Characterization of Near-eye Displays based on Wavefront Measurements

Abstract

Near-eye displays (NEDs) aim to deliver an immersive user experience. For virtual reality (VR) NEDs, this requires the display of virtual images with high optical resolutions within wide fields of view. Moreover, ideally, the NEDs should also deliver natural depth perception, where the stereo (binocular disparity and vergence) and focus (retinal defocus blur and accommodation) cues are accurately created. In traditional stereo-displays, however, there is a mismatch between accommodation and vergence due to fixed (single) display focal depth. This results in visual discomfort in prolonged use. Various advanced display methods have been proposed to address this so-called vergence-accommodation conflict (VAC) problem, which can be broadly categorized as accommodation-enabling and accommodation-invariant displays. The challenge in such methods is that typically one needs to sacrifice optical resolution to deliver more accurate depth cues and vice versa. The ability to quantitatively characterize these quality factors is therefore important as it enables understanding the limitations of existing displays and further improving them based on the obtained quantitative measures.

In this thesis, a wavefront sensing based NED characterization framework is presented using a Shack-Hartmann wavefront sensor. The optical response of a NED is characterized by measuring emitted wavefronts from display light sources at different positions and wavelengths. This approach makes the gathered data realistic, and thus can be used for replacing simulated data. Wavefronts are measured from a VR display in three different colors and at multiple angles. The wavefronts are passed to a human eye model to simulate how they are seen by a human viewer. This simulation includes modulation by the eye lens, propagation inside the eye, and image formation on the retina. The measured wavefronts at the eye pupil plane provide information about the optical aberrations originating from the display, while the retinal image formation simulations quantify the optical performance of the display system. In addition to optical resolution, the other quality factor modeled through simulations is the accommodation response. The model is based on the Visual Strehl ratio based on the Optical Transfer Function (VSOTF), taking into account neural factors in the human visual system.

The wavefront sensing experiments and subsequent simulations for optical resolution and accommodation prediction are conducted for a commercial VR display module. The results reveal useful information on the visual angle- and wavelength-dependent aberrations and optical resolution as well as angle-dependent expected accommodation response, which are otherwise not directly available or accessible as part of the provided display specifications.