Are you curious how Kura Technologies' AR glasses achieve a stunning 150-degree field of view (FOV)? Or are you wondering how to push the boundaries of traditional AR optics to create an even more immersive experience? Let's delve into the exciting advancements in AR technology, focusing on innovative methods to achieve a wider FOV in AR glasses. We'll explore two cutting-edge solutions: the Mixed Waveguide Optics inspired by Ant Reality and the Hybrid Waveguide-Based Augmented Reality Display System, drawing insights from leading-edge technologies like those from Kura Technologies.
Field of View (FOV) Representation in Waveguide AR Glasses
Outline of the blog
1. Understanding the Limits of Traditional Optics
Birdbath Optics
Waveguide Optics
2. Introducing Solutions for Wider FOV
1) The Mixed Waveguide Solution - Ant Reality's Mixed Waveguide Technology:
A Detailed Explanation
2) Introducing a Cutting-Edge Hybrid Waveguide Solution
3. Metavision’s Larger FOV AR Approach
1. Understanding the Limits of Traditional Optics
Birdbath Optics
Geometry Constraints: The Birdbath configuration involves a display, a beamsplitter, and a concave mirror. Light from the display reflects off the beamsplitter to the concave mirror, which directs it to the user's eye. Due to the cubic size of the setup, the view aperture is limited. Additionally, the view distance (the distance between the display and the user's eyes) typically ranges between 15 to 25 cm. This setup results in a maximum vertical FOV of around 30 degrees and a maximum horizontal FOV of about 60 degrees in a 16:9 aspect ratio.
Physical Constraints:
Cubic Size: The need to house the display, beamsplitter, and mirror in a compact space limits the aperture size and, consequently, the FOV.
View Distance: The optical path length determines the minimum view distance, making it challenging to create a compact design suitable for wearable AR glasses.
Birdbath Optical Configuration for AR Glasses I Source: ResearchGate
Waveguide Optics
Components and Configuration: Waveguide systems consist of a projector and a light guide. The light guide allows for pupil expansion, resulting in a large light aperture and short view distance. Achieving a larger FOV, like 70 degrees, requires a light engine with a significant volume (around 3.4 cubic centimeters), making it challenging to reduce the size while maintaining performance. This is typically seen in traditional methods using Liquid Crystal on Silicon (LCoS) or Digital Light Processing (DLP) technology.
Limits of Waveguide Optics in AR Glasses I Source: Ant Reality
2. Introducing Solutions for Wider FOV
1) The Mixed Waveguide Solution - Ant Reality's Mixed Waveguide Technology: A Detailed Explanation
Introduction to Mixed Waveguide Technology: Mixed waveguide technology is an advanced optical system that employs a multilayer composite structure. This structure is composed of various layers of different materials and stacks, which also include multiple types of polarizing filters. The primary objective of this technology is to achieve a polarized-based folded optical path design, optimizing both focal length and module thickness.
Key Components and Design
1. Polarizing Filters and Layers: The multilayer structure includes polarizing filters that control and manipulate the polarization of light. These filters help in managing how light travels through the different layers, ensuring efficient light propagation and minimal loss.
2. Folded Optical Path: The design incorporates a folded optical path, which means that light is reflected multiple times within the waveguide. This folding helps in compressing the optical path length, making the system more compact. Unlike Pancake optics that use coaxial folding, mixed waveguides utilize a tilted optical path folding method based on total internal reflection (TIR). This method allows the light to be folded without affecting the transmission of external light, thus enabling an optical see-through augmented reality (AR) display.
3. Total Internal Reflection (TIR): The TIR-based folding ensures that light stays within the waveguide, reflecting off the internal surfaces. This technique maintains high efficiency and ensures that the external light passes through without interference, crucial for AR applications where a clear view of the real world is essential.
Source: Ant Reality
4. Absence of a Light Engine: One of the significant advantages of mixed waveguide technology is the elimination of the traditional light engine. Instead of relying on a bulky light engine, the entire folded optical path, including the light source, which uses micro-OLED screens, handles imaging. This results in a more compact and lightweight design.
Diagram of Mixed Waveguide I Source: Ant Reality
In our quest to push the boundaries of augmented reality (AR) technology, we have discovered a solution that theoretically surpasses the Mixed Waveguide solution discussed above. This innovative approach comes from an academic article available here. The new method leverages hybrid waveguide technology to achieve an extra-large field of view (FOV), addressing the limitations of traditional optics and offering a more advanced solution for AR glasses. We dig into the content and explain it with readable language; we hope to inspire and encourage more experts in the field to engage in this critical aspect of AR technology development.
