Augmented Reality Compact Display Engine Optics for Smart Glasses

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ROHM Semiconductor GmbH

MEV Elektronik Service GmbH

Würth Elektronik eiSos GmbH & Co. KG

TriLite Technologies GmbH

Smart glasses for augmented reality require compact and lightweight displays, that produce a bright, high-quality image. Find out how a tiny laser beam scanner and some clever software make this possible.

Smart glasses for augmented reality (AR) have been around for more than a decade but have failed to gain traction as a popular consumer technology. One of the main reasons for this lacklustre uptake has been the bulkiness of the projector and optics, preventing their smooth fit into standard glasses for everyday use.

The basis of AR is that virtual objects are projected over the viewed area and objects located in front of the person wear­ing the glasses. For this technology to achieve wide acceptance, the so called head mounted display (HMD) must be as lightweight, comfortable and unobtrusive as possible.

And now, new developments in the field of optics and electronics are set to revolutionize the smart glasses market. Smaller, lighter display systems will enable stylish eyewear that will provide next-level AR features and a realistic user-experience.

The micro-display engine driving AR glasses requires two main components: a projector and an 'optical combiner' that bends and combines light beams from multiple sources and directs them into the eye. The system must provide bright, high-resolution, high-quality images that are clearly visible under all daylight conditions.

AR applications utilize two main categories of micro-display engines

There are two primary types of combiners suitable for AR glasses: holographic and waveguide types. Which is better? In general, a holographic combiner achieves greater efficiency in transmitting from the light source but has a significantly smaller 'eye-box'. This is an imaginary box within which the user's eye can move while still seeing the image correctly.

A large eye-box makes it possible for any­one to wear a pair of AR glasses, so even if they don't fit exactly, the viewer still sees a perfect image. This is essential to achieving mass AR adoption by consumers. Conversely, waveguide combiners have a larger eye-box but are far less light efficient.

AR applications utilize two main categories of micro-display engines: panel-based and scanner-based optical engines. Panel-based display engines consist of a two-dimensional (2D) array of pixels similar to a conventional computer monitor. The downside of this approach is that the minimum pixel size is constrained by manufacturing and efficiency factors, limiting the minimum panel size for a given image resolution, making panel-based systems relatively bulky and heavy.

Scanner-based optical engines, such as a laser beam scanner (LBS), deflect laser beams using moveable micro-electro-mechanical system (MEMS) mirrors. This enables them to display the individual pixels of an image in a time-sequential manner, much like on an old-fashioned cathode ray tube TV. Compared to a panel-based system,

LBS scanning approach delivers substantial advantages in terms of display engine size and weight. Other advantages of LBS over panel-based display systems include higher brightness and contrast coupled with lower power consumption and latency. Utilizing sinusoidal scanning movements along both the x and y axes following the 'Lissajous trajectory' requires less power to drive the mirror since the image is built up simultaneously over the entire display area rather than gradually line-by-line.

LBS systems also exhibit lower optical distortions than panel-based displays and typically provide a larger field of view (FoV) – the factor which determines how much of the world around is visible. The optimal FoV for next-generation AR devices is 30° across its diagonal.

The Trixel 3 system is a ultra-compact micro-display engine

Let’s take a closer look at the world's smallest LBS projection display, the Trixel 3 micro-display engine from TriLite. It weighs less than 1.5 g and take up less than 1 cm x 1 cm x 1 cm of space, making it ideal for AR glass applications. At the heart of Trixel 3 there is an integrated RGB colour laser module from TriLite and custom-designed microlenses specifically optimized for AR applications. TriLite's software-driven hardware architecture and proprietary high-precision laser bonding techniques substantially reduce the cost of the module without any compromises in image quality or performance.

A fundamental characteristic of Trixel’s architecture is the key role played by software, clearly differentiating it from the more common hardware-only systems. This software-driven hardware approach enables fewer alignment steps and higher alignment tolerances during manufacture, delivering noticeable benefits in terms of assembly yields and manufacturing costs. To enable customers to quickly launch their products on to the market, TriLite has qualified a high-volume manufacturing services platform based around partnerships with key suppliers for assembly and components such as the MEMS mirror from Infineon.

TriLite’s patented laser timing algorithms achieve perfect overlapping of the RGB colour channels, which eliminates distortions in high quality images. The key concept is that by shifting the complexity of a projector from hardware to software, the optical system can be smaller, lighter and simpler.

Compact dimensions for virtually any pair of AR glasses

Trixel 3 is designed for compatibility with both diffractive waveguides and holographic optical elements as optical combiners. Dispelix has created leading diffractive waveguides, and by engaging with TriLite an ultra-compact display system for AR glasses has been demonstrated. This unique system combines TriLite's Trixel 3 LBS with Dispelix's diffractive waveguides, where the waveguide is a predominately transparent, thin piece of glass or plastic that bends and combines light.

Unlike many other systems, TriLite's approach does not require any additional relay optics located between the LBS projector and the waveguide, enabling the size of the system to be half that of competitors' solutions.

The combined system form factor utilizing Dispelix's ultra-thin waveguide enables such compact dimensions that it can be built into virtually any pair of AR glasses, regardless of size or style. The system provides crisp, bright images and text, even in direct strong sunlight conditions. Furthermore, the architecture is designed from the ground up for consumer AR applications, making it easy and affordable to implement in mass manufacturing, significantly reducing time to market.

For AR smart glasses to become a popular, widely used consumer product, they need to have displays that are compact, lightweight and able to produce high quality images. Leveraging TriLite’s history of building world lead­ing LBS system and proprietary design and software solutions, the latest generation Trixel 3 LBS has enabled TriLite to meet these requirements, whilst concurrently delivering low power consumption and easy manufacturability.

Trixel 3 has been intrinsically optimized for system integration, eliminating bulky relay optics. Mass production of Trixel 3 lightweight, comfortable smart glasses is set to bring high optical performance to consumer AR applications – watch this space!

* Dr. Peter Weigand is CEO at TriLite Technologies.

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