Tag Archive for ODG

Near-Eye Bird Bath Optics Pros and Cons – And IMMY’s Different Approach

Why Birdbaths Optics? Because the Alternative (Waveguides) Must Be Worse (and a teaser)

The idea for this article started when I was looking at the ODG R-9 optical design with OLED microdisplays. They combined an OLED microdisplay that is not very bright in terms of nits with a well known “birdbath” optical design that has very poor light throughput. It seems like a horrible combination. I’m fond of saying “when intelligent people chose a horrible design, the alternative must have seemed worse

I’m going to “beat up” so to speak the birdbath design by showing how some fundamental light throughput numbers multiply out and why the ODG R-9 I measured at CES blocks so much of the real world light. The R-9 also has a serious issue with reflections. This is the same design that a number of publications considered among the “best innovations” of CES; it seems to me that they must have only looked at the display superficially.

Flat waveguides such as used by Hololens, Vuzix. Wave Optics, and Lumus as well as expected from Magic Leap get most of the attention, but I see a much larger number of designs using what is known as a “birdbath” and similar optical designs. Waveguides are no secret these days and the fact that so many designs still use the birdbath optics tells you a lot about the issues with waveguides. Toward the end of this article, I’m going to talk a little about the IMMY design that replaces part of the birdbath design.

As a teaser, this article is to help prepare for an article on an interesting new headset I will be writing about next week.

Birdbath Optics (So Common It Has a Name)

The birdbath combines two main optical components, a spherical mirror/combiner (part-mirror) and a beam splitter. The name  “birdbath” comes from the spherical mirror/combiner looking like a typical birdbath. It is used because it generally is comparatively inexpensive to down right cheap while also being relatively small/compact while having  good overall image quality. The design fundamentally supports a very wide FOV, which are at best difficult to support with waveguides. The big downsides are light throughput and reflections.

A few words about Nits (Cd/m²) and Micro-OLEDs

I don’t have time here to get into a detailed explanation of nits (Cd/m²). Nits is the measure of light at a given angle whereas lumens is the total light output. The simplest analogy is to water hose with a nozzle (apropos here since we are talking about birdbaths). Consider two spray patterns, one with a tight jet of water and one with a wide fan pattern both outputting the exact same total amount of water per minute (lumens in this analogy). The one with the tight patter would have high water pressure (nits in this analogy) over a narrow angle where the fan spray would have lower water pressure (nits) over a wider angle.

Additionally, it would be relatively easy to put something in the way of the tight jet and turn it into a fan spray but there is no way to turn the fan spray into a jet. This applies to light as well, it is much easier to go from high nits over are narrow angle to lower nits over a wide angle (say with a diffuser) but you can’t go the other way easily.

Light from an OLED is like the fan spray only it covers a 180 degree hemisphere. This can be good for a large flat panel were you want a wide viewing angle but is a problem for a near eye display where you want to funnel all the light into the eye because so much of the light will miss pupil of the eye and is wasted. With an LED you have a relative small point of light that can be funneled/collimated into a tight “jet” of light to illuminate an LCOS or DLP microdisplay.

The combination of light output from LEDs and the ability to collimate the light means you can easily get tens of thousands of nits with an LCOS or DLP illuminated microdisplay were OLED microdisplays typically only have 200 to 300 nits. This is major reason why most see-through near eye displays use LCOS and DLP over OLEDs.

Basic Non-Polarizing Birdbath (example, ODG R-9)

The birdbath has two main optical components, a flat beam splitter and a spherical mirror. In the case a see-through designs, the the spherical mirror is a partial mirror so the spherical element acts as a combiner. The figure below is taken from an Osterhaut Design Group (ODG) patent which and shows simple birdbath using an OLED microdisplay such as their ODG R-9. Depending on various design requirements, the curvature of the mirror, and the distances, the lenses 16920 in the figure may not be necessary.

The light from the display device, in the case of the ODG R-9 is a OLED microdisplay, is first reflect away from the eye and perpendicular (on-axis) to the curved beam splitter so that a simple spherical combiner will uniformly magnify and move the apparent focus point of the image (if not “on axis” the image will be distorted and the magnification will vary across the image). The curved combiner (partial mirror) has minimal optical distortion on light passing through.

Light Losses (Multiplication is a Killer)

A big downside to the birdbath design is the loss of light. The image light must make two passes at the beam splitter, a reflective and transmissive, with a reflective (Br) and transmissive (Bt) percentages of light. The light making it through both passes is Lr x Lt.  A 50/50 beam splitter might be about 48% reflective and transmissive (with say a 4% combined loss), and the light throughput (Br x Bt) in this example is only 48% x 48%= ~23%. And “50/50” ratio is the best case; if we assume a nominally 80/20 beam splitter (with still 4% total loss) we get 78% x 18% = ~14% of the light making through the two passes.

Next we have the light loss of the spherical combiner. This is a trade-off of image light being reflected (Cr) versus being transmitted  (Ct) to the real world where Cr + Ct is less than 1 due to losses. Generally you want the Cr to be low so the Ct can be high so you can see out (otherwise it is not much of a see through display).

So lets say the combiner has Cr=11% and the Ct=75% with about 4% loss with the 50/50 beamsplitter. The net light throughput assuming a “50/50” beam splitter and a 75% transmissive combiner is Br x Cr X Bt = ~2.5% !!! These multiplicative losses lose all but a small percentage of the display’s light. And consider that the “real world” net light throughput is Ct x Bt which would be 48% x 75% = 36% which is not great and would be too dark for indoor use.

Now lets say you want the glasses to be at least 80% transmissive so they would be considered usable indoors. You might have the combiner Ct=90% making Cr=6% (with 4% loss) and then Bt=90% making Br=6%. This gives the real world transmissive about 90%x90% = 81%.  But then you go back and realize the display light equation (Br x Cr X Bt) becomes 6%x6%x90% = 0.3%. Yes, only 3/1000ths of the starting image light makes it through. 

