Collimation, Etendue, Nits (Background for Understanding Brightness)

Introduction

I’m getting ready to write a much requested set of articles on the pro’s and con’s of various types of microdisplays (LCOS, DLP, and OLED in particular with some discussion of other display types). I feel as a prerequisites, I should give some key information on the character of light as it pertains to what people generally refer too as “brightness”.  For some of my readers, this discussion will be very elementary/crude/imprecise, but it is important to have a least a rudimentary understanding of nits, collimation, and etendue to understand some of the key characteristics of the various types of displays.

Light Measures – Lumens versus Nits

The figure on the left from an Autodesk Workshop Page illustrates some key light measurements. Lumens are a measure of the total light emitted. Candelas (Cd) are a measurement of the light emitted over a solid angle. Lux measures the light per square meter that hits a surface. Nits (Cd/m2) measure light at a solid angle. Key for a near eye display, we only care about the light in the direction that makes it to the eye’s pupil.

We could get more nits by cranking up the light source’s brightness but that would mean wasting a lot of light. More efficiently, we could use optics to try and somehow steer a higher percentage of total light (lumens) to the eye. In this example, we could put lenses and reflectors to aim the light to the surface and we could make the surface more reflective and more directional (known as the “gain” of a screen). Very simply put, lumens is a measure of the total light output from a light source, nits is a measure of light in a specific direction.

Etendue

The casual observer might think, just put a lens in front of or a mirror behind and around the light source (like a car’s headlight) and concentrate the light. And yes this will help but only within limits. The absolute limit is set down by a physics law that can’t be violated known as “etendue.”

There are more detailed definitions, but one of the simplest (and for our purpose practical) principles is given in a presentation by Gaggione on collimating LED light stating that “the beam diameter multiplied by the beam angle is a constant value” [for an ideal element]. In simpler terms, if we put an optical element that concentrates/focuses the light, the angles of the light will increase. This has profound implications in terms of collimating light. Another good presentation, but a bit more technical, on etendue and collimation is given by LPI.

Another law of physics is that etendue can only be increased. This means that the light once generated, the light rays can only becomes more random. Every optical element will hurt/increase etendue. Etendue is analogous to the second law of thermodynamics which states that entropy can only increase.

Lambertian Emitters (Typical of LEDs/OLEDs)

LEDs and OLEDs used in displays tend to be “Lambertian Emitters” where the nits are proportional to the cosine of the angle. The figure on the right shows this for a single emitting point on the surface. A real LED/OLED will will not be a single point, but an area so one can imagine a large set of these emitting points spread two dimensionally.

Square Law and Concentrating Light

It is very important to note that the diagram above shows only a side view. The light rays are spreading as sphere and nits are a measure of light per unit area on the surface a sphere. If the linear spread by is reduced by X, the nits will then increase by X-squared.

Since for a near eye display, the only light that “counts” is that which makes it into a person’s eye, there is a big potential gain in brightness that comes not from making the light source brighter but by reducing the angles of the light rays in the form of collimation.

Collimating Light

Collimation is the process of getting light rays to be a parallel to each other as possible (within the laws of etendue). Collimation of light is required for projecting light (as with projector), making for very high luminance (nits) near eye displays, and for getting light work properly with a waveguide (waveguides require highly collimated light to work at all)

Show below is the classic issue with collimating light. A light source with the center point “2” and the two extreme points point at the left “1” and right “3” edge of a Lambertian emitter are shown. There is a lens (in blue) trying to collimate the light that is located at a distances equal to the focal length of the lens. There is also shown a reflector in dashed blue that is often used to capture and redirect the outermost rays that would bypass the lens.

The “B” figure shows happens when 3 light rays (1a, 2a, and 3a) from the 3 points enter the lens at roughly the same place (indicated by the green circle). The lens can only perfectly collimate the center 2a ray to become 2a’ (dashed line) which exits along with all other rays from the point 2 perfectly parallel/collimated. While rays 1a and 3a have their angle reduced (consistent with the laws of etendue, the output area is larger than the source light area) to 1a’ and 3a’ but are not perfectly parallel to ray 2a’ or each other.

