Digitally switchable multi-focal element for wearable displays

An innovative concept is proposed for an optical element which offers the capability of rapidly switching the optical power of the system among multiple foci. The switchable multifocal element consists of a custom-designed freeform lens offering multiple discrete foci and a programmable high-speed liquid crystal shutter (LCS). The freeform lens is divided into patterned zones, through which multiple distinct foci are produced. The LCS consists of patterned zones corresponding to those zones of the freeform lens, which can be programmably switched on and off. By combining the multi-focal freeform lens and the LCS in a time-multiplexed fashion, a switchable multifocal element with high-speed, large aperture and large range of tunable power was achieved. The multifocal element also meets the other requirement of an ideal tunable optical element such as low-voltage control, robustness, and compactness. A proof-of-concept twofocal head mounted display was designed to demonstrate one application of the new switchable multifocal element. The design can provide a FOV of 40 degrees and angular resolution of 1 arc minutes in visual space in an 8mm by 8mm exit pupil.