Introduction

My coverage of CES and SPIE AR/VR/MR 2023 continues, this time on MicroLEDs. MicroLEDs companies were abundant in the booths, talks, and private conversations at AR/VR/MR 2023.

The list on the right shows some of the MicroLED companies I have looked at in recent years. Marked with a blue asterisk “*” are companies I talked to at AR/VR/MR 2023, with Jade Bird Display (JBD), PlayNitride, Porotech, and MICLEDI having booths in the exhibition. The green bracket on the left indicates companies where I had seen a MicroLED display generating an image (not just one or a few LEDs). Inside the gold rectangle in the list above are MicroLED companies that system companies have bought. MicroLEDs are the display technology where tech giants Meta, Apple, and Google place their bets for the future.

A much more extensive list of companies involved in MicroLED development can be found at microled-info.com, a site dedicated to tracking the MicroLED industry. Microled-info’s parent company, Metalgrass, also organized the MicroLED Association, and I spoke at their Feb. 7th Webinar (but you have to join the association to see it).

The efficiency of getting the Lambertian light that most LEDs emit through a waveguide to the eye is a major issue I have studied for years and will be covered first. Then after covering recent MicroLED prototypes and discussions, I have included an appendix with background information in the subsections “What is a MicroLED company,” “Microdisplay vs. Direct View Pixel Sizes,” and “Multicolor, Full Color, or True Color.”

MicroLEDs and Waveguides; Millions of Nits-In to Thousands of Nits-Out with Waveguides

When first hearing of MicroLEDs outputting millions of nits, you might think it must be overkill to deliver thousands of nits to the eye for outdoor use with a waveguide. But due to pupil expansion and light losses, only a tiny fraction of the light-in makes it to the eye. The figure (right) diagrams the efficiency issues with waveguides using a diffractive waveguide.

Most LEDs output diffuse (roughly) Lambertian light, whereas waveguides require collimated light. Typically, micro-optics such as microlens arrays (MLA) are on top of the MicroLEDs’ semi-collimate the light. These optics increase the nits; typically, the nits quoted for the MicroLED display are after micro-optics. A waveguide’s small entrance area severely limits the light due to a physics property known as “etendue,” causing it to be called “etendue loss.” Then there are the losses due to the pupil expansion/replication structures (diffraction gratings in the case of diffractive waveguides, semi-reflective “facets” in the case of reflective waveguides). Finally, the light-in from the small entrance area ends up spread out over the much larger exit area to support seeing the image over the whole FOV as the eye moves.

Multiple Headsets Using Diffractive Waveguides with JBD MicroLED

I found it an interesting dichotomy that while all the other prototypes I have seen using Jade Bird Display (JBD) MicroLEDs, including Vuzix, Oppo, TCL, Dispelix, and Waveoptics (before being acquired by Snap), JBD themselves showed a prototype 3-chip color cube projector with a Lochn “clone” (with lesser image quality) of a Lumus 2D expanding reflective waveguide in their booth (I was asked not photograph). Then in the Playnitride booth, they featured Lumus reflective waveguides. I should note that while efficiency is a major factor, other design factors, including cost, will drive different decisions.

Reflective (Lumus) Waveguides are More Efficient than Diffractive Waveguides with MicroLEDs

According to Lumus, their 2-D reflective (Lumus) waveguides result in a 3 to 9 times larger entrance area, and their semi-reflective facets lose less light than diffraction gratings. The net result is that reflective waveguides can be 5 to >10 times more optically efficient than diffractive waveguides with the same microLEDs, a major advantage in brightness and power (= less heat and longer battery life). This efficiency advantage appears to have been playing out at AR/VR/MR 2023.

Playnitride prominently showed their MicroLEDs using Lumus 2D and older 1D reflective waveguides in their booth (below left and middle). Their full-color QD-MicroLEDs only output about 150K nits (compared to the millions of others’ single-color native LEDs), so they needed a more efficient waveguide. Playnitride uses Quantum Dot conversion of blue LEDs to give red and green.

Lumus CTO Dr. Yochay Danziger brought a 2D expanding waveguide with input optics that he held up to Porotech’s MicroLEDs. I captured a quick handheld (and thus not very good) shot (with ND filters to reduce the very bright image) of Porotech’s green MicroLED via Lumus’s handheld waveguide (above right).

Lumus was the only company featured in the Schott Glasses booth at AR/VR/MR 2023. The often-asked question about Lumus is whether they can make them in volume production. The Schott Glass representative assured me they could make Lumus’s 2-D waveguides in volume production.