If the size of the light source were larger such that 1 and 3 are farther apart, the angles of rays 3a’ and 1a’ would be more severe and less collimated. Or if the light source were smaller, then the light would be more highly collimated. This illustrates how the emitting area can be traded for angular diversity by the laws of etendue.

Illuminating a Microdisplay (DLP or LCOS) Versus Self Emitting Display (OLED)

Very simply put, what we get conceptually by collimating a small light source (such as set of small RGB LEDs) is a bundle of individual highly collimated light sources to illuminate each pixel of a reflective microdisplay like DLP or LCOS. The DLP or LCOS device pixel mirrors then simply reflect light with the same characteristics with some losses and scattering due to imperfections in the mirror.

The big advantage in terms of intensity/nits for reflective mirodisplays is that they separate the illumination process from the light modulation. They can take a very bright and small LEDs and then highly collimate the light to further increase the nits. It is possible to get many tens of thousands of nits illuminating a reflective microdisplay.

An OLED microdisplay is self emitting and the light is Lambertian which as show above is somewhat-diffuse. Typically OLED microdisplay can emit only about 200 to at most 400 nits for long periods of time (some lab prototypes have claimed up to 5,000 nits, but this is unlikely for long periods of time). Going brighter for long periods of time will cause the OLED materials to degenerate/burn-up.

With the OLED you are somewhat stuck with the type of light, Lambertian, as well as the amount of light. The optics have to preserve the image quality of the individual pixels. If you want to say collimate the Lambertian light, it would have to be done on the individual pixels with miniature optics directly on top of the pixel  (say a microlens like array) to have a small spot size (pixel) to collimate. I have heard several people theorize this might be possible but I have not seen it done.

Next Time Optical Flow

Next time I plan to build on these concepts to lay out the “optical flow” for a see-through (AR) microdisplay headset. I will also discuss some of the issues/requirements.

 

23 comments

  1. Hawkeye says:

    Thanks for this- looking forward to the next ones! Why do you think these major companies- Apple, Sony, ODG etc are all shifting to OLED over LCOS? LCOS affords a lot more brightness to use/lose in the design while the only advantage for OLED seems to be ability to shape the screen better? I would understand the appeal of Waveguide since it can be directly integrated into a lens (a la a pair of glasses)

    • KarlG says:

      I think you are mixing apples and oranges a bit in talking about direct view displays and microdisplays.

      In direct view displays, the comparison is LCDs’ versus OLEDs. OLEDs have a big advantage in “black”/contrast, and generally are absorb ambient light a bit better. OLEDs generally are significantly thinner (a major factor is the backlight on LCDs), have better color and peak brightness (if only a small percentage of the pixels are on), and generally more power efficient for “typical” content. LCDs are better at lifetime and average brightness (if much of the display is bright),and lower cost. I highly recommend Displaymate’s information on the various direct view displays: http://www.displaymate.com/mobile.html . For the premium phones, the advantages of OLED, particularly size/thin and power consumption outweigh the cost.

      In Microdisplays, the comparison is to OLED on Silicon or LCOS. Generally LCOS has a 3X or more cost advantage (lots of variables in terms of pixel size), can be more than 50X brighter (in nits), and supports smaller pixels (by up to 2.5X linearly or >6x in area). LCOS drawbacks are that it needs an illumination path which since it is reflective makes things complicated (although Himax’s frontlit uses waveguide illumination to eliminate this problem at a cost and probably only works for lower brightness solution) and LCOS has Field Sequential Color (Himax has color filter LCOS for lower resolution and lower brightness applications). Usually, the decision is dictated by cost and brightness and the need for collimated light (OLEDs are not) and so LCOS dominates in see-through displays and I have not really seen much of a change in the trend. The ODG-9 and ODG-8 while they use OLEDs, I don’t consider to be really see-through as they block so much real world light because the display is so dim.

      I plan to answer more about these comparisons in the upcoming article.

      For near-eye displays it is a whole different game.

      • x321y says:

        “apples and oranges a bit in talking about direct view displays and microdisplays.”

        Exactly , that’s happening in “macro-pixel land” and my bet would be the significant expense of the standard optical stack , the proprietary nature of certain company’s reflective polarizer not aiding the LCD cause afterall, despite its very strong perfomance lead over competitors.

        • KarlG says:

          I’m not quite sure what you are getting at with your comments. If you could elaborate, it would be helpful.

