Birdbath: Currently The Most Popular Optical Solution

Augmented Reality (AR) systems use various optical solutions to overlay digital information on the real world. The industry adopts prism scheme, off-axis optical scheme, free surface scheme, birdbath scheme and optical waveguide scheme. The early prism scheme and off-axis optical scheme no longer have advantages in volume or field of view, and currently birdbath is one of the most popular optical schemes.

The birdbath optical scheme is a type of optical waveguide used in some augmented reality (AR) systems. It is named after the shape of the curved reflective surface, which resembles a birdbath.

In the birdbath optical scheme, a micro-display, such as an LCD or OLED screen, is used to generate the virtual image. The light emitted from the micro-display enters the curved reflective surface, which is typically shaped like a hemisphere or a section of a sphere.

The curved surface reflects the light multiple times, causing it to bounce around within the waveguide. This causes the light to travel along a predetermined path until it exits the surface and reaches the user's eye. The multiple reflections create a virtual image that appears to be floating in front of the user.

The birdbath optical scheme offers several advantages for augmented reality (AR) systems, including:

  1. Wide field of view: The curved reflective surface used in the birdbath optical scheme enables a wide field of view for AR systems. This means that the user can see virtual objects overlaid on a large portion of their real-world surroundings, which can enhance the immersion and usability of the system.
  2. Compact design: The birdbath optical scheme allows for a compact and lightweight AR system. The curved reflective surface is relatively small compared to the field of view it provides, which can make the system more portable and wearable.
  3. High brightness: The birdbath optical scheme can provide high brightness levels, which is important for AR systems that are used in varying lighting conditions. The multiple reflections of the light within the waveguide can help to increase the brightness of the virtual image.
  4. Simple design: The birdbath optical scheme has a relatively simple design compared to some other optical waveguide technologies. This can make it easier and more cost-effective to manufacture and integrate into AR devices.

However, there are also some limitations to the birdbath optical scheme, such as chromatic aberration and lower optical efficiency, which can impact image quality and overall system performance. These limitations must be carefully considered and addressed during the design and development of AR systems using this technology. There are several disadvantages associated with the birdbath optical scheme, including:

  1. Chromatic aberration: Chromatic aberration is a common issue with the birdbath optical scheme, which can cause color fringing and distortion around the edges of the virtual image. This can be addressed through careful design of the reflective surface and the use of specialized coatings and filters, but it can add complexity and cost to the system.
  2. Limited eye relief: Eye relief refers to the distance between the user's eye and the AR display. In the birdbath optical scheme, the eye relief can be limited due to the curved reflective surface. This can make it difficult for users with glasses or other visual impairments to use the system comfortably.
  3. Lower optical efficiency: The multiple reflections of the light within the waveguide in the birdbath optical scheme can lead to a lower optical efficiency, meaning that a significant portion of the light emitted from the micro-display is lost during the reflection process. This can lead to reduced brightness and image quality, which can be addressed through the use of brighter displays and improved reflective coatings.
  4. Limited field of view uniformity: The curved reflective surface in the birdbath optical scheme can create non-uniformities in the field of view, meaning that different parts of the virtual image may appear differently to the user. This can impact the overall image quality and user experience.

The optical technology in the AR field is advancing, and a series of optical schemes have emerged. Among them, birdbath has the advantages of low cost, moderate field of view, high light efficiency, low requirements for micro displays, and scalable production. Although it has its inevitable limitations and may not be the ultimate optical solution, it is still one of the best solutions currently available.

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