I plan on covering Lumus’s new smaller (than their two year old Maximus 2D waveguide) Z-Lens 2D waveguide in an upcoming article. In the meantime, I discussed the Z-Lens in the CES 2023 Video with SadlyItsBradley.

Other Optics (ex., Bird Bath, Freeform, and VR-Pancake) and Micro-OLEDs

I want to note here that while MicroLEDs are hundreds to over a thousand times brighter than Micro-OLEDs, they are likely well more than five years away from having anywhere near the same color control and uniformity. Thus designs that favor image quality over brightness using optical designs that are much more efficient than waveguides, such as Bird Bath, Freeform, and VR-pancake optics, will continue to use Micro-OLEDs or LCDs for the foreseeable future. Micro-OLEDs are expected to continue getting brighter, with some claiming they have roadmaps to get to about 30K nits.

Jade Bird Display (JBD) Based AR Glasses

Jade Bird Display (JBD) is the only company I know to be shipping MicroLEDs in production. All working headsets I have seen use JBD’s 640×480 green (only) MicroLEDs, including ones from Vuzix (Ultralite and Shield), Oppo, and Waveoptics (shown in 2022 before being acquired by Snap). JBD is developing devices supporting higher pixel depth and higher resolution.

Also, as background to MicroLEDs in general, as well as JBD and the glasses using their MicroLEDs, there is my 2022 blog article AWE 2022 (Part 6) – MicroLED Microdisplays for Augmented Reality and the associated video with SadlyItsBradley. Additionally, there is my 2021 article on JBD and WaveOptics in News: WaveOptics & Jade Bird Display MicroLED Partnership.

The current green MicroLEDs support only 4 bits per pixel or 16 (2⁴) brightness levels and will show contour lines with a smooth shaded area. I hear that JBD’s future designs will support more levels. While I have seen continuous improvement in the pixel-to-pixel brightness differences through the year, and while they are the most uniform MicroLED devices I have seen, there is still visible “grain” in what should be a solid area.

Vuzix

At CES 2023, Vuzix showed off the small size possible with their Utralite™ glasses (left side below) which weigh only 38 grams (not much more than most conventional glass). A tray full of display engines on public display was there to emphasize that they were in production. The comparison of light engines (below left) shows how compact the MicroLED green and color cube projector engines are compared with Vuzix’s older (but true color) DLP design with similar resolution. I discussed Vuzix’s Ultralite and Shield in the CES 2023 video with SadleyItsBradley.

The Vuzix Shield and Ultralite share the same small green MicroLED engine. The combination of the engine and Vuzix waveguide are capable of up to 4,100 nits which is bright enough to enable outdoor use. The power consumption of MicroLEDs is roughly proportional to the average pixel value (APV). Paul Travers, CEO of Vuzix, says that the Ultralites consume very little power and can work for two days in typical use on a charge. Vuzix has also improved their in-house developed waveguides, significantly reducing the forward projection (“eye-glow”).

Vuzix has been very involved with several MicroLED companies, as discussed with SadlyItsBradley in our AWE 2022 Video.

Oppo

At AR/VR/MR 2023, Oppo showed me their JBD green MicroLED based glassed with a form factor similar to the Vuzix Ultralite. The overall image quality and resolution seem similar on casual inspection. The Vuzix waveguides diffraction gratings seem less noticeable from the outside, but I have not compared them side by side in the same conditions.

TCL and JBD X-Cube Color

At CES 2023, TCL demonstrated a multicolor 3-Chip (R, G, and B) combined with an X-Cube prototype (using a Lochn reflective waveguide). Vuzix, in a 2020 concept video, and Meta (Facebook), in a 2019 patent application, have shown using three waveguides to combine the three primary colors (below right). I discussed the TCL glasses with JBD color X-Cube design and some of the issues with X-Cubes in the CES video with SadleyItsBradley.

The TCL glasses appear to be using a diffraction grating waveguide that is very different from others I have seen due to the way the exit grating has very big steps in the transmission of light (right). This waveguide differs from the reflective waveguide JBD was showing in their booth or other diffractive waveguides. I have seen diffractive waveguides that were none uniform but never with such large steps in the output gratings. While I didn’t get a chance to see an image through the TCL glasses, the reports I got from others were that the image quality was not very good.

Goertek/Goeroptics Design and Manufacturing JBD Projection Engines

In the CES 2023 TCL video, I discussed some of the issues associated X-Cube color combining and the problems with aligning the three panels. At the AR/VR/MR conference, the Goeroptics division of Goertek showed that they were making both green-only and Color X-Cube designs for JBD’s MicroLEDs (slide from their presentation below). While Goertek may not be a household name, they are a very large optics and not-optics design and OEM for many famous brands, including giants such as Apple, Microsoft, Sony, Samsung, and Lenovo.

