We present the design and implementation of an optical see-through head-mounted display (HMD) with addressable focus cues utilizing a liquid lens. We implemented a monocular bench prototype capable of addressing the focal distance of the display from infinity to as close as 8 diopters. Two operation modes of the system were demonstrated: a vari-focal plane mode in which the accommodation cue is addressable, and a time-multiplexed multi-focal plane mode in which both the accommodation and retinal blur cues can be rendered. We further performed experiments to assess the depth perception and eye accommodative response of the system operated in a vari-focal plane mode. Both subjective and objective measurements suggest that the perceived depths and accommodative responses of the user match with the rendered depths of the virtual display with addressable accommodation cues, approximating the real-world 3-D viewing condition.

Most existing stereoscopic head mounted displays (HMDs), presenting a pair of stereoscopic images at a fixed focal distance, lack the ability to correctly render the naturally coupled accommodation and convergence cues. Psychophysical studies have shown that such displays may cause many adverse consequences such as visual fatigue, diplopic vision, degraded oculomotor response, and depth perception errors. In this paper, we present a see-through HMD with addressable focal planes utilizing a novel active optical element - a liquid lens. The element, with a varying optical power from -5 to 20 diopters, is able to address the focal distance of the HMD from infinity to the near point of the eye. A monocular prototype was built from off-the-shelf elements and experimental results are presented to validate the proposed designs. We also describe both subjective and objective measurements of the accommodation responses of the viewer to the focal distances presented by the prototype.