Near‐to‐eye display with diffractive exit pupil expander having chevron design

Abstract

Abstract— Near‐to‐eye displays (NEDs) provide a unique way to perceive a larger image than the device itself. The user acceptance of commercially available NEDs has not been high partly because of reported physiological symptoms. Devices also tend to be thick and heavy, and therefore uncomfortable to wear. To overcome these shortcomings and to make a very usable device, a new approach was needed. By using very thin plastic light guides with diffractive structures on the surfaces, many of the known obstacles can be notably reduced. These exit pupil expanders (EPEs) enable a light and thin design for see‐through NEDs. The so‐called chevron EPE was designed to further improve the design and usability aspects of NEDs. The diffractive EPE has typically one incoupling grating area that delivers light into the light guide symmetrically towards the left and right eyes. By using slanted or overhanging gratings, the incoupling is very asymmetric. If the incoupling area is divided into two parts, each having opposite slanting angles, the EPE plate can be cut in half, and the left and right parts can be separated. The plates can be further tilted to a chevron shape following more closely the human face and mimicking the conventional eyeglass design. The reflection of the light from the tilted plate is directed out from the image‐forming optics, and therefore the contrast is improved. The privacy of the usage is improved because at the output the secondary beams are harder to observe. Because the device is brought closer to face, it enables better weight balance. The chevron geometry partially prevents the ambient light from the sides to enter the diffractive structures, and thus the unwanted reflections are also reduced. Furthermore, the half‐size of the chevron EPE parts from the undivided EPE results in a better production yield.