Retinal Scanning Display (RSD) is a near-to-eye scanned beam display technology. An exit pupil expander (EPE) or numerical aperture (NA) expander is used in RSDs to create a large display exit pupil. A novel EPE approach that uses two microlens arrays (MLA) is presented in this paper. The approach is based on cascading two identical microlens arrays spaced precisely at one focal length distance with sub-micron registration tolerances relative to each other. We demonstrated a dual MLA based EPE that produced excellent exit pupil uniformity and better than 90% diffraction efficiency for all three wavelengths in a color display system. Registration was performed with sub-micron precision using farfield alignment techniques. Both numerical and experimental results are presented, and three fabrication technologies: grayscale lithography, photoresist reflow, and isotropic etching, are compared.
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).
The numerical aperture of the light emanating from display pixels in a given display system determines 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. Accounting for positional freedom of the eye and relative display placement defines the required expansion of the limited input NA for producing the desired exit pupil size for the display system. Although there are many approaches to performing this task of NA conversion, or exit pupil expansion, many of them exhibit wavelength dependencies, causing the envelope of the intensity profile in the exit pupil plane to be non- overlapping or inconsistent versus pixel location, resulting in uniformity or color variations and often efficiency loss. This paper will look at a microlens array approach to exit pupil expansion for color display systems that will be shown as wavelength independent in terms of diffraction envelope.
High-resolution (i.e., large pixel-count) and high frame rate dynamic microdisplays can be implemented by scanning a photon beam in a raster format across the viewer's retina. A resonant horizontal scanner and a linearly driven vertical scanner can create a 2-D raster for video display. The combined motion of the two scanners form a sinusoidal raster in the vertical direction and cause non-uniform line spacing for the case of bidirectional scanning as if the forward and return half-period raster lines are pinched near the edge of the display screen. Raster pinch effect degrades the image quality, especially for multi-beam scanning systems. What is needed is a vertical scanner that creates a stairstep motion instead of linear motion. A third scanner can be added to the system to create an approximation to a staircase motion in the vertical axis and correct for the non-uniform raster spacing. The raster pinch scanner requirements, mechanical and magnetic designs with FEA analysis, and preliminary test results are discussed in this paper.
A Retinal Scanning Display (RSD) utilizes scanning mirrors and optics to produce a flying spot that forms a raster image directly on the retina of the eye. A high-frequency resonant horizontal scanner and a linear ramp vertical scanner function together to produce video typically at a 60Hz frame rate. Although the raster can be formed by Unidirectional Writing (using only the forward half-period of the horizontal scan function) and one flying spot, it is desirable to achieve Bidirectional Writing (utilizing the full period of the Horizontal scan function) with multiple scanned spots for the purpose of increased efficiency of the display with a limited horizontal scanner frequency. This paper will look at the limitations and requirements for the scanning functions to make this possible.