LCD-based coherent wearable projection display for quasi-accommodation-free imaging

This paper presents a novel coherent head mounted display (HMD) which provides nearly accommodation-free images. It is based on a liquid crystal display (LCD) illuminated by coherent light from a laser diode which projects the LCD-image directly onto the eye's retina. The paper describes the theoretical considerations involved in optimizing the above display for the best possible depth of focus. The theoretical analysis is verified through modulation transfer function (MTF) and readability measurements using an optical bench model of the proposed system. It is shown that the MTF-values are larger than 0.5 for eye power changes up to 5 diopters (D) and for spatial frequencies up to 32 cyc/ mm. This value corresponds to the maximum resolution of today's state of the art microdisplays used in HMD systems. In addition, text readability measurements indicated that text targets, which correspond to 15-pt-sized aria] font text viewed from a distance of 50 cm, can be read for eye accommodation ranging from 0 up to 5 D. As the normal adult human eye has a range of accommodation of about 5 D, the proposed coherent display can be considered quasi-accommodation-free. (C) 2003 Elsevier Science B.V. All rights reserved.