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  • Writer's pictureMaymay

The Two Main Optical Solutions in Consumer AR Devices: Birdbath and Waveguide Technologies

The choice of lens and optical technology is a defining characteristic of AR smart glasses, shaping their price, form factor, display quality, and overall user experience. Among the various options available, Birdbath and Waveguide optical designs have emerged as the two mainstream solutions in the consumer market. The waveguide lens design, in particular, has seen significant advancements and mass manufacturing scalability, leading to a surge of lightweight and thin AR smart glasses, especially in the Chinese market. Understanding the differences between these two optical solutions and their impact on the smart glasses market in recent years is crucial for grasping the current trends and future directions of AR technology.


Outline of the Blog

1.     Introduction

2.     Birdbath Optical Design

3.     Transition from Birdbath to Waveguide Technology in AR Glasses

4.     Waveguide Optical Design (Geometric Waveguides & Diffractive Waveguides)

5. Industry Trends

6.     RayNeo X2: A Case Study in Diffractive Waveguide and MicroLED Integration

7.     Does Metavision Have an Existing Waveguide Design AR Glass?

 

Birdbath Optical Design

The Birdbath optical design projects light from a display source onto a beamsplitter glass. This glass partially reflects the light towards the user while allowing the rest to pass through, creating an overlay of digital images on the physical world. The light is then redirected by a concave mirror (combiner) back into the user's eye, integrating virtual content seamlessly with the real world.


Birdbath optical design

Source: DisplayModule

 

Glasses Utilizing This Optical Design:

Xreal Air 2 Pro

Xreal Air 2

Rokid Max2

 

Key Features

These glasses integrate the Birdbath optical solution and MicroOLED displays, chosen for their high resolution and contrast. MicroOLED technology provides superior pixel density and image quality, crucial for creating clear and sharp augmented images in a compact form factor.


Field of View (FOV)

Typically, the Birdbath optical solution offers a medium FOV. The Xreal Air 2, for instance, has a 46-degree FOV, which is competitive but smaller than some advanced designs. The Microsoft HoloLens 2, which utilizes diffractive waveguide optics combined with Liquid Crystal on Silicon (LCOS) displays, offers a 52-degree FOV. This design allows for a larger FOV and high image quality. In comparison, the Meta Quest Pro in VR mode features a 90-degree FOV, achieved through the use of pancake optics and high-resolution displays. In terms of Field of View (FOV), glasses format products typically can't compete with headset format products.


Light Loss

The Birdbath optical design experiences significant light loss at various stages. Initially, the beamsplitter partially reflects and transmits light from the display source, leading to a 50% light loss. Further losses occur when the light reflects off the concave mirror (combiner) and passes back through the beamsplitter to reach the user's eye. These multiple reflections and transmissions reduce the overall brightness, necessitating darker lenses to compensate. This results in lenses that resemble sunglasses, potentially diminishing the augmented reality experience in low-light conditions.


Form Factor

While the Birdbath optical design allows for a more compact form factor, it also introduces higher ghosting effects and requires darker lenses. The use of MicroOLED displays contributes to the slim profile of these AR glasses, making them more portable and user-friendly. However, the compact design results in multiple reflections within the optical components, causing unwanted secondary images known as ghosting. Additionally, the darker lenses needed to counteract light loss can reduce the transparency of the AR glasses, affecting the clarity of the real-world view.


Birdbath AR glasses are more like sunglasses because they require darker lenses to compensate for light loss. In contrast, waveguide optical AR glasses need highly transparent lenses to maintain clarity and a seamless blend of digital and real-world views.

 

Transitioning from Birdbath to Waveguide Technology in AR Glasses

Birdbath designs, with their high image quality, quickly gained a foothold in the market. From early products by ODG to numerous current Birdbath-based models, they have leveraged their superior image quality, mainly for viewing purposes. These products often use a split design, keeping the overall weight around 80g, ensuring portability. However, they have larger optical modules, thicker lenses, lower transparency, and limited capability to view real-world scenes clearly.


Waveguide technology stands out for its lightweight, high transparency, and large FOV (compared to Birdbath optical design). Initially more expensive and used in enterprise-level products like HoloLens and Magic Leap, waveguides did not fully showcase their lightweight advantage in these early models. As costs have decreased, more consumer-grade AR glasses have adopted waveguide technology, achieving a high level of integration and a weight of under 80g. This advancement makes waveguide technology crucial for developing lightweight AR glasses suitable for daily wear.

 

Waveguide Optical Design

Waveguide technology has significantly advanced AR glasses with its innovative capabilities and sleek form factor. In the consumer AR market, the two most mainstream waveguide solutions are geometric waveguides and diffractive waveguides. Both of these technologies have unique mechanisms and benefits, making them popular choices among AR manufacturers.