Why the ODG R-9 Is Only About 4% to 5% “See-Through”

Ok, now back to the specific case of the ODG R-9. The ODG R-9 has an OLED microdisplay that most like has about 250 nits (200 to 250 nits is commonly available today) and they need to get about 50 nits (roughly) to the eye from the display to have a decent image brightness indoors in a dark room (or one where most of the real world light is blocked). This means they need a total throughput of 50/250=20%. The best you can do with two passes through a beam splitter (see above) is about 23%.  This forces the spherical combiner to be highly reflective with little transmission. You need something that reflects 20/23=~87% of the light and only about 9% transmissive. The real world light then making it through to the eye is then about 9% x 48% (Ct x Bt) or about 4.3%.

There are some other effects such as the amount of total magnification and I don’t know exactly what their OLED display is outputting display and exact nits at the eyepiece, but I believe my numbers are in the ballpark. My camera estimates for the ODG R-9 came in a between 4% and 5%. When you are blocking about 95% of the real world light, are you really much of a “see-through” display?

Note, all this is BEFORE you consider adding say optical shutters or something like Varilux® light blocking. Normally the birdbath design is used with non-see through designs (where you don’t have the see-through losses) or with DLP® or LCOS devices illuminated with much higher nits (can be in the 10’s of thousands) for see through designs so they can afford the high losses of light.

Seeing Double

There are also issues with getting a double image off of each face of plate beam splitter and other reflections. Depending on the quality of each face, a percentage of light is going to reflect or pass through that you don’t want. This light will be slightly displaced based on the thickness of the beamsplitter. And because the light makes two passes, there are two opportunities to cause double images. Any light that is reasonably “in focus” is going to show up as a ghost/double image (for good or evil, your eye has a wide dynamic range and can see even faint ghost images). Below is a picture I took with my iPhone camera of a white and clear menu through the ODG R-9. I counted at least 4 ghost images (see colored arrows).

As a sort of reference, you can see the double image effect of the beamsplitter going in the opposite direction to the image light with my badge and the word “Media” and its ghost (in the red oval).

Alternative Birdbath Using Polarized Light (Google Glass)

Google Glass used a different variation of the birdbath design. They were willing to accept a much smaller field of view and thus could reasonably embedded the optics in glass. It is interesting here to compare and contrast this design with the ODG one above.

First they started with an LCOS microdisplay that was illuminated by LEDs that can be very much brighter and more collimated light resulting in much higher (can be orders of magnitude) starting nits than an OLED microdisplay can output. The LED light is passed through a polarizing beam splitter than will pass about 45% P light to the LCOS device (245). Note a polarizing beam splitter passes one polarization and reflect the other unlike a the partially reflecting beam splitter in the ODG design above. The LCOS panel will rotate the light to be seen to S polarization so that it will reflect about 98% (with say 2% loss) of the S light.

The light then goes to a second polarizing beam splitter that is also acting as the “combiner” that the user sees the real world through. This beam splitter is set up to pass about 90% of the S light and reflect about 98% of the P light (they are usually much better/more-efficient in reflection). You should notice that they have a λ/4 (quarter wave = 45 degree rotation) film between the beam splitter and the spherical mirror which will rotate the light 90 degrees (turning it from S to P) after it passes through it twice. This  λ/4 “trick” is commonly used with polarized light. And since you don’t have to look through the mirror, it can be say 98% reflective with say another 3% loss for the λ/4.

With this design, about 45% (one pass through the beamsplitter) of the real world makes it through, but only light polarized the “right way” makes it through which makes looking at say LCD monitors problematical. By using the quarter wave film the design is pretty efficient AFTER you loose about 55% of the LED light in polarizing it initially. There are also less reflection issues because all the films and optics are embedded in glass so you don’t get these air to glass index mismatches of off two surfaces of a relatively thick plate that cause unwanted reflections/double images.

Google Glass design has a lot of downsides too. There is nothing you can do to get the light throughput of the real world much above 45% and there are always the problems of looking through a polarizer. But the biggest downside is that it cannot be scaled up for larger fields of view and/or more eye relief. As you scale this design up the block of glass becomes large, heavy and expensive as well as being very intrusive/distorting in looking through a big thick piece of glass.

Without getting too sidetracked, Lumus in effect takes the one thick beam splitter, and piece-wise cuts it into multiple smaller beam splitters to make the glass thinner. But this also means you can’t use the spherical mirror of a birdbath design with it and so you require optics before the beam splitting and the light losses of the the piece-wise beam splitting are much larger than a single beamsplitter.

Larger Designs

An alternative design would mix the polarizing beamsplitters of the Google Glass design above with the configuration of ODG design above.  And this has been done many times through the years with LCOS panels that use polarized light (an example can be found in this 2003 paper). The spherical mirror/combiner will be a partial non-polarizing mirror so you can see through it and a quarter waveplate is used between the spherical combiner and the polarizing beam splitter. You are then stuck with about 45% of the real world light times the light throughput of the spherical combiner.

A DLP with a “birdbath” would typically use the non-polarizing beam splitter with a design similar to the ODG R-9 but replacing the OLED microdisplay with a DLP and illumination. As an example, Magic Leap did this with a DLP but adding a variable focus lens to support focus planes.

BTW, by the time you polarized the light from an OLED or DLP microdisplay, there would not be much if any of an efficiency advantage sense to use polarizing beamsplitters. Additionally, the light from the OLED is so diffused (varied in angles) that it would likely not behave well going through the beam splitters.