          Certainly, there has been more progress with bigger/direct view size pixels than microdisplays. This is clearly evident with 4K TVs become inexpensive where 4K microdisplay cost thousands of dollars just for the display device.

          OLED direct view displays have their pro’s and con’s in terms of technical challenges. LCDs work with a pretty much static charge driving the LCDs with near zero current except at switching. They get away with a simple capacitor and transistor per pixel. AMOLED require not only the per-pixel control, but they must provide power to drive each pixel. By necessity they have stuff to put with each transistor. OLEDs have the advantage though in that they don’t have to be transparent so you can put stuff behind the transistors that would block light or even go to multiple layers. LCDs have gotten so inexpensive, even at 4K resolution that is is almost ridiculous. OLED are still a bit behind in terms of cost, but when viewed in historic context and adjusting for inflation are still a bargin.

          OLEDs are sufficiently bright for direct view applications and their light characteristics are also nearly ideal for direct view. This is not the case for near-eye use where the characteristics of their light (Lambertian with fairly wide spectrum primaries) prevents their use with many optical structures. Additionally the OLED microdisplays are no where near bright enough for AR/see-through use where a lot of light is needed as the optical combining throws away a lot of light. For non-see-through (ala VR) use OLEDs are fine, but not so with AR/see-through.

          Karl

          • frankenberry says:

            Nothing is more demanding for see through applications than military aviation .

            eMagin’s OLED DPD technology is making great strides that will be making their way into the consumer market .

            In April eMagin’s High Brightness OLED Microdisplays was Selected for Aviation Upgrade Program :

            http://ir.emagin.com/phoenix.zhtml?c=96135&p=irol-newsArticle&ID=2261851

            eMagin is also currently being prototyped to replace the LCD micro displays in the F-35 due to the persistent green glow problem :

            https://www.flightglobal.com/news/articles/paris-rockwell-collins-prototypes-fix-for-f-35-gre-438687/

            This technology should be entering the consumer market soon via the Everysight (Elbit) Raptor which uses similar tech as the commercial aviation model the Elbit Skylens .

            https://everysight.com

            http://elbitsystems.com/pr-new/elbit-systems-skylens-wearable-hud-begins-flying-final-configuration-onboard-atr-7242-aircraft/

            The eMagin DPD OLED tech was developed under the ManTech program . Please note the words – Affordable , Full Color & High Brightness :

            ManTech Enables Affordable
            Manufacture of Full Color High
            Brightness OLED Microdisplays

            The Challenge:
            Current fielded full color microdisplays cannot simultaneously achieve high contrast and high brightness to clearly display see-through imagery required for high ambient conditions without obscuring the scene. Fielded Organic Light Emitting Diodes (OLEDs) can achieve this for monochrome, but refinements to the manufacturing process, are necessary to improve manufacturing efficiency and luminance critical for military applications. Additionally, all silicon backplanes for OLED microdisplays are currently manufactured off-shore .

            Impact:
            • Established capability of on-shore foundry for advanced backplane wafers for OLED deposition
            • Increased luminance of manufactured microdisplays by >10X to >1,300-fL
            • Increased efficiency of manufactured microdisplays by >14X to 7 cd/A
            • Integrating early prototypes into High Definition Aviation Digital Display (HiDADD) Apache vision system upgrade tech demo (FY16), F-35 visor (FY16), and Soldier Visual Interface Technology (SVIT) tech demo (FY17)

            p 22

            https://www.dodmantech.com/JDMTP/Files/2016_ManTech_Brochure.pdf

          • KarlG says:

            Thanks for the update.

            Interesting about the green (only) high brightness device. I wonder how bright they can go with a green-only device. Longer term, I could see the monochrome/green-only high brightness being threatened by Micro-iLED.

            We will have to see what the price comes in at. As I have said, everyone I know that has looked at make a volume product with OLED has ended up using Sony (except for eMagin’s own night vision ski goggles). “Consumer pricing” is a very relative term.