2) Introducing a Cutting-Edge Hybrid Waveguide Solution
Addressing the Pain Points: Total Internal Reflection (TIR) and 2D Exit Pupil Expansion (EPE)
1)Total Internal Reflection (TIR):
In simpler terms, Total Internal Reflection (TIR) is a phenomenon that occurs in waveguide-based AR systems, where light is kept inside the waveguide by bouncing off its internal surfaces. This bouncing keeps the light confined within the waveguide, ensuring the image remains completely clear and bright. However, TIR only happens under certain conditions:
· Angle of Incidence: The light must hit the boundary at an angle greater than the critical angle (θc) specific to the materials involved.
· Refractive Index: TIR occurs when light travels from a medium with a higher refractive index (n1) to a medium with a lower refractive index (n2).
These conditions limit the range of angles at which light can travel within the waveguide without escaping. Because of this, the field of view (FOV) is restricted, meaning users can see less of the augmented reality content without moving their heads.
How Light is Confined in the Waveguide Core I Source: ResearchGate
2) 2D Exit Pupil Expansion (EPE):
The exit pupil size is critical for ensuring that the augmented image remains visible as the user’s eye moves. Without effective 2D EPE, the exit pupil is small, causing parts of the image to disappear when the user looks around. Achieving 2D EPE in waveguide systems has been challenging due to the difficulty in maintaining the TIR condition during the expansion process. Traditional methods struggle with these limitations, leading to incomplete images and a restricted FOV.
Exit Pupil Size Representation in Waveguide AR Glasses I Source: Digilens
The Solution: Hybrid Waveguide Technology
The hybrid waveguide technology is designed to overcome the limitations posed by TIR and the need for effective 2D Exit Pupil Expansion (EPE). Here's how it addresses these challenges:
Dual Projectors
The hybrid waveguide system employs two projectors to generate separate left and right parts of the output image. This division allows for a significant increase in the FOV. Each projector handles a portion of the visual field, effectively doubling the potential FOV compared to single-projector systems. This method breaks the constraints imposed by TIR, as light from each projector is confined to specific regions within the waveguide, ensuring that TIR conditions are met without limiting the FOV.
To achieve this, the field of view is split into two halves, and the light of each sub-field propagates within its corresponding waveguide. The grating periods of the two waveguides are different, which makes the TIR condition be met for both sub-fields. In the wave vector domain (k-domain) diagram, different grating periods mean different shift distances for the diffraction process. Appropriate grating periods can make the two parts of the curved rectangle fall within the annular region after in-coupling. This ensures that light from each projector remains confined within the waveguide and meets the TIR condition without escaping.
By carefully designing the grating periods, the system can maintain high optical efficiency and clarity while expanding the FOV. This approach allows for a significant increase in the horizontal FOV without compromising the overall optical performance of the AR glasses. The dual projector setup is a critical innovation in overcoming the limitations of traditional waveguide-based systems and achieving a larger, more immersive FOV for augmented reality experiences.
K-domain analysis diagram I Source: Hybrid waveguide based augmented reality display system
Waveguide optical system – Singular projector I Source: Digilens
Adding Extra Grating
By adding extra grating, it is possible to achieve 2D EPE. A design that can provide large FOV as well as 2D EPE has been proposed. In this design, the in-coupling grating can guide the incident light into the waveguide towards both left and right directions. Then the light is redirected by two folding gratings and leaks out from the waveguide at the out-coupling grating. Though the horizontal FOV can be expanded significantly in this design, the vertical FOV is still seriously limited by the TIR condition. If the vertical FOV is too large, the TIR condition will be broken after the redirection process.
General 1D EPE grating waveguide system
Principle of large FOV grating waveguide system with 2D EPE
Half-Mirror Arrays
The hybrid waveguide system overcomes the vertical FOV limitation by employing half-mirror arrays instead of conventional folding gratings. These half-mirrors are strategically placed within the waveguide to reflect light, enabling efficient 2D EPE.
The half-mirror arrays play a crucial role in maintaining the TIR condition while expanding both horizontal and vertical FOVs. They allow light to be redirected multiple times within the waveguide, ensuring that the light stays confined and maintains its brightness and clarity. This method eliminates the vertical FOV limitations posed by traditional gratings.
