MR Pt. 4 – FlexEnable’s Dimming, Electronic Lenses, & Curved LCDs

23 February 2023 at 21:00

Introduction – Combining 2023’s CES and AR/VR/MR

As I wrote last time, I met with over 30 companies, about 10 of which twice between CES and SPIE’s AR/VR/MR conferences. Also, since I started publishing articles and videos with SadlyItsBradley on CES, I have received information about other companies, corrections, and updates.

FlexEnable is developing technology that could affect AR, VR, and MR. FlexEnable offers an alternative to Meta Materials (not to be confused with Meta/Facebook) electronic dimming technology. Soon after publishing CES 2023 (Part 1) – Meta Materials’ Breakthrough Dimming Technology, I learned about FlexEnable. So to a degree, this article is an update on the Meta Materials article.

Additionally, FlexEnable also has a liquid crystal electronic lens technology. This blog has discussed Meta/Facebook’s interest in electronically switchable lens technology in Imagine Optix Bought By Meta – Half Dome 3’s Varifocal Tech – Meta, Valve, and Apple on Collision Course? and Meta’s Cambria (Part 2): Is It Half Dome 3?.

FlexEnable is also working on Biaxially Curved LCD technology. In addition to direct display uses, the ability to curve a display as needed will find uses in AR and VR. Curved LCDs could be particularly useful in very wide FOV systems. I discussed this briefly (discussed R6’s helmet having a curved LCD briefly in out AR/VR/MR 2022 video with SadlyItsBradley)

FlexEnable – Flexible/Moldable LC for Dimming, Electronic Lenses, Embedded Circuitry/Transistors, and Curved LCD

FlexEnable has many device structures for making interesting optical technologies that combine custom liquid crystal (LC), Tri-acetyl cellulose (TAC) clear sheets, polymer transistors, and electronic circuitry. While Flexenable has labs to produce prototypes, its business model is to design, develop, and adapt its technologies to its customers’ requirements for transfer to a manufacturing facility.

TAC films are often used in polarizers because they have high light transmittance and low birefringence (variable retardation and, thus, change in polarization). Unlike most plastics, TAC can retain its low birefringence when flexed or heated to its glass point (becomes rubbery but not melted) and molded to a biaxial curve. By biaxially curving, they can match the curvature of lenses or other product features.

FlexEnable’s Biaxially Curvable Dimming

Below is the FlexEnable approach to dimming, which is similar to how a traditional glass LC device is made. The difference is that they use TAC films to enclose the LC instead of glass. FlexEnable has formulated a non-polarization-based LC that can either darken or lighten when an electric field is applied (the undriven state can be transparent or dark). For AR, a transparent state, when undriven, would normally be preferred.

To form a biaxially curved dimmer, the TAC material is heated to its glass point (around 150°C) for molding. Below is the cell structure and an example of a curved dimmer in its transparent and dark state.

*FlexEnable biaxially shapeable electronic dimming*

The Need for Dimming Technology

As discussed in CES 2023 (Part 1) – Meta Materials’ Breakthrough Dimming Technology, there is a massive need in optical AR to support electronically controlled dimming that A) does not require light to be polarized, and B) has a highly transparent state when not dimming. Electronic dimming supports AR headset use in various ambient light conditions, from outside in the day to darker environments. It will make the virtual content easier to see without blasting very bright light to the eyes. Not only will it reduce system power, but it will also be easier on the eyes.

The Magic Leap has demonstrated the usefulness of electronic dimming with and without segmentation (also known as soft edge occlusion or pixelated dimming) with their Magic Leap 2 (and discussed with SadlyItsBradley). Segmented dimming allows the light blocking to be selective and more or less track the visual content and make it look more solid. Because the segmented dimming is out of focus can only do “soft edge occlusion,” where it dims general areas. “Hard-edge occlusion,” which would selectively dim the real work for each pixel in the virtual world, appears impossible with optical AR (but trivial in VR with camera passthrough).

The biggest problem with the ML2 approach is that it used polarization-based dimming that blocks about 65% of the light in its most transparent state (and ~80% after the waveguide). I discussed this problem in Magic Leap 2 (Pt. 3): Soft Edge Occlusion, a Solution for Investors and Not Users. The desire (I would say needed, as discussed in the Meta Materials article here) for light blocking in AR is undeniable, but blocking 80% of the light in the most transparent state is unacceptable in most applications. Magic Leap has been demonstrating that soft edge occlusion improves the virtual image.

Dimming Range and Speed

Two main factors affect the darkening range and switching speed: the LC formulation and the cell gap thickness. For a given LC formula, the thicker the gap, the more light it will block in both the transmissive and the dark state.

Like with most LC materials, the switching speed increases roughly inversely proportional to the square cell gap thickness. For example, if the cell gap is half as thick, the LC will switch about 4 times faster. FlexEnable is not ready to specify the switching speeds.

The chart on the right shows some currently possible dimming ranges with different LC formulas and cell thicknesses.

Segmented/Pixelated Dimming

Fast switching speeds become particularly important for supporting segmented dimming (ex., Magic Leap 2) because the dimming switching speed needs to be about as fast as the display. Stacking two thin cells in series could give both faster switching with a larger dynamic range as the light blocking would be roughly squared.

FlexEnable supports passive and active (transistor) circuitry to segment/pixelate and control the dimming.