            I can definitely see a big advantage for OLED at night over LCD. But I also wonder why they could not fix the green glow some other way. I don’t see how OLED microdisplays will be bright enough for daytime use. For $400K they could afford to put both in, (one for day one for night :-))

            From what I read these are the goal for getting this funding. The “high brightness” they want to achieve is 10X what can be done today or 1,300 FT (equals 4,454 nits) suggesting today they are are about 400 to 500 which is what I have seen on eMagin’s site. You can get 10’s of thousands of nits today off LCOS and DLP with LED illumination. Even with a 10X improvement, it is not enough for a see-through daytime HUD as you end up dumping much of the image light in the combining optics, so you need to stare more nits. As far as affordable goes, this is military/government affordable, like lower in cost than the $400K F-35 helmet HUD.” I think it is good for the military to invest in some OLED development, but it still has a lot of issues, primarily related to brightness and longevity/burn-in.

          • frankenberry says:

            Karl : Interesting about the green (only) high brightness device. I wonder how bright they can go with a green-only device.

            At 2015 3Q CC CEO Andrew Sculley ( I believe this was old tech) :

            “The monochrome green can go well above 20,000 nits to about 24,000 nits. ”

            https://seekingalpha.com/article/3682376-emagins-eman-ceo-andrew-sculley-q3-2015-results-earnings-call-transcript?page=3

            Karl: I could see the monochrome/green-only high brightness being threatened by Micro-iLED.

            eMagin has received a NOA on 6/30/17 for their Micro-LED patent app :

            One interesting note is that their patent does not appear to be confined to micro-displays but will be applicable to all display sizes .

            22. The device of claim 21, wherein the device is a display.
            23. The device of claim 21, wherein the display is a micro display.

            https://www.google.com/patents/US20160293586

            Karl: We will have to see what the price comes in at.

            Kopin CEO has stated $50 on their new 2K x 2K OLED Lightening micro display

            I have a comprehensive Analyst report from January 2017 that I am not at liberty to share that references a $50 selling price per display for the eMagin 2K x 2K .

            Of course this is in reference to a VR application display . While a VR application does not require the brightness of AR I would point out that Kopn’s 2K x 2K will be stuck at the very low end which will effect their ability to achieve High Dynamic Range HDR while eMagin will be able to achieve this with their higher brightness displays. CEO on last CC – “Everyone we speak with about the VR market wants above 3,000 nit. Hence, we are the only company that has demonstrated this capability. ”

            I believe cost reductions will be achieved with the new DPD technology eMagin has developed with the military under ManTech as well as implementation of new backplane tooling . In the recent 2Q 2017 CC Q & A , the CEO talks about a new metals tool and a new anode deposition tool . In his opening remarks he indicates the timelines _ “we have acquired a new metals deposition tool and during the process of completing the design specs for another tool used in our process. ” I believe this is an intermediate step toward a larger volumes in the future when multiple lines/tooling are brought up with a manufacturing partner . This should coincide with the market ramping which is still a couple years away .

            I can’t put my finger on in but I saw you will be speaking at a Conference this fall in which some Representatives from eMagin will also be speaking . Perhaps you will be able to get more information .

          • KarlG says:

            Thanks for the information. That is an awful lot wrapped into a question. I will try an “unpack” below.

            First, I am speaking at the the Dispay Summit on October 4-5th. I didn’t see eMagin on the agenda.

            The XLT green is on their website at being “XLT Monochrome Green: over 5,000 nits (up to 20,000 nits!)” which is not really a spec., but an interesting statement. I would like to understand the qualifiers. Is this for the whole display or for a percentage of the display at a time. Often OLEDs are spec’ed for maximum brightness over a small region at a time which is perfectly fine for some applications particularly heads-up displays that are suppose to be mostly see-through. I would also want to know if this is a small sampling and/or the 20,000 nits can be sustained for long periods of time. So depending on the qualifiers, it could be bright enough. It would still be an application for which Micro-iLED might target.

            It is a HUGE different to making a full color display versus a single color in terms of brightness. Also I’m not sure that 3,000 nits is necessary for most of VR (not see through) as so much light is block, and it is not nearly enough for outdoors AR where so much light will be loss in the “combiner” optics. I would definitely agree that eMagin is currently way ahead of Kopin in brightness (what Kopin demonstrated at CES had good image quality but was not very bright, no measurement but I heard someone say they thought is was 100 to 200 nits). Particularly with OLEDs you have to talk brightness, the amount of display “on” at a time, and lifetime simultaneously.

