## 1 Introduction  
Augmented reality (AR) is a relatively new technology that allow superimposing virtual objects (computer-generated graphics) over the real world. The term AR was born in the 1990s from [PERSON] and [PERSON] [1], at which time AR was classified for the first time into the virtuality continuum \"space\" that includes other hierarchical levels such as the environments, augmented virtuality and virtual environments. In this classification, AR is closely-related to the real environment that is augmented--not replaced--with computer-generated objects. This is in contrast with augmented virtuality that expands virtual environments with real elements. In general, when real and virtual objects are simultaneously accessible in a display, we refer to this as mixed reality [2]. Likewise, the concept of AR may be applied to all those technologies, e.g., PCs, laptops, tablets, smartphones and wearables that combine and register (align) 3D physical and virtual objects in tangible environments,and in real-time [3]. The importance and strength of an AR system lies in its ability to provide information to users that otherwise would not be available to the handler's senses and help them in solving tasks at the same time [4]. Moreover, a branch of AR supported from portable devices is named mobile augmented reality (MAR). A MAR system has the same characteristics of an AR system and otherwise display the augmented contents on a mobile device. As a result of their nature, the most important MAR devices are smartphones, tablets and smartphones. Currently the MAR world is predominantly associated with hand-held devices such as smartphones and tablets, due to their high portability, reduced dimensions (in particular mobile phones) and social acceptance. However, in the near future, they are expected to be replaced by smartglasses (SGs), since they enable users to visualize computer-generated contents without the use of hands and on a front display [5; 6]. Some reports suggest there will be an exponential growth of the AR and Virtual Reality (VR) market and of the number of SG distributed by 2025 [7; 8]. The first SG protype was developed in 1997 for urban exploration and consisted of several devices (wearable see-through display, handheld computer, GPS receiver, desktop computer in the back, etc.) brought to the user [9]. It was a cumbersome system in contrast with the actual SG available devices. Currently, smartglasses are essentially head-wearable miniature mobile computer that provide a see-through display, at eye level, where the augmented content (e.g., image, text, video) is projected without occluding the real-word view. Several companies are developing SG devices for AR with large differences in design, incorporating technology and existing functions. In particular, we can differentiate SGs by optical/video see-through display, available sensors (accelerometer, gyroscope, magnetometer, GPS, etc. ), tangible interface (external controller, trackpad and button), operating system (Android, Windows, Linux), processors, memory, battery life, weight, field of view and price [10; 11]. These overall features are important factors that can affect the SG-wearing experience, leading to different acceptance levels by the users. Another important element that characterizes an AR system is the tracking and registration process that allow to superimpose the virtual object on the physical ones with several methods. This process can be sensor-based (inertial, magnetic, electromagnetic and ultrasonic method) and video-based (marker-based, feature-based method). Another one can be the hybrid method, which blends the previous systems to reduce their limitations [5].