Electronically Controlled Lenses

FlexEnable is also developing what are known as GRIN (Gradient Index) LC lenses. With this type of LC, the electric field changes the LC’s refraction index to create a switchable lens effect. The index-changing effect is polarization specific, so to control unpolarized light, a two-layer sandwich is required (see below left). As evidenced by patent applications, Meta (Facebook) has been studying GRIN and Pancharatnam–Berry Phase (PBP) electronically switchable lenses (for more on the difference between GRIN and PBP switchable lenses, see the Appendix). Meta application 2020/0348528 (Figs. 2 and 12 right) shows using a GRIN-type lens with a Fresnel electrode pattern (what they call a Segmented Lens Profile or SPP). The same application also discusses PBP lenses.

*FlexEnable (left) and Meta Patent Application 2020/0348528* Figs. 2 and 12 *(right)*

The switchable optical power of the GRIN lens can be increased by making the cell gap thicker, but as stated earlier, the speed of LC switching will reduce by roughly the square of the cell gap thickness. So instead, a Fresnel-like approach can be used, as seen diagrammatically in the Meta patent application figure (above right). This results in a thinner and faster switching lens but with Fresnel-type optical issues.

When used in VR (ex., Meta’s Half Dome 3), the light can be polarized, so only one layer is required per switchable lens.

There is a lot of research in the field of electronically switchable optics. DeepOptics is a company that this blog has referenced a few times since I saw them at CES 2018.

Miniature Electromechanical Focusing – Cambridge Mechatronics and poLight

At CES, I met with Cambridge Mechatronics (CML)and poLight, which have miniature electromechanical focusing and optical image stabilization devices used in cell phones and AR cameras. CML uses Shape Memory Alloy wire to move conventional optics for focusing and stabilization. poLight uses piezoelectric actuators to bend a clear deformable membrane over a clear but soft optical material to form a variable lens. They can also tilt a rigid surface against the soft optical material to control optical image stabilization and pixel shifting (often called wobulation) I plan to cover both technologies in more detail in a future article, but I wanted to mention them here as alternatives to LC control variable focus.

Polymer Transistors and Circuitry

FlexEnable has also developed polymer semiconductors that they claim perform better than amorphous silicon transistors (typically used in flat panel displays). Higher performance translates into smaller transistors. These transistors can be used in an active matrix to control higher-resolution devices.

Biaxially Curved LCD

Combining FlexEnable’s technologies together, including curved LCD, circuitry, and polymer semiconductors results in their ability to make biaxially curved LCD prototype displays (right).

Curved displays and Very Wide FOV

Curved displays become advantageous in making very wide FOV displays. At AWE 2022, Red 6 had private demonstrations (discussed briefly in my video with SadlyItsBradley) of a 100-degree FOV with no pupil swim (image distorting as the eye moves) military AR headset incorporating a curved LCD. Pulsar, an optical design consulting company, developed the concept of using a curved display and the optics for the new Red 6 prototype. To be clear, Red 6/Pulsar used a curved glass LCD display, not one from FlexEnable, but it shows that curved displays become advantageous.

Conclusions

In the near term, I find the non-polarized electronic dimming capabilities most interesting for AR. While FlexEnable doesn’t claim to have the light-to-dark range of Meta Materials, they appear to have enough range, particularly on the transparent end, for some AR applications. We must wait to see if the switching speeds are fast enough to support segmented dimming.

To have electronic dimming in a film that can be biaxially curved to add to a design will be seen by many to have design advantages over Meta Material’s rigid lens-like dimming technology. Currently, it seems that, at least on specs, Meta Materials has demonstrated a much wider dynamic range from the transparent to the dark state. I would expect that Flexenable’s LC characteristics will continue to improve.

Electronically changeable lenses are seen as a way to address vergence accommodation conflict (VAC) in VR (such as with Meta’s Half-Dome 3). They would be combined with eye tracking or other methods to move the focus based on where the user is looking. Supporting VAC with AR would be much more complex to prevent the focus changing in the real world a pre-compensation switchable lens would have to cancel out the effect on the real world. This complexity will likely prevent them from being used for VAR in optical AR anytime soon.

Biaxially curved LCDs would seem to offer optical advantages in very wide FOV applications.

Appendix: GRIN vs. Pancharatnam-Berry phase lenses

Simply put, the LC itself acts as a lens with a GRIN lens. The voltage across the LC and the LC’s thickness affects how the lens works. Pancharatnam-Berry phase (PBP) lenses use an LC shutter (uniform) to change the polarization of light that controls the effect of a film with the lens function recorded in it. The lens function film will act or not act based on the polarization of the light. As stated earlier, Meta has been considering both GRIN and PBP lenses (for example, both are shown in Meta application 2020/0348528)

For more on how GRIN lenses work, see Electrically tunable gradient-index lenses via nematic liquid crystals with a method of spatially extended phase distribution.

For more on PBP lenses, see the Augmented reality near-eye display using Pancharatnam-Berry phase lenses and my article, which discusses Meta’s use in the Half-Dome 3.

GRIN lenses don’t require light to be first polarized, but they require a sandwich of two cells. PBP in an AR application would require the real-world light to be polarized, which would lose more than 50% of the light and cause issues with looking at polarized light displays such as LCDs.

The PBP method would likely support more complex lens functions to be recorded in the films. The Meta Half-Dome 3 used a series of PBP lenses with binary-weighted lens functions (see below).