Curved display screens and variable-focus liquid crystal lenses seem to be the trends of AR/VR in the future. In fact, companies are already providing such technologies, which are expected to have an important impact on AR/VR. AR/VR optics expert Karl Guttag pointed out that he saw an organic electronics company focusing on flexible displays and sensors at CES 2023: FlexEnable, which has demonstrated curved LCDs, electronically controlled liquid crystal lenses and dimming solutions. Guttag believes that the company's electronic dimming solution has the potential to become an alternative technology to Meta Materials, which can bring electric zoom effects to VR, and has attracted the attention of companies such as Meta in recent years.
In addition, FlexEnable's LCD screen is a two-axis curved surface solution, which means that the screen can be made into a curved shape. In AR/VR head-mounted displays, such a design may bring a wider FOV. Next, let’s take a deeper look at what key technologies FlexEnable has developed that can be used for AR/VR.
About Dual Axis Surface Technology
FlexEnable's business covers custom liquid crystals, triacetyl cellulose (TAC) light-transmitting films, polymer transistors, and electronic circuits. The company has labs that produce prototypes, but its business model is largely design, development and customization, with manufacturing out to outside vendors.
TAC film has high light transmission and low birefringence characteristics, so it is often used in polarizers, which can realize polarization change by changing retardation. Unlike most plastic materials, TAC maintains a low birefringence when bent, or heated to the glass transition point (rubbery but not melted), so it can be well molded into biaxial curved structures to match AR/VR The curvature or other characteristics of a lens.
About Dimming Technology
FlexEnable's dimming solution is similar to the manufacturing method of glass LCD screens. The difference is that it is encapsulated with TAC film instead of glass. From a structural point of view, TAC films are added to both sides of the LCD screen. In the process of forming a biaxial curved surface, the TAC material needs to be heated to the glass transition point (about 150°C) to shape it.
In terms of details, FlexEnable's dimming technology is based on a non-polarized liquid crystal solution, which is characterized by the brightness being driven by electricity, and the unpowered state can be transparent or dark (the AR glasses can choose a high-transmittance solution), and the specific effect is as shown in the figure below.
The necessity of dimming technology
Dimming is important for optical AR because it allows AR glasses to be used in a variety of lighting environments, from bright outdoors to dark indoors. Simply put, this function can make AR images look clearer, and can well control the light entering the human eye, thereby reducing power consumption and improving visual comfort. For optical AR, usually electronic dimming needs to have two major characteristics, one is that the light will not be polarized, and the other is that it has high light transmittance in the non-dimming state.
Magic Leap 2 supports segmented electronic dimming function, which can selectively block light, which can better follow AR content and make it look more three-dimensional. However, because the segmental dimming intelligently realizes soft-edge occlusion (soft-edge occlusion), it will cause an area to be darkened. In contrast, hard-edge occlusion can more precisely control the brightness of each pixel, but at this stage, optical AR can hardly achieve this, unless it is a mixed reality solution based on VST perspective.
Another problem with ML2 is that it uses polarization-based dimming. Polarized light means light loss, so the AR glasses still block 65% of ambient light in the undimmed state (blocking 80% after adding the optical waveguide). It seems that adding a hood to the AR glasses does not solve the problem of contrast and clarity very well.
Dimming range and speed
It is understood that FlexEnable's technology has a wide dimming range, from about 0% to nearly 90%. In fact, the dimming range and response speed are mainly affected by two factors, one is the liquid crystal solution, and the other is the width of the liquid crystal layer gap. The wider the gap, the more light is blocked in the light-transmitting state and the dimming state.
Similar to most liquid crystal materials, screen response speed is roughly inversely proportional to the size of the square liquid crystal gap. If the gap width is halved, the response speed of the LCD screen will increase by about 4 times. Currently, FlexEnable has not announced the response speed of its liquid crystal solution.
Fast response is very important for the segmental dimming function of AR glasses, because the speed of dimming should be synchronized with the change of display pixels. If two thin liquid crystal layers are stacked in series, the response speed and dynamic range can be improved at the same time, and the problem of light blocking can be compensated.
Electron lens
FlexEnable supports passive and active (transistor) two segment/pixel dimming methods. It has developed a liquid crystal gradient index (GRIN) liquid crystal lens, which can change the refractive index of the liquid crystal through an electric field, thereby achieving the lens dimming effect. The scheme is only suitable for polarized light, and the control of unpolarized light requires a double-layer sandwich structure. GRIN lens does not need to polarize light first, but requires a double liquid crystal sandwich structure, while PBP needs to polarize ambient light, which means more than 50% light loss, and there are problems when combined with polarized light screens such as LCD.
In recent years, Meta has been researching focusing lens technology based on GRIN and PBP schemes. For example, its patent (2020/0348528) has demonstrated a GRIN structure lens combined with Fresnel electrodes. This scheme is also called Segmented lens configuration (SPP).
By widening the liquid crystal layer gap, the light switching efficiency of the GRIN lens can be further improved, but this will reduce the switching speed (response speed). Therefore, a solution close to the Fresnel lens can be adopted, the advantage is that the dimming lens can be made thinner and the response speed is faster, but it has the same disadvantages as the Fresnel lens.
When used in VR (such as Meta Half Dome 3), the light can be polarized, so each dimmable lens only needs a single layer structure. Half Dome 3 employs a series of PBP lenses that function as binary weighted lenses.
In addition to the above-mentioned dimming liquid crystals, companies such as poLight, Cambridge Mechatronics (CML), and DeepOptics also provide various dimming and focusing solutions.
Polymer Transistors and Circuits
Guttag pointed out that FlexEnable has also developed polymer semiconductors, which feature smaller transistors and claim to outperform the amorphous silicon transistors used in flat-panel displays.
Biaxial Curved LCD
If FlexEnable's technology is integrated (including curved LCD, electronic circuits and polymer semiconductors), it will be possible to manufacture biaxial curved LCD prototype displays.
For AR/VR, the advantage of curved display is that it can increase the width of FOV. During AWE 2022, Red 6 demonstrated a military AR headset based on a curved LCD solution, which claimed to achieve a 100° FOV without significant pupil swimming (the image distorts with eye movement). It is reported that the prototype of Red 6's AR headset was designed by Pulsar, which uses an LCD screen made of curved glass, which is a conceptual design.
in conclusion
Although FlexEnable does not claim that its dimming range is equivalent to that of Meta Materials, for some AR application scenarios, its dimming range seems to be sufficient and the light transmittance is good enough. What is uncertain at present is whether the response speed of this solution is fast enough to support segmental dimming well.
Some people believe that designing the film as a biaxial curved surface structure for electronically controlled dimming has advantages over Meta Materials rigid dimming lenses. In terms of specifications, Meta Materials has demonstrated a sufficiently wide dynamic range, and the future FlexEnable LCD solution may continue to improve on this basis. In addition, the dual-axis curved LCD enables a wider FOV.
In addition to dynamic dimming, switchable lenses can also be used to achieve dynamic zoom, thereby alleviating VAC issues. However, the application of dynamic dimming in AR is more complicated, and it may be difficult to implement in the short term. Because it needs to be combined with technologies such as eye tracking, but also to pre-compensate/offset the effect of zoom on the physical environment (transmitted ambient light). Reference: KG