Porotech, Ostendo, and Innovation Semiconductor color tunable LEDs

I met Porotech in their private suite at CES and their booth at AR/VR/MR 2023. They have already received much attention on this blog in CES 2023 (Part 2) – Porotech – The Most Advanced MicroLED Technology, AWE 2022 (Part 6) – MicroLED Microdisplays for Augmented Reality, and my CES 2023 video with SadlyIsBradley on Porotech. They have been making a lot of news in the last year with their development of single-color InGaS red, green, and blue MicroLEDs and particularly their single emitter color tunable LED (what Porotech calls DynamicPixelTuning ^® or DPT ^®)

Below is a very short video I captured in the Porotech booth with a macro lens of their DynamicPixelTuning demo. I apologize for the camera changing focus when I switched from still to video mode with the blooming due to the wide range of brightness as the color changes. The demo shows the whole display changing color, as Porotech does not have a backplane that can change colors pixel by pixel.

At CES 2023, I was reminded by Ostendo, best known for the color-stacked MicroLEDs technology, that they had developed tunable color LEDs several years ago. Sure enough, six years ago, Ostendo presented the paper III-nitride monolithic LED covering full RGB color gamut in the Journal of the SPIE in February 2016. I have not seen evidence that Ostendo has come close to pursuing it beyond the single LED prototype stage, as Porotech has done with their DynamicPixelTuning.

The recent startup Innovation Semiconductor (below) is developing technology to integrate the control transistor circuitry into the InGaS substrate and avoid the more common hybrid InaS, and CMOS approaches almost all others are using. They are also developing a “V-grove” technology for making color-tunable LEDs. Innovation Semi cites work by the University of California at Stata Barbara (see paper 1 and paper 2 ) plus their own work that suggests that V-groves may be a more efficient way to produce color-tunable LEDs than the approach taken by Porotech and Ostendo.

A major concern I have with Innovation Semi’s approach to integrating the control transistors in GaN is whether they will be able to integrate enough control circuitry without making the devices too expensive and/or making the pixel size bigger.

PlayNitride (Blue with QD Conversion Spatial Color)

PlayNitride demonstrated its full-color MicroLED technology, which uses blue LEDs with Quantum Dot (QD) conversion to produce red and green. At 150K nits, they are extremely bright compared to Micro-OLEDs but are much less bright than native red, green, and blue MicroLEDs from companies including JBD and Porotech.

As discussed earlier, PlayNitride showed their MicroLEDs working with Lumus waveguides. But even though Lumus waveguides are more efficient than diffractive waveguides, 150K nits from the display are not bright enough for practical uses. They are about 1/10th the brightness of the native MicroLEDs of JBD and Porotech, and their pixels are bigger.

PlayNitride was the only company showing fairly high-resolution (1K by 1K and 1080P) full-color single-chip MicroLED microdisplays. Furthermore, these are only prototypes. Still, the green and red were substantially weaker than the blue, as seen in the direct (no waveguide) macro photograph of PlayNitrides MicroLED below. Also, the red was more magenta (mixed red and blue).

Looking at the 2X zoom, one sees the “grain” associated with the pixel-to-pixel brightness differences in all colors common to all MicroLEDs demonstrated to date. Additionally, in the larger reddish wedge pointed at by the red arrow, there are color differences/grain at the pixel level.

Known issue with QD spatial color conversion and microdisplays

While quantum dot (QD) color conversion of blue and UV LEDs has been proposed as a method to make full-color MicroLEDs for many years, there are particular issues with using QD with very small microdisplay pixels. Normally the QD layer required for conversion stays roughly the same thickness as the pixels become smaller, resulting in a very tall stack of QD compared to the pixel size. It then requires some form of microscopic baffling to prevent the light from adjacent LEDs from illuminating the wrong color.

Some have tried using thinner layers of QD and then relied on color filters to “clean up” the colors, but this comes with significant losses in efficiency and issues with heat. There are also issues with how hard the QD material can be driven before it degrades, which will limit brightness. Using spatial color itself has the issue of pixel sizes becoming too big for use in AR.

Many of these issues will be very different for making larger direct-view and VR pixels. The thickness of the QD layers becomes a non-issue as the pixels get bigger and spatial color has long been used with larger pixels. We have already seen where different OLED technologies have been used based on pixel size and application; for example, color-filtered OLEDs won out in large-screen TVs, whereas native color OLED subpixels are used in smartphone phones, smartwatches, and microdisplay OLEDs.

