OKU Laboratory

1000-volume/s high-speed volumetric display

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In response to increasing interest in virtual reality (VR) and augmented reality (AR) in recent years, various head-mounted displays (HMD) have been developed and have attracted attention. However, HMDs in widespread use today suffer from (1) vergence-accommodation conflict (VAC) and (2) insufficient display frame rate. The former is a contradiction between the eye’s focus adjustment and convergence. It said to be the cause of fatigue and symptoms of a sickness called virtual reality sickness. On the other hand, considering the superimpositions on the dynamic object in the real world through a see-through HMD, the current HMDs cause a discrepancy between the object and the annotation due to the slow refresh rate of the display. This problem causes discomfort to the viewer. Therefore, we proposed and developed a display principle to achieve a refresh rate of 1000Hz without VAC.

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The proposed display is based on a volumetric display, in which the depth of the display surface is changed at high speed and a different image is presented at each depth to present three-dimensional information, and it is possible to achieve this goal if the speed is increased. To achieve the high-speed volumetric display, the depth of the display surface needs to be changed rapidly, and this is achieved with a tunable acoustic gradient index (TAG) lens, which is a resonant liquid lens with a focal length that oscillates rapidly at about 70 kHz. And also, the images displayed at each depths need to be changed at high speed. This is achieved by using Digital Micromirror Device(DMD), which controls the reflection of light by changing the angle of micro mirror and enables fast image display. By combining these devices, the above problems are solved simultaneously.

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This display uses a focal length selection technique in which LED is synchronized with a TAG lens that oscillates continuously at 14.5us per cycle and emits 500ns of light at specific times during one cycle. Since the timing of the LED emission can be electrically controlled almost instantaneously, the focal length of the TAG lens can be selected within 14.5us, which is the oscillation cycle. In addition, by incorporating and synchronizing a DMD capable of image updates at up to 6900 fps, we built a prototype system that allows to present one monochrome image every 159.5us to a specific depth. Using this system, we successfully presented a volume consisting of a set of six monochrome images, each presented at a different depth, with an update rate of 1ms (≈159.5×6).