            Going with single color, particularly green, certainly makes the technical challenge much easier in terms of brightness, although Green is usually a harder color to generate/emit. Per unit area there is 1/3 of the circuitry giving more area for green emission. A big advantage is that for say a heads up combiner they can use a green notch filter mirror that reflects green and thus a higher percentage of green will be reflected without blocking the see-through light (other than taking a notch out of the part of the spectrum which will cause a slight color shift).

            Kopin did NOT (as far as I can find) actually say “Kopin CEO has stated $50 on their new 2K x 2K OLED Lightening micro display”. What I was able to be find is that “In terms of cost, this will depend very much upon the volume of the order. However, Fan envisions a path to reach $50 per panel for volume production in the future, which would allow for companies to create more cost competitive consumer VR headsets.” Envisioning a path, could mean in 3 to 5 years at 1 million units a month where the “path” to get there requires first buying a lot of units at $200+ each. It does indicate that they don’t at $50 at any volume today or in the near future at today’s likely volumes. For a price to matter you have to tie down the time frame, volume, and specification.

            Citing a dependent claim from their application with a NOA does not say anything. I think found the patent with the independent claim. I did not go through the file wrapper to see if the claims were modified/narrowed from the application before being allow (some of the language of claim 16 is ambiguous, like “in a manner which does not cause significant damage to the structure”). That said, for claims 22 and 23 to mean anything to a non-microdisplay that display must do EVERYTHING in claim 16 below (which is not how they build larger displays):

            >”16. A method of fabricating a light emitting device, comprising the steps of: forming a compound stacked semiconductor structure over a substrate, wherein the semiconductor structure including: an n-type type semiconductive layer formed on the substrate comprising a material selected from the group consisting of III-V and II-VI compounds; a p-type semiconductive layer overlying the n-type semiconductive layer comprising a material selected from the group consisting of III-V and IV-VI compounds; electrically coupling a first electrode with the n-type semiconductive layer; and electrically coupling a second electrode with the p-type semiconductive layer; forming an insulating layer comprising dielectric material over the semiconductor structure in a manner which does not cause significant damage to the structure; forming conductive vias through the insulating layer; and forming a metal oxide thin film transistor backplane over the insulating layer and conductive vias in a manner which does not cause significant damage to the semiconductor structure.”< You may find me a bit too skeptical, but eMagin has been making big claims for being low cost since I started in displays in 1998 (eMagin was founded around 1996). I have sent companies to look at eMagin several times and they always have come back with massive sticker shock. As recently as 2017 CES I did an informal pole of companies using OLED microdisplays and with the exception of the captive night vision ski goggles, they were all using Sony OLED microdisplays. Once again, tie things down on the time frame, volume, and spec.

          • frankenberry says:

            My bad , at the Display Summit there will be W. Lee Hendrick from Rockwell Collins speaking about Integrated Digital Vision System (IDVS) this is an AR device with eMagin displays :

            The first prototypes utilized two high resolution OLED micro-displays with see-though free-form prisms for near-eye display. The next generation IDVS will incorporate high definition waveguide displays for better see-through quality and higher brightness.

            http://www.displaysummit.com/integrated-digital-vision-system-idvs/

            IDVS Brochure :

            https://www.rockwellcollins.com/Products_and_Services/Defense/Displays_and_Controls/Ground/Helmet-mounted_Displays/Integrated_Digital_Vision_System.aspx

          • frankenberry says:

            Here is a video with the Integrated Digital Vision System with waveguide in case you overlooked it in the brochure .

            It would be interesting to know if they are using OLED with this newer version as they were with prism version .

            Day to Night – pretty cool

            https://www.youtube.com/watch?v=-YjingaUlG4

          • KarlG says:

            Yes I saw the video. You probably know IDVS is WUXGA (1920×1200) and that eMagin happens to make an WUXGA device in a military version. For night vision you don’t need a lot of nits and black level is very important. Along with the other information, it all adds up to it being highly likely that the IDVS is using an eMagin OLED microdisplay. So it looks likely you will “get your wish” that Rockwell is using eMagin for this product.

            It is also interesting that some recent patent filings for waveguides (WO2016130509 and US20140140654) are joint between SGB-Labs/Digilens and Rockwell. This might suggest they might be using SGB-Labs for waveguide technology.

