We know that the core of AR glasses is the optical module and the optical module consists of micro-display, optical components and fixed components. The optical components can be divided into prism, free surface, Birdbath and waveguide in terms of technical implementation.It is time to talk about Birdbath technology.

       The birdbath combines two main optical components, a spherical mirror/combiner (part-mirror) and a beam splitter. The name  "birdbath" comes from the spherical mirror/combiner looking like a typical birdbath.

      Birdbath projects light from the OLED into the Beamsplitter, which maintains a 45-degree Angle with the OLED light source plane. The Beamsplitter has reflective (Br) and transmissive (Bt) properties that allow light to be partially reflected as a percentage of Br, the light returned by the Beamsplitter is redirected to the eye by the concave mirror

Advantage

• It is used because it generally is comparatively inexpensive to down right cheap while also being relatively small/compact while having  good overall image quality. The design fundamentally supports a very wide FOV.

Weakness

• There are also issues with getting a double image off of each face of plate beam splitter and other reflections. Depending on the quality of each face, a percentage of light is going to reflect or pass through that you don’t want. This light will be slightly displaced based on the thickness of the beamsplitter. And because the light makes two passes, there are two opportunities to cause double images.

• A big downside to the birdbath design is the loss of light. The image light must make two passes at the beam splitter, a reflective and transmissive, with a reflective (Br) and transmissive (Bt) percentages of light. The light making it through both passes is Lr x Lt. When you are blocking about 95% of the real world light, are you really much of a “see-through” display?

• It has a basic privacy issue because other people can see the display from the outside, from all directions.

      However DisplayModule's birdbath optics module has perfect solution for all weakness.  It has not double images with 26% light transmittance and can eliminate 90% of user information leakage.