The numerical aperture of the light emanating from display pixels in a given display system determines the exit pupil size. In retinal scanning displays, the exit pupil is defined by the scanner optics, creating a rastered, projected image at an intermediate plane, typically resulting in an exit pupil approximately the size of an eye's pupil. Positional freedom of the eye and relative display placement define the required expansion of the limited input NA for producing the desired exit pupil size for the display system. Currently Microvision utilizes an optical element comprised of two Microlens Arrays (MLAs) in tandem to expand the NA. The dual-MLA system has demonstrated exit pupil size that is independent of color; and uniformity of the beamlet structure is quite Top-Hat like. To further improve the perceived image quality, Microvision has now refined the optical system to minimize interference effects in the Exit Pupil plane that were caused by the coherent nature of the light source. We describe here a single refractive double-sided aspheric element that diminishes this interference effect by converting an input Gaussian beam profile to a Top-Hat profile. We also discuss the theory behind the use of a Gaussian-to-Top-Hat Converter, the tradeoffs associated with its use, as well as experimental results showing the uniformity improvements when using a Top-Hat converter element in conjunction with the MLA-based Exit Pupil Expander. In addition, we report the progress of environmental testing of the Exit Pupil Expander (EPE).
A high-frequency resonant horizontal scanner and a linearly driven vertical scanner at display frame rates can create a 2-D raster for video display. The combined motion of the two scanners forms a sinusoidal raster in the vertical direction where the raster line spacing is uniform only at the center and becomes progressively nonuniform towards the left and right edges of the display screen. Nonuniformities degrade the image quality and can be corrected by the addition of a third scanner to the system. Last year we reported the requirements and some of the early results in our MEMS-based raster correction scanner development effort. Since then, a lot of progress was made and the scanner was successfully incorporated into an SXGA resolution helmet-mounted display system. In this paper we report the results of thick copper coil development, new coil and magnet design for electromagnetic actuator, thermal flatness testing, new mounting design, and finally the performance measurements for the HMD system with a raster correction scanner.
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