            You can couple OLEDs to waveguides, but it is not clear how well they work. Its very hard to get any objective data on waveguides (“little things” like coupling efficiency and spectral requirements/performance). You might get a lot of “waveguide glow” but this is not as important for very sparse content on a night vision HUD type display. You are not trying for “movie quality” but rather just providing key information.

            I still remain more than a little skeptical about the “consumer pricing.” eMagin is over 20 years old and still looses about $2M every quarter and has an accumulated net loss of over $200M. They get favorable treatment from the U.S. government (in the form of grants) and its contractors (because of the military requirement for domestic sources) as they are a U.S. company but this benefit is not extended by consumer product companies. Ironically perhaps, if they were on the cusp of a a consumer products breakthrough, I will expect to see much larger money flowing (in say the $100M’s of dollars).

          • kramapple says:

            AMD is a likely partnership candidate and I have written plenty about this . Roy Taylor has stated on more than 1 occasion that they are working with EMAN .

            At D.I.C.E. Europe 2016 – AMD’s Roy Taylor shows EMAN VR device and states that they have been working EMAN 18 months .

            @ 16:40

            https://youtu.be/BfOyFnqOdyk?t=1000

            The manufacturing partnership negotiations are complex and taking time due to the fact that they are 3 way between the Manufacturer , EMAN , and EMAN’s customers . This has been stated in recent CC’s .

            In any event , I don’t believe EMAN is in anyway behind the curve as mass volume for AR and high resolution VR is still a couple years away .

        • kramapple says:

          “Ironically perhaps, if they were on the cusp of a a consumer products breakthrough, I will expect to see much larger money flowing (in say the $100M’s of dollars).”

          May be seeing the start of that . AWN Investment Co Inc ., KOPN’s #2 Institutional Shareholder, purchased 3,261,000 shares of EMAN in 2Q 2017 .

          http://www.nasdaq.com/symbol/eman/institutional-holdings

          I expect EMAN may do another financing raise timed with their announcement of a manufacturing partner that they have been working toward .

          Furthermore , many are unaware of EMAN’s advancements and are quick to dismiss them .

          • KarlG says:

            Furthermore , many are unaware of EMAN’s advancements and are quick to dismiss them .

            eMagin is a 20+ year old “startup” that has lost over $200M and has for a number of years been losing about $2M per quarter. Perhaps the discounting of what they say has been earned.

            I have seen a lot of “lab prototype” announcements that have not made it into products (from eMagin and many others) and have developed a thick skin. Specifically eMagin has been saying that consumer products are just around the corner since at least as far back as 1998 (as long as I have known of eMagin).

          • frankenberry says:

            Yeah , I wonder what AWM is smoking .

          • frankenberry says:

            Given Sony’s investment in Digilens , it would appear that OLED’s may work fine with Digilens waveguides .

            https://techcrunch.com/2017/01/19/digilens-grabs-22-million-from-sony-foxconn-as-it-looks-to-print-next-gen-ar-displays/

            So it appears eMagin in Military , Sony in consumer .

            eMagin will have a brightness advantage – Sony cost for now .

          • KarlG says:

            “Work fine” can be relative. The image quality you need for say a HUD with low image content is pretty different from what you might want to watch video.

            I would also throw Samsung and LG into the mix as they both had a background OLEDs and semiconductors. I also think I have heard rumors that there are Chinese companies looking at OLEDs of various sizes (kind of a no-brainier).

  2. x321y says:

    I’m not the first poster, I just wanted to underscore how BOM comparisons don’t seem to account for polarization recycling and TIR elements of LCD backlights.

    Macro-pixel LCD-s seem to be the better candidate for bridging the macro-micro gap (because color sequential modes) , whether if such transition is even possible, I don’t know.

    • KarlG says:

      I definitely agree about considering the whole BOM and weighing up all the plus and minuses. But it turns out even considering the polarization/recycling and backlighting, LCDs are ridiculously inexpensive. Today you an buy a 55″ 4K LCD TV at Best Buy for about $425. You have to figure the display with backlight including polarization and other films/diffusers/etc. is about $100 as a component with the LCD panel with its color filter and thin film transistors on glass costing the most.