In the wave vector domain, half-mirror arrays effectively manage the k-vectors of the incident light, flipping them to ensure that the TIR condition is always met. This flipping process allows the light to propagate efficiently within the waveguide, preventing the loss of light and ensuring high image quality.
To ensure that the light of each field can exit the waveguide at a specific area of the out-coupling grating and pass through the exit pupil, some requirements for the size of the half-mirror arrays need to be met. These requirements ensure that the light is correctly directed and maintains the TIR condition throughout its path within the waveguide.
By using half-mirror arrays, the hybrid waveguide system achieves a wide FOV of 88° horizontally and 53° vertically, resulting in a diagonal FOV of 94.7°. This advanced design maintains high light efficiency and minimal distortion, providing an immersive AR experience with superior image quality.
Front View of Hybrid Waveguide Layout with Half-Mirror Arrays I Source: Hybrid waveguide-based augmented reality display system
Collimating System Design
In the hybrid waveguide system, the collimating optical system is designed to optimize the use of light from the projectors. The collimating lens must handle a significant portion of the FOV while maintaining high image quality. To achieve this, the projection system is tilted, reducing the required FOV for each collimating lens.
The tilted projection system ensures that light from each projector is directed into the waveguide at the optimal angle. This configuration allows the system to achieve a horizontal FOV of 88° and a vertical FOV of 53°, resulting in a diagonal FOV of 94.7°. The design also minimizes distortion, ensuring that the displayed image is clear and free of artifacts.
Gratings Design
The hybrid waveguide system uses surface-relief gratings for in-coupling and out-coupling light. These gratings are designed using rigorous coupled wave analysis (RCWA) to optimize light diffraction and ensure high efficiency. The in-coupling gratings introduce light into the waveguide, while the out-coupling gratings ensure that the expanded light exits the waveguide correctly.
The gratings are manufactured on a thin glass substrate using nanoimprint technology, allowing for high accuracy and low-cost production. The in-coupling gratings have a sawtooth structure to achieve high diffraction efficiency, while the out-coupling gratings have a symmetrical structure to ensure consistent performance for light from both directions.
2D Layout of Hybrid Waveguide with In-Coupling and Out-Coupling Gratings and Half Mirrors I Source: Hybrid waveguide-based augmented reality display system
Waveguide Design
The waveguide design is crucial for achieving a wide FOV and maintaining high image quality. The hybrid waveguide system uses a single-layer waveguide with half-mirror arrays to achieve effective 2D EPE. The waveguide is made from HOYA-FD60W glass, which has a high refractive index of 1.817 at 532 nm, ensuring that the TIR condition is met.
The waveguide includes two in-coupling gratings and a single out-coupling grating. The in-coupling gratings are circular with a diameter of 2 mm, matching the pupil of the projection system. The waveguide also includes 27 half-mirrors, which are arranged in a right trapezoid shape to optimize light propagation and minimize energy loss.
The hybrid waveguide system achieves a wide FOV by carefully designing the waveguide and incorporating advanced optical components. This design ensures that the system delivers high image quality and a wide FOV, providing an immersive AR experience for users.
3D View of Whole System I Source: Hybrid waveguide-based augmented reality display system
Conclusion - Hybrid Waveguide-Based Augmented Reality Solution
The hybrid waveguide-based augmented reality display system represents a significant advancement in AR technology. By addressing the limitations of traditional optics and introducing innovative components like dual projectors and half-mirror arrays, this system achieves a wide FOV and high image quality. As AR technology continues to evolve, such hybrid waveguide systems are poised to play a crucial role in developing next-generation AR glasses with enhanced user experiences.
Despite these advancements, transitioning from prototype to consumer-ready products remains challenging. Whether it's Kura’s 150-degree FOV glasses, Ant Reality’s 90-degree AR glasses, or the Hybrid Waveguide AR solutions, these technologies will take time to become widely available in the consumer market. Even industry leaders like Meta are rumored to be facing challenges in achieving FOVs greater than 30 degrees in their next-generation AR glasses.
Metavision’s Approach
Metavision offers wide FOV AR glasses solutions featuring a FOV of 70 degrees or more. However, these solutions require an additional box to supply the necessary power and integrate essential ICs. We warmly welcome you to contact us for more discussion and detailed information about our advanced AR glasses tailored for industrial use.
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