MICLEDI Reconstituted InGaS Wafers

MICLEDI is a spinout of the IMEC research institute in Belgium in 2019 with a booth at AR/VR/MR 2023. They are fabless with a mix of MicroLED technologies they have developed (right). They claim to have single color per die, spatial color (colors side by side), and stacked color technology. They have also developed GaN and Aluminum Gallium Phosphor (AlinGAP) red. After some brief discussions in their booth and going through their handout material, their MicroLEDs seem like a bit of a grab bag of technology for license without a clear direction.

The one technology that seems to set MICLEDI apart is for taking 100, 150mm, or 200mm GaN or AlinGap EPI wafers and making a “reconstituted” wafer with pick and placed known good dies. These reconstituted wafers can be “flip chipped” with today’s 300mm CMOS wafers. Today, almost all LED manufacturing is on much smaller wafers than mainstream production CMOS. For development today, companies are flipping small GaN wafers with spaced-out sets of LED arrays onto a larger CMOS wafer and throwing away most of the CMOS wafer.

Stacked MicroLEDs

While I didn’t see MIT at CES or AR/VR/MR 2023, MIT made news during AR/VR/MR with stacked color MicroLEDs. I don’t know the details, but it sounds similar to what Ostendo discussed, at least as far back as 2016 (see lower left). MICLEDI (above) has also developed a stated color LED technology where the LEDs are side by side.

The obvious advantage of stacked color is that the full color is smaller. But the disadvantage is that the LEDs and other circuitry above block light from lower LEDs. The net result is that stacked LEDs will likely be much brighter than Micro-OLEDs but much less bright than other MicroLED technologies. Also concerning is that while red is the color with the least efficiency today, it seems to end up on the lowest layer.

With their mid-range brightness, stacked MicroLEDs would likely be targeted at non-waveguide optics designs. Ostendo has been developing its optical design, which tiles multiple small MicroLEDs to give a wider FOV.

Conclusions

Many giant and small companies are betting that MicroLEDs will be the future of MicroDisplay technology for VR and AR. At the same time, one should realize that none of the technologies is competitive today regarding image quality with Micro-OLED, LCOS, or DLP. There are many manufacturing and technical hurdles yet to be solved. Each of the methods for producing full-color MicroLEDs has advantages and disadvantages. The race in AR is to support full-color displays and higher resolution at high brightness as, low power, and small size. I can’t see how multiple monochrome displays using X-Cubes, Waveguides, or other methods are long-term AR solutions.

I often warn people that if someone does a demo first, that does not mean they will be in production first. Some technical approaches will yield a hand-crafted one-off demo faster but are not manufacturable. The warning is doubly true when it comes to color MicroLEDs. It is easier to rule out certain approaches than to say which approach or approaches will succeed. For MicroDisplay MicroLEDs used in AR, I think native LEDs will win out over color-converted (ex., QD) blue LEDs. A different MicroLED technology will likely be better for direct-view displays.

It will be interesting to see the market adoptions of the new small form factor but green-only AR glasses. While they meet the form factor requirement of looking like glasses with acceptable weight, they don’t have great vision correction solutions, and being green-only will limit consumer interest.

A continuing issue with be which optics work best with MicroLEDs. Part of this issue will be affected by the degree of collimation of the light from the LEDs. The 2-D reflective waveguides developed by Lumus have a significant efficiency advantage, but still, many more companies are using diffractive waveguides today.

Appendix: MicroLED Background Information

What is a MicroLED Company?

To have a successful MicroLEDs is more than making the LEDs; it is about making a complete display and the ability to control it accurately at an affordable cost.

What constitutes a “MicroLED company” varies widely from a completely fabless design company to one that might design and fab the LEDs, design the (typically) CMOS control backplane, and then do the assembly and electrical connection of the (typically) Indium Gallium Nitride (InGaS) LEDs onto the CMOS backplane. Almost every company has a different “flow” or order in which they assemble/combine various component technologies. For example, shown below is the flow given by JBD, where they appear to be applying the Epi-lay to grow the LEDs on top of the CMOS wafer; other companies would form the LEDs first on the InGaN wafer and then bond the finished transistor arrays onto the finished CMOS control devices.

There is no common approach, and there are as many different methods as there are companies with some flows radically different from JBD’s. Greatly complicating matters is that most InGaN fabrication is done on 150mm to 200mm diameter wafers. In contrast, mainstream CMOS today is made on 300mm wafers which least to a variety of methods to address this issue, some of which are better suited to volume manufacturing than others.