IMMY – Eliminating the Beamsplitter

The biggest light efficiency killer in the birdbath design is the combined reflective/transmissive passes via the beamsplitter. IMMY effectively replaces the beamsplitter of the birdbath design with two small curved mirrors that he correct for the image being reflected off-axis from the larger curved combiner. I have not yet seen how well this design works in practice but at least the numbers would appear to work better. One can expect only a few percentage points of light being lost off of each of the two small mirrors so that maybe 95% of the light from the OLED display make it to the large combiner. Then you have the the combiner reflection percentage (Cr) multiplying by about 95% rather than the roughly 23% of the birdbath beam splitter.

The real world light also benefits as it only has to go through a single combiner transmissive loss (Ct) and no beamsplitter (Bt) loses. Taking the OGD R-9 example above and assuming we started with a 250 nit OLED and with 50 nits to the eye, we could get there with about an 75% transmissive combiner. The numbers are at least starting to get into the ballpark where improvements in OLED Microdisplays could fit at least for indoor use (outdoor designs without sunshading/shutters need on the order of 3,000 to 4,000 nits).

It should be noted that IMMY says they also have “Variable transmission outer lens with segmented addressability” to support outdoor use and variable occlusion. Once again this is their claim, I have not yet tried it out in practice so I don’t know the issues/limitations. My use of IMMY here is to contrast it with the classical birdbath designs above.

A possible downside to the the IMMY multi-mirror design is bulk/size has seen below. Also noticed the two adjustment wheel for each eye. One is for interpupillary distance to make sure the optics line up center with the pupils which varies from person to person. The other knob is a diopter (focus) adjustment which also suggests you can’t wear these over your normal glasses.

As I have said, I have not seen IMMY’s to see how it works and to see what faults it might have (nothing is perfect) so this is in no way an endorsement for their design. The design is so straight forward and a seemingly obvious solution to the beam splitter loss problem that it makes me wonder why nobody has been using it earlier; usually in these cases, there is a big flaw that is not so obvious.

See-Though AR Is Tough Particularly for OLED

As one person told me at CES, “Making a near eye display see-through generally more than double the cost” to which I would add, “it also has serious adverse affects on the image quality“.

The birdbath design wastes a lot of light as do every other see-through designs. Waveguide designs can be equally or more light wasteful than the birdbath. At least on paper, the IMMY design would appear to waste a less than most others. But to make a device say 90% see through, at best you start by throwing away over 90% of the image light/nits generated, and often more than 95%.

The most common solution to day is to start with LED illuminated LCOS or DLP microdisplay so you have a lot of nits to throw at the problem and just accept the light waste. OLEDs are still orders of magnitude in brightness/nits away from being able to compete with LCOS and DLP with brute force.

 

CES 2017 AR, What Problem Are They Trying To Solve?

Introduction

First off, this post is a few weeks late. I got sick on returning from CES and then got busy with some other pressing activities.

At left is a picture that caught me next to the Lumus Maximus demo at CES from Imagineality’s “CES 2017: Top 6 AR Tech Innovations“. Unfortunately they missed that in the Lumus booth at about the same time was a person from Magic Leap and Microsoft’s Hololens (it turned out we all knew each other from prior associations).

Among Imagineality’s top 6 “AR Innovations” were ODG’s R-8/R-9 Glasses (#1) and Lumus’s Maximus 55 degree FOV waveguide (#3). From what I heard at CES and saw in the writeups, ODG and Lumus did garner a lot of attention. But by necessity, theses type of lists are pretty shallow in their evaluations and I try to do on this blog is go a bit deeper into the technology and how it applies to the market.

Among the near eye display companies I looked at during CES include Lumus, ODG, Vuzix, Real Wear, Kopin, Wave Optics, Syndiant, Cremotech, QD Laser, Blaze (division of eMagin) plus several companies I met with privately. As interesting to me as their technologies was there different takes on the market.

For this article, I am mostly going to focus on the Industrial / Enterprise market. This is were most of the AR products are shipping today. In future articles, I plan to go into other markets and more of a deep dive on the the technology.

What Is the Problem They Are Trying to Solve?

I have had an number of people asked me what was the best or most interesting AR thing I saw at CES 2017, and I realized that this was at best an incomplete question. You first need to ask, “What problem are they trying to solve?” Which leads to “how well does it solve that problem?” and “how big is that market?

One big takeaway I had at CES having talked to a number of different company’s is that the various headset designs were, intentionally or not, often aimed at very different applications and use cases. Its pretty hard to compare a headset that almost totally blocks a user’s forward view but with a high resolution display to one that is a lightweight information device that is highly see-through but with a low resolution image.

Key Characteristics

AR means a lot of different things to different people. In talking to a number of companies, you found they were worried about different issues. Broadly you can separate into two classes:

  1. Mixed Reality – ex. Hololens
  2. Informational / “Data Snacking”- ex. Google Glass

For most of the companies were focused on industrial / enterprise / business uses at least for the near future and in this market the issues include:

  1. Cost
  2. Resolution/Contrast/Image Quality
  3. Weight/Comfort
  4. See-through and/or look over
  5. Peripheral vision blocking
  6. Field of view (small)
  7. Battery life per charge

For all the talk about mixed reality (ala Hololens and Magic Leap), most of the companies selling product today are focused on helping people “do a job.” This is where they see the biggest market for AR today. It will be “boring” to the people wanting the “world of the future” mixed reality being promised by Hololens and Magic Leap.

You have to step back and look at the market these companies are trying to serve. There are people working on a factory floor or maybe driving a truck where it would be dangerous to obscure a person’s vision of the real world. They want 85% or more transparency, very lightweight and highly comfortable so it can be worn for 8 hours straight, and almost no blocking of peripheral vision. If they want to fan out to a large market, they have to be cost effective which generally means they have to cost less than $1,000.

To meet the market requirements, they sacrifice field of view and image quality. In fact, they often want a narrow FOV so it does not interfere with the user’s normal vision. They are not trying to watch movies or play video games, they are trying to give necessary information for person doing a job than then get out of the way.

