A Novel Birdbath Eyepiece for Light Field AR Glasses

Natural and comfortable visual experience is critical to the success of metaverse, which has recently drawn worldwide attention. However, due to the vergence-accommodation conflict (VAC), most augmented reality (AR) or virtual reality (VR) displays on the market today can easily cause visual discomfort or eyestrain to users. Being able to resolve the VAC, light field display is commonly believed to be the ultimate display for metaverse. Similar to conventional near-eye AR displays, a near-eye light field AR display consists of three basic components: light source, projection unit, and eyepiece. Although the same light source can be used in both kinds of displays, the projection unit and the eyepiece of a near-eye light field AR display call for a new design to preserve the structure of light field when it reaches user's retina. The primary focus of this paper is on the eyepiece design for a near-eye light field AR display. In consideration of the compact form factor and wide field of view, the birdbath architecture, which consists of a beam splitter and a combiner, is selected as the basis of our eyepiece design. We optimize the birdbath eyepiece for the light field projection module produced by PetaRay Inc. The birdbath eyepiece receives the light field emitted from the light field projection module and projects it fully into the user's eye. Our design preserves the structure of light field and hence allows virtual objects at different depths to be properly perceived. In addition, the eyepiece design leads to a compact form factor for the near-eye light field AR display. Specifically, our eyepiece is designed by optimizing the tradeoff between the eyebox and the depth of focus (DOF) of the near-eye light field AR display. The resulting DOF allows the user to have a clear and sharp perception of any virtual object in the working range, which is from 30 cm to infinity. In addition, we optimize the entrance pupil position and the F-number of the eyepiece according to the exit pupil position and the divergence angle of the light field projection module. This way, the eyepiece is able to preserve the structure of light field, meaning that the angular relation between light rays coming from the same object point in space is preserved. To demonstrate the performance of our birdbath eyepiece, we use the Human Eye Model-Liou & Brennan (JOSA A 08/97) to simulate the image formation process.