      The problem with LCDs is that it is transmissive, it limits scaling before it iris’s out. This is why microdisplays with LC use reflective LCOS so you can “hide” the circuitry under the pixel. They have make color filter LCOS (particularly Himax) to eliminate field sequential color, but the problem is as the pixels gets small the colors tend to bleed together (neighboring sub-pixels of one color affect the others). In terms of size, I think OLED has an advantage for scaling down as the circuitry could be hidden under the pixel. The problem for OLED is that the properties of its light are not good for see-through display optics.

  3. Formosa says:

    I am looking forward to seeing your next topic. Especially “The problem for OLED is that the properties of its light are not good for see-through display optics”, in my guess, this is the reason why why Hololens is so expensive and can not really mass production.

    See-through waveguide optics seems only support very narrow band of spectrum. The direct RGB laser light source should be more suitable for see-through wavegudie optics. So maybe using Laser with LCOS could be a match for near eye display for see-through waveguide optics design.

    Except the Laser with LCOS, what do you think about LBS, like Microvision MEMS? Is there any chance for LBS to match with see-through waveguide optics?

    • KarlG says:

      In theory a narrower spectrum would be better for diffraction based waveguides. Another issue with OLED light is the lack of collimation.

      Lasers, at least very narrow band one have their own issues, particularly speckle. The direct diode lasers have a few nanometers of line width which should help (I have never tried then in a headset).

      LBS has its own issues, on the general front the resolution is very low (about 1/2 in each direction what they claim). The fact is is always in focus is a double edge sword.

      On the plus side of LBS headset: The light is always in focus and will be with our without your glasses if you need them. Additionally, people will some eye conditions that would otherwise make them blind apparently see the laser light.

      On the downside of LBS headsets: the light is so focused that if you have any “floaters” in your eyes (and most people older than 40 do) will case a shadow that blocks part of the image. Then there are issues of speckle as well as the power consumption which is much higher than other techniques as well as being much lower in effective resolution.

      So my general assessment of LBS for headsets is that it may be applicable to a very specialized niche medical use market.

  4. Hawkeye says:

    Hi Karl- love your posts. Was wondering what you think the best AR microdisplay is in terms of a combination of price/ slimness / brightness / contrast

    The Kopin/EMagin ones are priced way too high for consumer/commercial use (mainly focused on military). The issue with the Vuzix waveguide’s is the field of view is so small and also very expensive.
    Sony’s OLED display that ODG is using seems to be a good choice.

    The future seems to be some combo of waveguide/freeform prism that will be the one to break through…

    • KarlG says:

      Thanks,

      Every technology today has an Achilles Heal (major weak point). There really is no “best” only some technologies are better for some applications than others.

      Right now OLED microdisplays are much more expensive. They are inherently more expensive to make than LCOS and maybe DLP. Their fundamental image quality is great, particularly in that they have near perfect blacks and are not field sequential color.

      OLED’s key drawbacks is that they have very limited brightness and they emit Lambertian light. LCOS and DLP can easily be 10X brighter in nits and thus can afford the typical 10X or more optical losses associated with a truly “see through” AR.

      I don’t really consider ODG’s R9 and R8 to be good/true AR/see-through displays. They have to block almost all the real world light because the display is so very dim. They are using a Birdbath design that is horrible in light efficiency for both the real world and the display and has some pretty bad ghost images. In my simple measurements they only let about 5% off the real world light through, not quite as much as the Solar Eclipse glasses, but more like very dark sunglasses. If you really want to be “see-though” indoors it should be more like 85% or more see-through.

      The image quality of waveguides is pretty poor by any objective measure. I think there are some fundamental physics problems with using diffractive waveguides due to scatter/error off the diffraction gratings. Lumus which uses segmented mirrors has some advantages but it is not very optically efficient and you can see the segments as very fine, but still visible, lines between the segments. It is hard to get the waveguides beyond about a 55 degree (diagonal) FOV.

      Freeform prisms seem to mostly be used in lower cost applications (at least in AR today). People think they look bulky and they can be heavy as the FOV grows.

      Another way to go would be to use refractory (lens) optics with a simple plate beam splitter (ex. ODG R7).

      There are been about 100 different designs over the last 20 years for AR display. If it were easy, somebody would have already solved it.

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