Looking In Different Places For the Information

I am often a hard audience. I’m not interested in the marketing spiel, I’m looking for what is the target market/application and what are the facts and figure and how is it being done. I wanting to measure things when the demos in the boths are all about trying to dazzle the audience.

As a case in point, let’s take ODG’s R-9 headset, most people were impressed with the image quality from ODG’s optics with a 1080p OLED display, which was reasonably good (they still had some serious image problems caused by their optics that I will get into in future articles).

But what struck me was how dark the see-through/real world was when viewed in the demos. From what I could calculate, they are blocking about 95% of the real world light in the demos. They also are too heavy and block too much of a person’s vision compared to other products; in short they are at best going after a totally different market.

Industrial Market

Vuzix is representative of the companies focused on industrial / enterprise applications. They are using with waveguides with about 87% transparency (although they often tint it or uses photochromic light sensitive tinting). Also the locate the image toward the outside of the use’s view so that even when an image it displayed (note in the image below-right that the exit port of the waveguide is on the outside and not in the center as it would be on say a Hololens).

The images at right were captured from a Robert Scoble interview with Paul Travers, CEO of Vuzix. BTW, the first ten minutes of the video are relatively interesting on how Vuzix waveguides work but after that there is a bunch of what I consider silly future talk and flights of fancy that I would take issue with. This video shows the “raw waveguides” and how they work.

Another approach to this category is Realwear. They have a “look-over” display that is not see through but their whole design is make to not block the rest of the users forward vision. The display is on a hinge so it can be totally swung out of the way when not in use.

Conclusion

What drew the attention of most of the media coverage of AR at CES was how “sexy” the technology was and this usually meant FOV, resolution, and image quality. But the companies that were actually selling products were more focused on their user’s needs which often don’t line up with what gets the most press and awards.

 

ODG R-8 and R-9 Optic with a OLED Microdisplays (Likely Sony’s)

ODG Announces R-8 and R-9 OLED Microdisplay Headsets at CES

It was not exactly a secret, but Osterhout Design Group (ODG) formally announce their new R-8 headset with dual 720p displays (one per eye) and R-9 headset with dual 1080p displays.  According to their news release, “R-9 will be priced around $1,799 with initial shipping targeted 2Q17, while R-8 will be less than $1,000 with developer units shipping 2H17.

Both devices use use OLED microdisplays but with different resolutions (the R-9 has twice the pixels). The R-8 has a 40 degree field of view (FOV) which is similar to Microsoft’s Hololens and the R-9 has about a 50 degree FOV.

The R-8 appears to be marketed more toward “consumer” uses with is lower price point and lack of an expansion port, while ODG is targeting the R-9 to more industrial uses with modular expansion. Among the expansion that ODG has discussed are various cameras and better real world tracking modules.

ODG R-7 Beam Splitter Kicks Image Toward Eye

With the announcement comes much better pictures of the headsets and I immediately noticed that their optics were significantly different than I previously thought. Most importantly, I noticed in the an ODG R-8 picture that the beam splitter is angled to kicks the light away from the eye whereas the prior ODG R-7 had a simple beam splitter that kicks the image toward the eye (see below).

ODG R-8 and R-8 Beam Splitter Kicks Image Away From Eye and Into A Curved Mirror

The ODG R-8 (and R-9 but it is harder to see on the available R-9 pictures) does not have a simple beam splitter but rather a beam splitter and curve mirror combination. The side view below (with my overlays of the outline of the optics including some that are not visible) that the beam splitter kicks the light away from the eye and toward partial curved mirror that acts as a “combiner.” This curve mirror will magnify and move the virtual focus point and then reflects the light back through the beam splitter to the eye.

On the left I have taken Figure 169 from ODG’s US Patent 9,494,800. Light from the “emissive display” (ala OLED) passes through two lenses before being reflected into the partial mirror. The combination of the lenses and the mirror act to adjust the size and virtual focus point of the displayed image. In the picture of the ODG R-8 above I have taken the optics from Figure 169 and overlaid them (in red).

According to the patent specification, this configuration “form(s) at wide field of view” while “The optics are folded to make the optics assembly more compact.”

At left I have cropped the image and removed the overlay so you can see the details of the beam splitter and curved mirror joint.  You hopefully can see the seam where the beam splitter appears to be glued to the curved mirror suggesting the interior between the curved mirror and beam splitter is hollow. Additionally there is a protective cover/light shade over the outside of the curved mirror with a small gap between them.

The combined splitter/mirror is hollow to save weight and cost. It is glued together to keep dust out.

ODG R-6 Used A Similar Splitter/Mirror

I could not find a picture of the R-8 or R-9 from the inside, but I did find a picture on the “hey Holo” blog that shows the inside of the R-6 that appears to use the same optical configuration as the R-8/R-9. The R-6 introduced in 2014 had dual 720p displays (one per eye) and was priced at $4,946 or about 5X the price of the R-8 with the same resolution and similar optical design.  Quite a price drop in just 2 years.

ODG R-6, R-8, and R-9 Likely Use Sony OLED Microdisplays

Interestingly, I could not find anywhere were ODG says what display technology they use in the 2014 R-6, but the most likely device is the Sony ECX332A 720p OLED microdisplay that Sony introduced in 2011. Following this trend it is likely that the ODG R-9 uses the newer Sony ECX335 1080p OLED microdisplay and the R-9 uses the ECE332 or a follow-on version. I don’t know any other company that has both a 720p and 1080p OLED microdisplays and the timing of the Sony and ODG products seems to fit. It is also very convenient for ODG that both panels are the same size and could use the same or very similar optics.

