2. An overview of VR/AR technologies and peripherals Virtual reality (VR) simulation is to generate immersive environments from which users can experience unique insights into the way the real world works [15,16]. The concept of VR was brought up over fifty years ago when the first immersive human-computer interaction (HCI) mock-up named “Man-Machine Graphical Communication System” was invented [17]. The formal term of VR was put up in 1989 [18]. Since then, several taxonomies have been raised by scholars to expound where a rigorous VR concept should stay from along the continuum of reality to virtuality (RV). For example, Milgram's taxonomy (Milgram and Colquhoun, 1994) shown in Fig. 1 defines four levels of RV experience based upon the degree of blending that different electronic display systems can achieve; Benford's taxonomy [19] shown in Fig. 2 classifies four spaces according to the extent to which a group of users can access virtual objects from their local space and the extent to which a space is either synthetic or is based on the physical world. VR attempts to replace a user's perception of the surrounding world with a computer-generated artificial 3D environment. And such virtual 3D environment is not necessary to be established based on a real one. In the RV continuum based on Milgram's taxonomy, VR represents effort in creating a virtual environment (VE) with visual and immersive aids to let users feeling a “real” sensation. However, it can only provide a limited level of ‘realism’ due to a lack of sensory feedback to accommodate for perceptual and cognitive viewpoints [20]. As an emerging technology, AR integrates images of virtual objects into a real world. By inserting the virtually simulated prototypes into the real world and creating an augmented scene, AR technology could satisfy the goal of enhancing a person's perception of a virtual prototyping with real entities. This gives a virtual world an ameliorated connection to the real world while maintaining the flexibility of the virtual world [21]. In sum, the rigorous classification defined for VR and AR in this research is based on whether or not the visual sensations from the real world get to involve regardless of the establishment of immersion or the mechanisms of the display. While there is a blending of reality and virtuality, it can always be referred to Mixed Reality (MR) as a collective term. Such examples include a VR generated virtual environment fully superimposed on its related physical world, or the wide range of AR applications. In particular, AR, within the continuum of MR, generates an RV blended environment where the most of the visual sensation comes from the real world, and the virtual elements contributed less. The percentages of reality and virtuality can be reverted for augmented virtuality (AV) which is not discussed in this paper. Ever since the first mature wearable VR/AR device (i.e., Google Glass, Forte VFX1) on the market, the reality of mobile VR/AR devices seems to be inevitable and have the potential to enrich the way information is accessed and presented. Technology developers (including hardware and software) from around the world have been trying to work with big brand marketers to build more tangible and auditory VR/ AR solutions to deliver the best solutions matching clients' requirements and objectives (Fig. 3). Instead of just being able to interact with 3D contents in a pure computer-generated environment, users nowadays are capable of realizing a highly immersive, holistic and realistic experience underpinned by synthesized digital and physical world information presented using more sophisticated software and hardware. As shown in Fig.4, the paramount for the sensation of immersion into VR/AR are a high frame rate (at least 95 fps) and low latency. Furthermore, a pixel persistence lower than 3 ms could prevent users feel sick when moving their head around. Nowadays, the gap between the real world and its digital counterparts is becoming narrower. The tremendous potential that VR/AR could lead to a number of important changes in human life and activity has been witnessed from a wide range of application areas such as education and training [22], engineering [23], architectural and urban design [24], heritage and archaeology [25], medical science [26], entertainment [27] and so forth. In order to understand the frontier of VR/AR technologies from a broad range of application areas and uncover the possibilities of using VR/AR in construction safety, this paper provides a thorough review of mainstream literature published between 2000 and 2017. The selected arFig. 1. Reality-virtuality (RV) Continuum [87]. ticles were classified according to a four-level taxonomy: (1) technology Augmented Reality Virtual Reality Physical Reality Telepresence Synthetic (Generated fro m computer data) Physical (Generated from the real world) Local (Remain in the physical world) Remote (Leave your body behind) Fig. 2. Classification of Shared Spaces According to Transportation and Artificiality (adapted from [19]). X. Li et al. Automation in Construction 86 (2018) 150–162 151 characteristics; (2) application domains; (3) safety enhancement mechanisms; and (4) safety assessment and evaluation. The rigorous classification of the literature could provide a critical framework for analysis and summarization of the state-of-the-art work in AR/VR-CS research. This study sets forth a list of gaps derived from the in-depth review and comes up with the prospective research works. These works look to assist both researchers and industrial practitioners with appreciating the research and practice frontier of VR/AR-CS and soliciting the latest VR/AR applications

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