North’s Focals LBS AR Glasses Overview
North (formerly Thalmic Labs) announced a few days ago (Oct 23, 2018) the “Focals” laser beam scanning-based AR glasses (aside: a set of confusing names). The best article I saw on the “Focals” was from The Verge. The Verge, back in February 2018 covered a similar (now canceled) concept from Intel call the Vaunt.
A set of low-resolution (~300 by 300 pixels), small field of view (~15 degrees), custom fitted AR glasses, and costing $999 might otherwise be a nothing announcement. But with both Amazon and Intel have invested in North/Thalmic it is garnering them some attention from the press.
The Focals use laser beam scanning (LBS) to generate a low-resolution image (described by The Verge as 300 by 300 pixels or about the resolution of an Apple watch) with a holographic film to redirect the light back toward the eye. The basic “physics” behind the Focals with red, green and blue lasers are the same as the single (red) color Intel Vaunt AR glasses that I describe in two articles (article 1 and article two) in Feb. 2018.
Laser Beam Scanning (LBS) and Its Midas Touch Consequences
LBS is the “King Midas Touch” (the king whose touch turned everything into gold) of displays; it seems great until you understand the consequences. I have been describing the multitude of problems with LBS displays since this blog started in 2011. The “superpowers” of “infinite focus of laser beams” bring with them all manner of difficult to solve problems. The problems associated with using them in AR glasses include:
- Optics directing the laser light into the eye and the problems it causes with the view
- Low resolution and low frame rate electro-mechanical scanning
- Extremely small eye-box/pupil (the image disappears unless perfectly lined up with eye)
- The complexity of combining three (or more) lasers into a single highly coaxial and tight set of beams for color
- Cost of lasers versus LEDs
- Controlling the brightness of a beam with a highly variable velocity
- Casting shadows on the retina due to “floaters” in the eye
Holographic Film to Bend The Light Toward The Eye
The reason for the holographic film is that if they use a simple mirror reflection, with the angle of incidence equal to the angle of reflection, the light will miss the eye (see figure on the right). So they use a holographic film to act as a tilted mirror at an angle to redirect the light at a sharper angle.
Thalmic/North has several patents for embedding the holographic film in molded glasses. But embedding the film has its issues including how much you can curve it. North only supports a limited range of diopter (focus) corrections and no astigmatism nor bi-focal support.
The film is going to have a negative effect on the view through it and will likely have similar problems as diffractive waveguides such as those used by Intel and Magic Leap (see my articles on diffractive waveguides including here). As described by The Vaunt, “The photopolymer material that serves as the display location isn’t noticeable for the most part, but when it catches the light, it looks like the glasses need to be wiped down.”
Laser Scanning AR’s Tiny Eye Box
North faced the same tiny eye box issue as Intel with their similar “Vaunt” glasses. Both the Verge article on the Intel Vaunt and the Verge article on the North Focals describe needing to have the custom fitting and the need to look directly at the image to see the image at all.
The fundamental problem is that with laser scanning direct writing on the retina, the laser beams have to be aimed to go directly through the pupil of the eye. If the laser beam aim is off or the eye moves even slightly, then the laser beams miss the pupil, and no image is seen at all.
The problem is discussed in several patent applications assigned to bothThalmic/North and Intel. The figures below illustrate the problem from their applications/patents hoping to improve it. Both of these try to replicate the laser beam so that when the eye moves relative to the beam, the laser light will still enter the pupil. Since both the Focals and the Vaunt have a tiny eye box, neither of these techniques were successfully implemented.
Tiny Field of View (FOV)
In the “Marketing Versus Reality” department, we have North’s “concept” image showing a huge clock with the date filling the FOV. On top of North’s stock image, I have taken a photo from the Verge article then rotated and scaled it to overlay in order to show the actual size of the holographic film. The image will then have to fit inside the circle of the film as is roughly indicated by the dotted line square. This is the difference between the ~60-degree FOV implied by the marketing concept and the ~ 15-degree FOV that they can actually support.
The Verge had to run a correction that they called them “North Labs,” but with a company name of “North” and a product named “Focals,” name confusion is inevitable. Based on what they are planning on selling: expensive, custom fit, low resolution, and small FOV AR glasses, I don’t think we will have to worry about the name problem for too long.
What I don’t understand, is why big companies like Amazon and Intel throw money into this type of concept without understanding the basic physics involved. You would think it would be far cheaper to do some good due diligence first. Ah well, at least they spent a lot less on the Focals than has been spent on Magic Leap. I want to see companies showering money on startups that will have a chance of bringing a good product to market.
I would like to thank Ron Padzensky for reviewing and making corrections to this article.