Sony had a 9.6 micron pixel on a 1024 by 768 OLED microdisplay back in 2011 so for Sony the pixel pitch has gone from 9.6 in 2011 to 8.2 microns on the 1080p device. This is among the smallest OLED microdisplay pixel pitches I have seen but still is more than 2x linearly and 4x in area bigger than the smallest LCOS (several companies have LCOS pixels pitches in the 4 micron or less range).

It appears that ODG used an OLED microdisplay for the R-6 then switched (likely for cost reasons) to LCOS and a simple beam splitter for the R7 and then back to OLEDs and the splitter/mirror optics for the R-8 and R-9.

Splitter/Combiner Is an Old Optic Trick

This “trick” of mixing lenses with a spherical combiner partial mirror is an old idea/trick. It often turns out that mixing refractive (lenses) with mirror optics can lead to a more compact and less expensive design.

I have seen a beam splitter/mirror used many times. The ODG design is a little different in that the beam splitter is sealed/mated to the curved mirror which with the pictures available earlier make it hard to see. Likely as not this has been done before too.

This configuration of beam splitter and curve mirror even showed up in Magic Leap applications such as Fig. 9 from 2015/0346495 shown at right. I think this is the optical configuration that Magic Leap used with some of their prototypes including the one seen by “The Information.

Conclusion/Trends – Turning the Crank

The ODG optical design while it may seem a bit more complex than a simple beam splitter, is actually probably simpler/easier to make than doing everything with lenses before the beam splitter. Likely they went to this technique to support a wider FOV.

Based on my experience, I would expect that ODG optical design will be cleaner/better than the waveguide designs of Microsoft’s Hololens. The use of OLED microdisplays should give ODG superior contrast which will further improve the perceived sharpness of the image. While not as apparent to the casual observer, but as I have discussed previously, OLEDs won’t work with diffractive/holographic waveguides such as Hololens and Magic Leap are using.

What is also interesting that in terms of resolution and basic optics, the R-8 with 720p is about 1/5th the price of the military/industrial grade 720p R-6 of about 2 years ago. While the R-9 in addition to having a 1080p display, has some modular expansion capability, one would expect there will be follow-on product with 1080p with a larger FOV and more sensors in a price range of the R-8 in the not too distant future and perhaps with integration of the features from one or more of the R-9’s add-on modules; this as we say in the electronics industry, “is just a matter of turning the crank.”

Kopin Entering OLED Microdisplay Market

Kopin Making OLED Microdisplays

Kopin announced today that they are getting into the OLED Microdisplay business. This is particularly notable because Kopin has been a long time (since 1999) manufacture of transmissive LCD microdisplays used in camera viewfinders and near eye display devices. They also bought Forth Dimension Displays back in 2011, a maker of high resolution ferroelectric reflective LCOS used in higher end near eye products.

OLED Microdisplays Trending in AR/VR Market

With the rare exception of the large and bulky Meta 2, microdisplays, (LCOS, DLP, OLED, and transmissive LCD), dominate the AR/MR see-through market. They also are a significant factor in VR and other non-see-through near eye displays

Kopins entry seems to be part of what may be a trend toward OLED Microdisplays used in near eye products. ODG’s next generation “Horizon” AR glasses is switching from LCOS (used in the current R7) to OLED microdisplays. Epson which was a direct competitor to Kopin in transmissive LCD, switched to OLED microdisplays in their new Moverio BT-300 AR glasses announced back in February.

OLED Microdisplays Could Make VR and Non-See-Through Headsets Smaller/Lighter

Today most of the VR headsets are following Oculus’s use of large flat panels with simple optics. This leads to large bulky headsets, but the cost of OLED and LCD flat panels is so low compared to other microdisplays with their optics that they win out. OLED microdisplays have been far too expensive to compete on price with the larger flat panels, but this could change as there are more entrants into the OLED microdisplay market.

OLEDs Don’t Work With Waveguides As Used By Hololens and Magic Leap

It should be noted that the broad spectrum and diffuse light emitted by OLED is generally incompatible with the flat waveguide optics such as used by Hololens and is expected from Magic Leap (ML). So don’t expect to see these being used by Hololens and ML anytime soon unless they radically redesign their optics. Illuminated microdisplays like DLP and LCOS can be illuminated by narrower spectrum light sources such as LED and even lasers and the light can be highly collimated by the illumination optics.

Transmissive LCD Microdisplays Can’t Compete As Resolution Increases

If anything, this announcement from Kopin is the last nail in the coffin of the transmissive LCD microdisplay in the future. OLED Microdisplays have the advantages over transmissive Micro-LCD in the ability to go to higher resolution and smaller pixels to keep the overall display size down for a given resolution when compared to transmissive LCD. OLEDs consume less power for the same brightness than transmissive LCD. OLED also have much better contrast. As resolution increases transmissive LCDs cannot compete.

OLEDs Microdisplays More Of A Mixed Set of Pros and Cons Compared to LCOS and DLP.

There is a mix of pro’s and con’s when comparing OLED microdisplays with LCOS and DLP. The Pro’s for OLED over LCOS and DLP include:

  1. Significantly simpler optical path (illumination path not in the way). Enables optical solutions not possible with reflective microdisplays
  2. Lower power for a given brightness
  3. Separate RGB subpixels so there is no field sequential color breakup
  4. Higher contrast.

The advantages for LCOS and DLP reflective technologies over OLED microdisplays include:

  1. Smaller pixel equals a smaller display for a given resoluion. DLP and LCOS pixels are typically from 2 to 10 times smaller in area per pixel.
  2. Ability to use narrow band light sources which enable the use of waveguides (flat optical combiners).
  3. Higher brightness
  4. Longer lifetime
  5. Lower cost even including the extra optics and illumination

Up until recently, the cost of OLED microdisplays were so high that only defense contractors and other applications that could afford the high cost could consider them. But that seems to be changing. Also historically the brightness and lifetimes of OLED microdisplays were limited. But companies are making progress.

