# AR Displays: Next-Generation Technologies to Solve the Vergence-Accommodation Conflict
## 5 Multifocal Approach by LightSpace Technologies  
The proposition for the next-generation stereoscopic 3D wearable AR display from LightSpace Technologies is a multifocal display based on static volumetric display architecture. The key element of this display architecture is a liquid-crystal-based optical diffuser element, which has two distinct optical states--highly transparent ultra-low-haze optical state and highly light-scattering state, Figure 1[31; 32]. The distinguishing characteristic is the ability to drive the optical diffuser element between these two optical states in a controlled manner with a response time of \(\leq\)500 us [31], which along with ultra-low haze in the transparent state is the determining factor allowing formation of multi-diffuser element stacks to serve as a discretized projection volume. The principles of image output within the projection volume are shown in Figure 2. An ultrafast-refresh rate image projection microunit is synchronized with the driver electronics controlling optical state of each optical diffuser element forming the projection volume or stack. The 3D rendering pipeline executed on a separate dedicated processing unit renders the scene compliant with the depth-plane format. Transmitted graphical data in a manner of \"slice-by-slice\" is then projected towards projection volume, which reconfigures its optical state each time the respective image depth plane (image \"slice\") is projected. The default configuration of the projection volume has all but one optical diffuser element in the transparent state, in which they virtually do not interact with the incident light, whereas the single optical diffuser element in the scattering state receives the projected light and scatters it thus defining the depth, from which an image is originating. Consequently, by cycling through all diffuser elements at a sufficiently high rate, a flickerless 3D scene can be perceived. In a way the stack of optical diffuser elements acts as a spatial image depth-demultiplexer, allowing to convert a stream of 2D images into a volumetric image. Considering the small thickness of active layers (6-12 um), the image originating depth is well defined. For the utilization in near-to-eye or wearable displays, this miniature volumetric display has to be viewed through a magnifying eyepiece, which then expands the positions of the virtual focal planes in depth. The physical spacing of active layers within the stack as well as the optical strength of the eyepiece dictates the positions of actual focal planes (Figure 3). Figure 1: Example showing two optical states of liquid crystal based optical diffuser element: (**a**) Highly transparent ultra-low-haze optical state, (**b**) highly light scattering optical state. Figure 2: Block schematic depicting principle of volumetric 3D display based on a multi-plane optical diffuser technology. The core technology of the proposed multifocal near-to-eye stereoscopic display. It must be noted, that depth discretization into depth planes does not necessarily mean that image discontinuities will be noticeable. In this regard, multifocal displays have been widely studied--including from the point of view of optometry and human perception [15; 17; 18]. The research suggests, that depth blending or interplane antialiasing is fully applicable to hide discontinuities between image depth planes, as well as drive accommodation in between these focus planes, essentially ensuring continuous range for accommodation at a slight expense of reduced image sharpness [17; 18]. Quite obviously, the more optically closer the focal planes are located in the diopter space, the better the expected image quality. Nonetheless, as with all next-generation 3D display technologies, currently it is a game of compromises and, for practical implementation, an equilibrium between technological capabilities and perceived image quality have to be met. On one hand it is desirable to cover accommodative range from close distance of 30 or 25 cm to infinity (4 diopter range).