# Review of Microsoft HoloLens Applications over the Past Five Years
## 1 Introduction  
Virtual reality (VR), augmented reality (AR) and mixed reality (MR) have been emerging technologies for several years, but they are now able to generate realistic images, sounds and other sensations that allow users to experience a spectacular imaginary world [1]. It can be difficult to understand the difference between VR, AR and MR, at a glance. Therefore, the definitions of these and other, similar terms are required. VR is the most prevalent of these technologies. It allows people to feel completely immersed in an alternate reality. Using a head-mounted display (HMD) or headset, a user can manipulate objects while experiencing computer-generated visual effects and sounds. Alternatively, AR overlays digital information on real elements. Based in the real world, it creates a new layer of perception and complements reality or the environment. MR utilizes reality and digital elements. Next-generation sensors and imaging technologies can be used to interact and manipulate both physical and virtual items and environments. MR provides an environment in which users can see their surroundings while interacting with the virtual environment without taking off their headset.  
VR, AR and MR technologies are currently gaining importance in several fields (including healthcare, architecture and civil engineering, manufacturing, defense, tourism, automation and education) and changing the way we work in these fields. In the field of AR, it is critical to visualize both real and virtual content simultaneously. Significant progress has been made in this area and many AR devices, such as Google Glass, Vuzik Blade and Epson Moverio, have been developed [2]. HoloLens--a state-of-the-art AR device developed by Microsoft in 2016--is an MR-based HMD, i.e., a wearable device that allows users to interact with the environment using holograms while stimulating the senses as a whole [3]. HoloLens is a pair of perspective holographic glasses that includes a central processing unit, graphics processing unit and holographic processing unit, which can handle real-time spatial mapping and processing [4]. Further, its functions are more advanced than those of conventional AR equipment, such as stereoscopic three-dimensional (3D) display, gaze design, gesture design, spatial sound design and spatial mapping [5, 6].  
Based on these functions, various studies have been conducted to determine methods for using HoloLens more efficiently. For instance, a study on effectively controlling the HoloLens, using the gaze, gesture and voice control functions provided by the device, and studies on methods for utilizing this device in various industrial fields have been conducted [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Research on methods of visualizing the scanned image or data of an actual object as 3D contents [18; 19; 20] and correcting them [21; 22] have also been conducted. Moreover, some studies have evaluated the performance and utility of the HoloLens, while others have developed additional toolkits or hardware to improve the performance of the HoloLens [23; 24; 25; 26]. [PERSON] et al. [27] conducted a comprehensive study on the development stage and analysis model, simulation toolkit, system type and architecture type for MR and presented its practical issues for stakeholders considering other domains of MR. Nevertheless, there is a lack of analysis and review on how HoloLens is being used in various industries.  
Therefore, the purpose of this study was to review academic papers on the applications of HoloLens in medical and surgical aids and systems, medical education and simulation, industrial engineering, architecture, civil engineering and other engineering fields. For this purpose, 44 academic research articles published over the past five years (2016-2020) were analyzed. The current status and trends in research based on HoloLens were investigated by year and field and the type of visualization technology and functions provided to users were discussed.