OLED Microdisplay Competition

Kopin is long from being the first and certainly is not the biggest entry in the OLED microdisplay market. But Kopin does have a history of selling volume into the microdisplay market. The list of known competitors includes:

  1. Sony appears to be the biggest player. They have been building OLED microdisplays for many years for use in camera viewfinders. They are starting to bring higher resolution products to the market and bring the costs down.
  2. eMagin is a 23-year-old “startup”. They have a lot of base technology and are a “pure play” stock wise. But they have failed to break through and are in danger of being outrun by big companies
  3. MicoOLED – Small France startup – not sure where they really stand.
  4. Samsung – nothing announced but they have all the technology necessary to make them. Update: Ron Mertens of OLED-Info.com informed me that I was rumored that the second generation of Google Glass was considering a Samsung OLED microdisplay and that Samsung had presented a paper going back to 2011.
  5.  LG – nothing announced but they have all the technology necessary to make them.

I included Samsung and LG above not because I have seen or heard of them working on them, but I would be amazed if they didn’t at least have a significant R&D effort given their sets of expertise and their extreme interest in this market.

For More Information:

For more complete information on the OLED microdisplay market, you might want go to OLED-info that has been following both large flat panel and small OLED microdisplay devices for many years. They also have two reports available, OLED Microdisplays Market Report and OLED for VR and AR Market Report.

For those who want to know more about Kopin’s manufacturing plan, Chris Chinnock of Insight Media has an interesting article outlining Kopin’s fabless development strategy.

AR/MR Combiners Part 2 – Hololens

hololens-combiner-with-patent

Microsoft’s Hololens is perhaps the most most well known device using flat “waveguide” optics to “combine” the real world with computer graphics. Note there are no actual “holograms” anywhere in Hololens by the scientific definition.

At left is a picture from the Verge Teardown of a Hololens SDK engine and a from a US Patent Application 2016/0231568 I have added some red and green dots to the “waveguides” in the Verge picture to help you see their outlines.

diffraction grating is a type of Diffractive Optical Element (DOE) and has a series of very fine linear structures with a period/repeated spacing on the order of the wavelengths of light (as in extremely small). hololens-diffraction-gratingA diffraction grating acts like a lens/prism to bend the light and as an unwanted side effect the light also is split separated by wavelength (see top figure at left) as well has affecting the polarization of the light. If it were a simple grating, the light would symmetrically split the light in two directions (top figure at left) but as the patent points out if the structure is tilted then more of the light will go in the desired direction (bottom figure at left).   This is a very small structure (on the order of the wavelength of the light) must be formed on the surface of the flat waveguide.

Optical waveguides use the fact that once light enters glass or clear plastics at a certain angle or shallower, it is will totally reflect, what is known as Total Internal Reflection or TIR.  The TIR critical angle is around 45 degrees for the typical glass and plastics with their coatings used in optics.

hololens-patent-sideviewHololens use the diffraction grating (52 in Fig 3B above) to bend or “incouple” the light or the light so that it will TIR (see figure at right).   The light then TIR’s off of the flat surfaces around within the glass and hits off a triangular “fold zone” (in Fig. 3B above) which causes light to turn ~90 degrees down to the “exit zone” DOE (16 in Fig. 3B).  The exit zone DOE causes the angle of the light to be reduced so it will no longer TIR so it can exit the glass toward the eye.

Another function of the waveguides, particularly the exit waveguide 16 is to perform “pupil expansion” or slightly diffusing the light so that the image can be viewed from a wider angle.   Additionally, it is waveguide 16 that the user sees the real world through and invariably it has to have some negative effect from seeing the world through a slightly diffuse diffraction grating.

Hololens is far from the first to use DOE’s to enter and exit a flat waveguide (there are many examples) and they appear to have acquired the basic technology from Nokia’s efforts of about 10 years ago.   Other’s have used holographic optical elements (HOE) which perform similar functions to DOEs and still others have use more prismatic structure in the waveguides, but each of these alternatives solves some issues as the expense of others.

A big issue for the flat combiners I have seen to date has been chroma aberrations, the breaking up of white light into colors and out of focus and haze effects.   In bending the light at about 45 degrees is like going through a “prism” and the color separate, follow slightly different paths through the waveguide and are put back together by the exit grating.  The process is not perfect and thus there is some error/haze/blur that can be multiple pixels wide. Additionally as pointed out earlier, the user is invariably looking  at the real world through the structure meant to cause the light to exit the from the waveguide toward the eye and it has to have at least some negative effect.

There is a nice short 2013 article on flat combiners by (one author being a Google employee) that discusses some of the issues with various combiners including the Nokia one on which Hololens is base.  In particular they stated:

“The main problems of such architecture are the complexity of the master fabrication and mass replication as well as the small angular bandwidth (related to the resulting FOV). In order to mimic the holographic Bragg effect, sub-wavelength tilted structures with a high aspect ratio are needed, difficult to mass replicate for low cost volume production”  

Base on what I have heard from a couple of sources, the yield is indeed currently low and thus the manufacturing cost is high in making the Hololens combiner.   This may or may not be a solvable (in terms of meeting a consumer acceptable price) problem with volume production.

hololens-odg-comparisonWhile the Hololens combiner is a marvel of optical technology, one has to go back and try and understand why they wanted a thin flat combiner rather than say the vastly simpler (and less expensive maybe by over 10X) tilted flat combiner that say Osterhout Design Group (ODG), for example, is currently using.   Maybe it is for some planned greater advantage in the long term, but when you look at the current Hololens flat combiner, the size/width of the combiner would seem to have little effect on the overall size of the resulting device.  Interestingly, Microsoft has spent about $150 million in licensing fees to ODG.

