# AR Displays: Next-Generation Technologies to Solve the Vergence-Accommodation Conflict
## 3 Stereoscopic Head-Mounted AR Displays  
For professional tasks, a key component is true 3D spatial awareness, thus the augmenting digital content has to be aligned to the real 3D world precisely. Consequently, for this, both eyes of the observer have to be presented with an image. The most pressing issue with current AR displays is that utilization of basic stereoscopy has severe limitations. In this case, the sensation of 3D depth relies on binocular disparities as the strongest source of 3D depth [2]. Nevertheless, the image is at a fixed focal distance which requires fixed accommodation on the given focal plane, while the vergence of human visual system has a freedom for variation. As a consequence, naturally coupled accommodation and vergence of eyes becomes forcefully decoupled causing the vergence-accommodation conflict (VAC), possible effects of which are well known from VR headset experiences and can include eyestrain, fatigue, nausea, blurred vision, and similar, which is inhibitive, especially considering professional work for possibly long hours. Typically, people can tolerate a slight mismatch between vergence and accommodation, without any onset of previously mentioned adverse effects. This limit can vary from person to person and is dependent on the accommodation distance but, generally, it can be assumed that a vergence accommodation mismatch of 0.3 to 0.4 D should not bother display users [3]. Consequently, the head-mounted display manufacturers tend to place the focal plane at around 2 to 3 m apparent distance. For example, considering the comfort limit of \(\pm\)0.4 D, by placing the focal plane at 2.5 m distance, a viewer with normal vision should be able to observe a virtual space corresponding to a depth of 1.25 m to infinity without any noticeable adverse effects. Nevertheless, as the vergence is driven towards closer distances, the mismatch increases rapidly to an extent that the accommodation lock becomes lost causing a blurred image. This is one of the critical reasons, why the existing basic stereoscopic approach is ill-suited for close professional work. On one hand it can be argued that this could be easily overcome by placing the focal plane at a much closer distance--corresponding to the desired arm's reach (around 30 to 50 cm distance). Nevertheless, at such near distances, the tolerable comfort region ensures that only a narrow depth range can be viewed without experiencing onset of adverse effects. For example, if the focal plane is positioned at 40 cm, the effective comfort region is only 13 cm in depth, whereas at 30 cm focal distance this region decreases to only 7 cm depth. Quite obviously this is not convenient and cannot be offered as a viable solution for near-work-oriented AR display systems. Furthermore, when the digital 3D content is to be positioned in respect to the real world, using only a single focal plane does not allow matching it precisely. The real world has a virtually infinite number of depth planes, allowing the viewer to arbitrarily and continuously change focus, whereas a single focal plane of the stereoscopic display forces locking accommodation on this focal distance. If the digital content is placed in a space substantially away from the position of an image focal plane, the viewer cannot get a simultaneously sharp focus lock on the real world at said \"object's\" depth and the digital content, which can cause confusion and misalignment. Recently in a study from Pisa university it was shown that, due to this misalignment, conventional stereoscopic AR headsets cannot be reliably applied for high-precision work [4]. It was found, that people using the AR technology without being aware made larger mistakes than when performing bare-eyed. Currently it has become entirely accepted that a simple stereoscopic approach by exploiting only binocular disparities cannot yield a reliable and generally accepted display solution.