Microdisplay vs. Direct View Pixel Sizes

What companies call MicroLED displays varies from wall-size monitors and TVs that can be more than a meter wide down to microdisplays typically less than 25mm in diagonal. As the table on the right shows, a small pixel on an AR microdisplay is about 300 to 600 times smaller than a direct-view smartphone or smartwatch. Pixel sizes get closer when comparing waveguide-based AR to VR pixels.

VR headsets started with essentially direct-view cell phone-type displays with some cheap optics to enable the human eye to focus but have been driving the pixel size down to improve angular resolution. The latest trend is to use pancake optics which can use even smaller pixels to enable smaller headsets.

There is some “bridging” between AR and VR with display types. For example, large combiner “bug-eye” AR often uses direct-view type displays common in VR. Some pancake optics-based VR displays use the same Micro-OLED displays used with AR birdbath optics.

With the radically different pixel sizes, it should not be surprising that the best technology to support that pixel size could change. Small microdisplays used by waveguide-based AR require microdisplays with semiconductor (usually CMOS) transistors. TVs, smartphones, and smartwatches use various types of thin film transistors.

Particularly regarding supporting color with MicroLEDs, it should be expected that the technologies used for microdisplays could be very different from those used for direct-view type displays. For example, while quantum dots color conversion of blue or UV light might be a good method for supporting larger displays, it does not seem to scale well to the small pixel sizes used in AR.

Multicolor, Full Color, or True Color

While not “industry standard definitions,” for the sake of discussion, I want to define three categories of color display:

1. Multicolor – Provides multiple identifiable colors, including, at a minimum, the primary colors of red, green, and blue. This type of display is useful for providing basic information and color coding it. Photographic images will look cartoonish at best, and there are typically very visible “contour lines” in what should be smoothly shaded surfaces.
2. Full Color – This case supports a wide range of colors, and smooth surfaces will not have significant contours, but the color control across the display is not good enough for showing pictures of people.
3. True Color – The display is capable of reasonably accurate color control across the display. Importantly, faces and skin tones, to which human vision is particularly sensitive, look good. It a display is “true color,” it should also be able to control the “white point,” and whites will look white, and grays will be gray. There should be no visible contouring.

The images below are examples of “multicolor,” “full color,” and “true color” images.

It might seem to some that my definition of “full” versus “true” color is redundant, but I have seen many demonstrations through the years where the display can display color but can’t control it well. In 2012, I wrote Cynics Guide to CES – Glossary of Terms. I called this issue “Pixar-ized” because there were so many demos of cartoon characters showing color saturation but none showing humans, which requires accurate color control.

Pixar-ized – The showing of only cartoons because the device can’t control color well and/or has low resolution.  People have very poor absolute color perception but tend to be are very sensitive to skin tones and know what looks right when viewing humans, but the human visual systems is very poor at judging whether the color is right in a cartoon.  Additionally it is very hard to tell resolution when viewing a cartoon.

I will add to this category above “artistic” false/shifted color images (see Playnitride’s above). Sometimes this is done because the work to calibrate the prototype has not been completed, even though the display can eventually support full color. Still, it is often done to hide problems.

I should note that what can be acceptable to the eye with a single-color image can look very bad when combined with other colors. What are weak or dead pixels with a monochrome display will turn into colorized or color-shifted pixels that will stick out. Anyone with a single dead color within a pixel on display has seen how the missing color sticks out. The images below are a simplified Photoshop (simulation) of what happens if random noise and dim areas occur in the various colors. The left image shows the effect on the full-color image, and the right image shows the same amount of random noise and dimming (in green) with the monochrome green (note, the image on the right is the grayscale image and then converted to green and not just the green channel from the true color image). In the green-only image, you can see some noise and a slight dimming that might not even be noticeable, whereas, in the color image, it turns into a magenta-colored area.

In that same 2012 article, I wrote about “Stilliphobia,” the fear of showing still images. We are seeing that with displaying content that is very busy and/or with lots of motion to hide dead or weak pixels or random pixel values in the display. When I see a needlessly busy image or lots of motion, I immediately think they are trying to hide problems. Someone with a great-looking display should show pictures of people and smooth images for at least some content.

Most of today’s MicroLED displays are working on getting to multicolor displays and are far from true color. All MicroLED microdisplays I have seen to date have large pixel-to-pixel variations. No amount of calibration or mura correction will be enough to produce a good photographic image if the individual colors can’t be controlled accurately. The good news is that most of today’s AR applications only require a multicolor display.