious issue) and material issues that will likely make it tough to make physical pixels much smaller than 3 micron.

Even if there was a display resolution breakthrough (which I doubt based on the evidence), there are issues as to whether this resolution could make it through the optics. As the resolution improves the optics have to also improve or else they will limit the resolution. This is a factor that particularly concerns me with the waveguide technologies I have seen to date that appear to be at the heart of Magic Leap optics.

5 comments

  1. Taniya T. says:

    Magic Leap is the most exciting thing in AR right now and we still don’t know exactly how it works

    • KarlG says:

      It is certainly exciting in terms of the amount they have raised and how much they talk. I think I am getting very close to how it works. Magic Leap has a way of talking about what they want to do someday but that is not what they will have at first.

    • drtune says:

      Why is it exciting – based on some dubious Youtube videos? Demos for impartial observers and pictures of working hardware prototypes would be exciting, both of which would give us reason to believe they’ve solved the many obvious technical problems between what we have today and a commercially viable mass-market product (which is the only thing that would justify the huge money invested in them). They have a lot of engineering challenges to solve and this article explores the huge difficulties in implementing a fiber-scanning display. Good work.

  2. It’s really interesting how you view the resolution as the main the concern, while Yeoh was mainly focused on the distortion. I agree that resolution is a much more compelling issue right now. I would think that distortion could be corrected after the scan takes place, so it would be less of an issue. Is that possible?

    • KarlG says:

      I don’t know that resolution is my main concern, there is cost and other factors too.

      If you do “correction” in software by pre-processing the image you inherently lose resolution due to re-sampling. A major problem occurs if the scan has gaps in it where you can’t draw a pixel at all and if the scan is not predictable (random errors). The fiber is particularly unstable at the start of the scan when it is moving slow (has low momentum) and that was shown in the 2008 YouTube video’s scanning pattern I captured for the article.

      Basically you are at the will of the wiggling fiber and all it mechanical harmonics. The concept seem great for making a low resolution camera image, but it does not scale up well to high resolution. Even getting to something equivalent to 720p, no less 1080p is way beyond its capability.

      In many ways Fiber scanning is worse than CRT scanning in that you are trying to move a mechanical mass rather than an electron beam. A wiggling fiber is a bad starting point to try and make a high resolution display.

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