Solving ambient lighting challenges in AR optics

The development of augmented reality (AR) headset technologies that improve user experience continues to advance on multiple fronts. The goal is a headset that can seamlessly stitch together the real world with the virtual, while being indistinguishable from regular glasses, thus meeting the requirements of comfort and realism needed for sustained use.

Comfort is not only about the physical weight and fit of the headset, but also about the visual experience, minimizing eye strain and fatigue to levels equivalent to viewing the real world. Achieving this requires technological innovation throughout the optical system, from the display to the downstream optics. While these challenges also exist in VR headsets, they are arguably more pronounced in AR, because the in-use headset must cater to the changing real-world environment as well.

Consequently, one of the challenges for see-through AR headsets is ensuring that virtual objects are displayed with consistent contrast regardless of the changing brightness in the real world. In AR headsets today, the brightness of virtual objects is limited by low efficiencies of the optical system—only a small percentage of light from the display reaches the eye. The ambient brightness against which the virtual object must compete can rapidly change over several orders of magnitude depending on whether the user is indoors or outdoors on a sunny day.

As a result, a virtual object that appears relatively bright and high contrast in an office environment may appear at best semitransparent and washed out in a sunny outdoors setting, preventing the virtual object from occluding anything behind it. The problem is exacerbated when the virtual object is dark or black. The display cannot project the color black onto the real world.

To counteract this and ensure a comfortable, long-term visual experience, maintaining brightness, color accuracy, and achieving solid occlusion of real-world objects behind virtual ones are essential to provide the expected image quality and depth cues for the user.

Ultrathin and ultralight flexible liquid crystal cells can be used in AR glasses to improve performance and user comfort. FlexEnable’s technology enables push-pull lenses and ambient dimming.

 In principle, a brute-force approach to increasing image contrast in bright environments would be to increase display brightness by orders of magnitude. In practice, however, this would be so power hungry as to make the product unusable due to the resulting short battery life, thermal management, and user comfort. And it wouldn’t help with displaying dark or black virtual objects.

A more practical solution is to introduce a way of dimming the brightness of the outside world to the device itself. This can be added directly to the device optics to provide global dimming (like a pair of adjustable sunglasses), or better still, at local level (sometimes referred to as spatial dimming), where dimming occurs only into the region of the virtual image itself. The visibility of the real world would be undiminished, while the virtual object would appear solid and nontransparent.

Biaxially curved LC dimmer on TAC film. Photo credit: FlexEnable

For global dimming there are several candidate technologies, such as electrochromic- and liquid-crystal (LC)-based approaches, but for spatial dimming the high switching speed required necessitates an LC-cell. The cell comprises an LC mixture sandwiched between two optically ideal substrates, typically glass, each containing an electrode. Applying a voltage across the electrodes causes the LC molecules to rotate, thereby changing the proportion of light transmitted through the cell. A variety of architectures can be used, some including polarizers, and some that use dichroic dye molecules.

As pioneered by George Heilmeier at RCA labs in 1965, by adding a dye to an LC mixture, the orientational order of the LC molecules imposes an orientation on the dye molecules, and this can be adjusted by applying voltage. It switches the LC cell from a clear (transmissive) state to a dark (absorbing) state, so that the amount of light passing through can be controlled, creating a way to dim the light levels of the outside world.

This approach results in highly uniform, rapid-switching (10 ms) global light modulation films. By incorporating an active-matrix transistor array into the stack, the cells can be pixelated to achieve spatial dimming, thereby dynamically dimming only the pixels “behind” a virtual object.

Pixelated spatial dimming cells of this type could be manufactured on glass, using for example, silicon thin-film transistors to drive the pixels, but glass-based LC cells would not be an ideal solution for two main reasons: First, the glass itself is relatively heavy, adding unwanted weight to a device that needs to be as light as possible for comfort and prolonged use. Second, AR optics are typically curved in more than one direction (biaxially), like the surface of a sphere.

Although glass can be curved, this is not viable due to the very high tolerance required of the cell gap itself. That is why glass LC cells (for example, those used in LCD TVs) are manufactured flat. The idea of biaxially forming such glass cells after production is prohibitive because the temperatures needed to soften the glass would destroy both the LC and transistor materials.

FlexEnable has developed a technology to build LC cells and organic transistors on flexible bioplastic—triacetyl cellulose (TAC) film—instead of glass. TAC is low cost, widely available and already used in polarizers for displays because of its ideal optical properties: greater than 93 percent transmission, no birefringence, and near zero haze, making it perfect for LC optics. It is possible to use TAC film as a substrate for manufacturing organic transistors and liquid crystal cells for the first time, because the entire manufacturing process need not exceed 100 degrees C. This low manufacturing temperature is a property unique to organic transistors; silicon or other inorganic transistors require several hundred degrees C during production and cannot, then, be manufactured on optically ideal films such as TAC.

The result is spatial dimmers that are incredibly thin and light— a single 30-mm diameter dimmer weighs less than 0.3 g and is just 100-µm thick. In addition, because TAC can be softened at much lower temperatures than glass, the LC cell can be biaxially curved by thermoforming at temperatures that won’t damage the LC material or the organic transistors.

FlexEnable’s ambient dimming evaluation samples (flat or biaxially curved) are already being assessed by strategic partners interested in integrating our technology with other optical components. This promises to bring new capabilities to future AR headsets.

Paul Cain is Strategy Director for FlexEnable (www.flexenable.com). He has 20 years’ experience in the flexible and organic electronics industries, in both technical and strategic management roles. He holds 25 patents relating to processes and architectures that enable high-yield manufacture of flexible displays.