Conclusions

Now step back and look at the size of the whole Hololens structure with the concentric bands going around the users head.  There is inner band to grip the user’s head while the electronics is held in the outer band.  There is a large nose bridge to distribute the weight on the persons nose and a big curve shield (usually dark tinted) in front of the combiner.  You have to ask, did the flat optical combiner make a difference?

I don’t know reasons/rational/advantages of why Hololens has gone with a vastly more complex combiner structure.   Clearly at the present, it does not give a significant (if any) size advantage.   It almost looks like they had this high tech combiner technology and decided to use it regardless (maybe it was the starting point of the whole program).

Microsoft is likely investing several billion dollars into Hololens. Google likely spent over $1 billion on the comparatively very simple Google Glass (not to mention their investment in Magic Leap). Closely realated, Facebook spent $2b to acquire Oculus Rift. Certainly big money is being thrown around, but is it being spent wisely?

Side Comments: No Holograms Anywhere to be Found

What Microsoft calls “Holograms” are the marketing name Microsoft has given to Mixed Reality (MR).   It is rather funny to see technical people that know better stumble around saying things like “holograms, but not really holograms, . . .”  Unfortunately due to the size and marketing clout of Microsoft others such as Metavision has started calling what they are doing “holograms” too (but this does not make is true).

Then again probably over 99% of what the public thinks are “holograms” are not.  Usually they are simple optical combiner effects cause by partial reflections off of glass or plastic.

Perhaps ironically, while Microsoft talks of holograms and the product as the “Hololens” there are as best I can find no holograms used even static ones that could have been used in the waveguide optics (they use diffraction gratings instead).

Also interestingly, the patent application is assigned to Microsoft Technology Licensing, LLC., a recently separated company from Microsoft Inc.  This would appear to be in anticipation of future patent licensing/litigation (see for example).

Next Time on Combiners

Next time on this subject, I plan on discussing Magic Leap the $1.4 Billion invested “startup” and what it looks like they may be doing.   I was originally planning on covering it with Hololens, but it became clear that it was too much to try and cover in one article.

AR/MR Optics for Combining Light for a See-Through Display (Part 1)

combiners-sample-cropIn general, people find the combining of an image with the real world somewhat magical; we see this with heads up displays (HUDs) as well as Augmented/Mixed Reality (AR/MR) headsets.   Unlike Starwars R2D2 projection into thin air which was pure movie magic (i.e. fake/impossible), light rays need something to bounce off to redirect them into a person’s eye from the image source.  We call this optical device that combines the computer image with the real world a “combiner.”

In effect, a combiner works like a partial mirror.  It reflects or redirects the display light to the eye while letting light through from the real world.  This is not, repeat not, a hologram which it is being mistakenly called by several companies today.  Over 99% people think or call “holograms” today are not, but rather simple optical combining (also known as the Pepper’s Ghost effect).

I’m only going to cover a few of the more popular/newer/more-interesting combiner examples.  For a more complete and more technical survey, I would highly recommend a presentation by Kessler Optics. My goal here is not to make anyone an optics expert but rather to gain insight into what companies are doing why.

With headsets, the display device(s) is too near for the human eye to focus and there are other issues such as making a big enough “pupil/eyebox” so the alignment of the display to the eye is not overly critical. With one exception (the Meta 2) there are separate optics  that move apparent focus point out (usually they try to put it in a person’s “far” vision as this is more comfortable when mixing with the real word”.  In the case of Magic Leap, they appear to be taking the focus issue to a new level with “light fields” that I plan to discuss the next article.

With combiners there is both the effect you want, i.e. redirecting the computer image into the person’s eye, with the potentially undesirable effects the combiner will cause in seeing through it to the real world.  A partial list of the issues includes:

  1. Dimming
  2. Distortion
  3. Double/ghost images
  4. Diffraction effects of color separation and blurring
  5. Seeing the edge of the combiner

In addition to the optical issues, the combiner adds weight, cost, and size.  Then there are aesthetic issues, particularly how they make the user’s eye look/or if they affect how others see the user’s eyes; humans are very sensitive to how other people’s eye look (see the EPSON BT-300 below as an example).

FOV and Combiner Size

There is a lot of desire to support a wide Field Of View (FOV) and for combiners a wide FOV means the combiner has to be big.  The wider the FOV and the farther the combiner is from the eye the bigger the combiner has to get (there is not way around this fact, it is a matter of physics).   One way companies “cheat” is to not support a person wearing their glasses at all (like Google Glass did).

The simple (not taking everything into effect) equation (in excel) to computer the minimum width of a combiner is =2*TAN(RADIANS(A1/2))*B1 where A1 is the FOV in degrees and and B1 is the distance to farthest part combiner.  Glasses are typically about 0.6 to 0.8 inches from the eye and the size of the glasses and the frames you want about 1.2 inches or more of eye relief. For a 40 degree wide FOV at 1.2 inches this translates to 0.9″, at 60 degrees 1.4″ and for 100 degrees it is 2.9″ which starts becoming impractical (typical lenses on glasses are about 2″ wide).

For, very wide FOV displays (over 100 degree), the combiner has to be so near your eye that supporting glasses becomes impossible. The formula above will let your try your own assumptions.

Popular/Recent Combiner Types (Part 1)

Below, I am going to go through the most common beam combiner options.  I’m going to start with the simpler/older combiner technologies and work my way to the “waveguide” beam splitters of some of the newest designs in Part 2.  I’m going to try and hit on the main types, but there are many big and small variations within a type

gg-combinerSolid Beam Splitter (Google Glass and Epson BT-300)

These are often used with a polarizing beam splitter polarized when using LCOS microdisplays, but they can also be simple mirrors.  They generally are small due to weight and cost issues such as with the Google Glass at left.  Due to their small size, the user will see the blurry edges of the beam splitter in their field of view which is considered highly undesirable.  bt-300Also as seen in the Epson BT-300 picture (at right), they can make a person’s eyes look strange.  As seen with both the Google Glass and Epson, they have been used with the projector engine(s) on the sides.