Architectures of waveguide combiners

Source: SpringerOpen


Geometric Waveguides

Mechanism: Geometric waveguides use reflective optics, such as mirrors and prisms, to guide light. They offer superior image quality with higher contrast and color saturation due to the use of traditional optics. These waveguides are compatible with high-brightness displays like LCOS, DLP, and microLEDs.

Advantages:

  • Higher Optical Efficiency:  Better light transmission (compared to Diffractive Waveguide Optical Design) leads to reduced power consumption.

  • Established Manufacturing Processes: These waveguides leverage traditional manufacturing techniques, making them relatively easier to produce with high quality.

Challenges:

  • Complex Manufacturing: Despite using established processes, the manufacturing involves precise alignment of multiple optical elements, which can be complex and costly.

  • Light Leakage: These waveguides can suffer from light leakage, although typically less than diffractive waveguides.


Diffractive Waveguides

Mechanism: Diffractive waveguides utilize surface relief gratings (SRG) or holographic gratings to diffract light. They are favored for their minimal thickness, lightweight design, and full transparency.

Advantages:

  • Compact and Lightweight: They allow for sleeker, more stylish AR glasses.

  • Ease of Mass Manufacturing: Laser etching processes make diffractive waveguides easier to scale for mass production. However, diffractive waveguides have high requirements for the brightness of the display. This is because the diffraction process involves significant light loss, necessitating a bright display source to ensure a clear and vivid AR experience. Therefore, the development and improvement of high-brightness display technologies, such as MicroLED and LCOS, are crucial for the successful realization and adoption of diffractive waveguide AR glasses in the consumer market.

Challenges:

  • Color Dispersion: One of the main drawbacks is the "rainbow effect," where different colors diffract at different angles, causing color dispersion.

  • Lower Efficiency: Diffractive waveguides generally have lower optical efficiency, leading to higher power consumption for achieving the same brightness as geometric waveguides.

  • Image Quality: The presence of diffraction gratings can introduce artifacts and reduce overall image clarity compared to geometric waveguides.

Overall, both geometric and diffractive waveguides have unique advantages that cater to different aspects of AR glasses, from image quality to form factor. The choice between them often depends on the specific requirements of the AR application and the desired balance between performance and manufacturability.


Industry Trends

The AR industry prefers high-brightness displays such as microLEDs for diffractive waveguides to compensate for light loss and ensure a vivid AR experience. As advancements continue, the combination of microLED and diffractive waveguides is seen as the optimal solution for developing lightweight, daily-wear AR glasses. This combination provides high brightness and efficiency, making it suitable for consumer use​.


Rumor has it that Meta will launch three different types of AR glasses featuring AR displays with small, middle, and large FOV within the next three years. Two of these will adopt diffractive waveguides, while one will adopt geometric waveguides. These advancements promise to cater to a variety of user needs and preferences, enhancing the AR experience with varying degrees of field of view and technological sophistication. Let’s look forward to these exciting rumors becoming a great reality.


RayNeo X2: A Case Study in Diffractive Waveguide and MicroLED Integration

The RayNeo X2 AR glasses utilize a combination of full-color Micro-LED displays and diffractive waveguide technology. This combination offers several advantages over the Birdbath design, including a higher transparency rate, with the RayNeo X2 achieving over 85% transparency compared to the Birdbath's 15%-25%. The high transparency of diffractive waveguides allows AR glasses to have a design closer to ordinary glasses and provides a better view of external scenes, which is crucial for creating augmented reality content.

Micro-LED displays complement the low light efficiency of waveguide solutions by providing high brightness, making them an ideal match for diffractive waveguides. This combination ensures that the AR experience is bright and clear, even in challenging lighting conditions.


Does Metavision Have an Existing Waveguide Design AR Glass?

Yes, Metavision has successfully integrated waveguide optical solutions into its product lineup, most notably with the launch of the M51 AR glasses in 2022.

The AR glasses M51 utilize geometric waveguide technology, achieving an impressive 85% light transmission rate. This advanced optical design allows users to view augmented reality content seamlessly overlaid on the real world without significant obstruction, ensuring a clear and immersive experience. The M51 is equipped with an LCoS (Liquid Crystal on Silicon) display, which provides sharp and vibrant images, essential for high-quality AR applications.

The M51 AR glasses choose LCoS (Liquid Crystal on Silicon) displays due to their distinct advantages in delivering high-quality visuals. LCoS displays are known for their ability to produce sharp and detailed images with excellent color accuracy. This makes them particularly suitable for AR applications where clarity and precision are paramount. The reflective nature of LCoS technology allows for a higher resolution and better control over the light, resulting in a more vibrant and lifelike augmented reality experience.

 

We are also in the development of new AR glasses with waveguide optical design. Join us and let’s shape the future of augmented reality together!



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