# AR Displays: Next-Generation Technologies to Solve the Vergence-Accommodation Conflict
## 4 Overview of Proposed Next-Generation 3D Display Solutions
### Methods of Image Combination  
The mentioned 3D display methods (Table 1) in principal can be applied as in virtual reality, as well as in augmented reality display devices. The key part which differentiates between both is the means of image combination. AR devices have to superimpose a virtual content onto the real world, while VR devices do not. In the most basic form, the optical image combiner is a semi-transparent mirror, while recently holographic waveguides as a means of obtaining flat-looking optical design have shown promise. First holographic image waveguides that reached head-mounted AR displays had a narrow viewing angle, were rather inefficient, and prone to optical image artifacts. Nevertheless, the technology is maturing at a fast pace, while no breakthroughs can be expected in respect to conventional beam splitter designs. One of the major problems with optical image combiners is in their efficiency, as inevitably in both cases much of the light is lost (due to diffraction efficiency or unwanted reflection/transmission), and if the light source is not particularly bright, the resulting image can be of insufficient brightness to be viewed, for example, outdoors, requiring the light from the ambient scene to be attenuated. Alternatively, especially when considering a holographic type of display, so-called holographic mirror image combiners have been successfully used [6; 7]. Essentially these are volume gratings incorporating possibly different optical functions and selectively reflecting narrow bandwidth light, thus being highly transparent to the ambient light.  
As a different alternative, digital image combination is also viewed as a viable and in some ways a superior method to optical image combination. Essentially, by supplying a virtual reality headset with cameras, the view of the ambient scene can be recorded and digitized with subsequent digital fusion with the computer-generated content. First of all, this makes a system simpler from the optics point of view; secondly, the brightness of the image source can vary greatly as the attenuation of ambient brightness can be accomplished digitally; and thirdly, digital fusion solves the opacity/transparency problem of virtual content. Nevertheless, generally, it is expected that the acceptance of inability to directly view the ambient scene will be treated as a safety risk, especially in case of using technology for critical tasks. Furthermore, by utilization of basic cameras, no depth information can be reliably recorded, thus the user does not receive monocular depth cues (if the technology supports them). Though, this can be overcome by supplying the visual cameras with a depth sensing camera and using the depth data to provide the user with monocular focus cues also for the ambient surroundings. With rapid progress in neural networks, methods of depth extraction from stereoscopic image pairs could also be a possible way of providing monocular depth cues to the recorded ambient scene [30]. It must be said, that not all 3D display technologies can be successfully paired with holographic image waveguides, thus conventional beam-splitter optics and holographic mirror technology is expected to prevail.