Google glass has only about a 13 degree FOV (and did not support using a person’s glasses) and about 1.21 arc-minutes/pixel angular resolution with is on the small end compared to most other headset displays.    The BT-300 about 23 degree (and has enough eye relief to supports most glasses) horizontally and has dual 1280×720 pixels per eye giving it a 1.1 arc-minutes/pixel angular resolution.  Clearly these are on the low end of what people are expecting in terms of FOV and the solid beam quickly becomes too large, heavy, and expensive at the FOV grows.  Interesting they are both are on the small end of their apparent pixel size.

meta-2-combiner-02bSpherical/Semi-Spherical Large Combiner (Meta 2)

While most of the AR/MR companies today are trying to make flatter combiners to support a wide FOV with small microdisplays for each eye, Meta has gone in the opposite direction with dual very large semi-spherical combiners with a single OLED flat panel to support an “almost 90 degree FOV”. Note in the picture of the Meta 2 device that there are essentially two hemispheres integrated together with a single large OLED flat panel above.

Meta 2 uses a 2560 by 1440 pixel display that is split between two eyes.  Allowing for some overlap there will be about 1200 pixel per eye to cover 90 degrees FOV resulting in a rather chunkylarge (similar to Oculus Rift) 4.5 arc-minutes/pixel which I find somewhat poor (a high resolution display would be closer to 1 a-m/pixel).

navdy-unitThe effect of the dual spherical combiners is to act as a magnifying mirror that also move the focus point out in space so the use can focus. The amount of magnification and the apparent focus point is a function of A) the distance from the display to the combiner, B) the distance from the eye to the combiner, and C) the curvature.   I’m pretty familiar with this optical arrangement since the optical design it did at Navdy had  similarly curved combiner, but because the distance from the display to the combiner and the eye to the combiner were so much more, the curvature was less (larger radius).

I wonder if their very low angular resolution was as a result of their design choice of the the large spherical combiner and the OLED display’s available that they could use.   To get the “focus” correct they would need a smaller (more curved) radius for the combiner which also increases the magnification and thus the big chunky pixels.  In theory they could swap out the display for something with higher resolution but it would take over doubling the horizontal resolution to have a decent angular resolution.

I would also be curious how well this large of a plastic combiner will keep its shape over time. It is a coated mirror and thus any minor perturbations are double.  Additionally and strain in the plastic (and there is always stress/strain in plasic) will cause polarization effect issues, say whenlink-ahmd viewing and LCD monitor through it.   It is interesting because it is so different, although the basic idea has been around for a number of years such as by a company called Link (see picture on the right).

Overall, Meta is bucking the trend toward smaller and lighter, and I find their angular resolution disappointing The image quality based on some on-line see-through videos (see for example this video) is reasonably good but you really can’t tell angular resolution from the video clips I have seen.  I do give them big props for showing REAL/TRUE video’s through they optics.

It should be noted that their system at $949 for a development kit is about 1/3 that of Hololens and the ODG R-7 with only 720p per eye but higher than the BT-300 at $750.   So at least on a relative basis, they look to be much more cost effective, if quite a bit larger.

odg-002-cropTilted Thin Flat or Slightly Curved (ODG)

With a wide FOV tilted combiner, the microdisplay and optics are locate above in a “brow” with the plate tilted (about 45 degrees) as shown at left on an Osterhout Design Group (ODG) model R-7 with 1280 by 720 pixel microdisplays per eye.   The R-7 has about a 37 degree FOV and a comparatively OK 1.7 arc-minutes/pixel angular resolution.

odg-rr-7-eyesTilted Plate combiners have the advantage of being the simplest and least expensive way to provide a large field of view while being relatively light weight.

The biggest drawback of the plate combiner is that it takes up a lot of volume/distance in front of the eye since the plate is tilted at about 45 degrees from front to back.  As the FOV gets bigger the volume/distance required also increase.
odg-horizons-50d-fovODG is now talking about a  next model called “Horizon” (early picture at left). Note in the picture at left how the Combiner (see red dots) has become much larger. They claim to have >50 degree FOV and with a 1920 x 1080 display per eyethis works out to an angular resolution of about 1.6 arc-minutes/pixel which is comparitively good.

Their combiner is bigger than absolutely necessary for the ~50 degree FOV.  Likely this is to get the edges of the combiner farther into a person’s peripheral vision to make them less noticeable.

The combiner is still tilted but it looks like it may have some curvature to it which will tend to act as a last stage of magnification and move the focus point out a bit.   The combiner in this picture is also darker than the one in the older R-7 combiner and may have additional coatings on it.

ODG has many years of experience and has done many different designs (for example, see this presentation on Linked-In).  They certainly know about the various forms of flat optical waveguides such as Microsoft’s Hololens is using that I am going to be talking about next time.  In fact,  that Microsoft’s licensed Patent from ODG for  about $150M US — see).

Today, flat or slightly curved thin combiners like ODG is using probably the best all around technology today in terms of size, weight, cost, and perhaps most importantly image quality.   Plate combiners don’t require the optical “gymnastics” and the level of technology and precision that the flat waveguides require.

Next time — High Tech Flat Waveguides

Flat waveguides using diffraction (DOE) and/or holographic optical elements (HOE) are what many think will be the future of combiners.  They certainly are the most technically sophisticated. They promise to make the optics thinner and lighter but the question is whether they have the optical quality and yield/cost to compete yet with simpler methods like what ODG is using on the R-7 and Horizon.

Microsoft and Magic Leap each are spending literally over $1B US each and both are going with some form of flat, thin waveguides. This is a subject to itself that I plan to cover next time.