### Augmented Reality (AR)  
From a technical point of view, AR is a technology superimposing digital, computer-generated information onto the physical world to enrich humans' perspectives about the surrounding environment. This innovates regarding the interaction of humans with digital information and the real world. There are different types of augmented information, which are visual augmentation [8], audio [10], haptic feedback [11] and multimodal feedback [12]. AR applications based on visual augmentation are currently dominant in the manufacturing context. However, there is an emerging interest in multimodal AR applications, which mainly implement visual augmentation with another sensing feedback. Although the research interest in AR technology has rapidly evolved and been intensively investigated over the past 20 years, the first immersive reality prototype can be dated back to 1968. In that year, the first head-mounted display (HMD) device connecting to the computer, which provided the earliest of humankind's experience into augmented reality named \"Sword of Damocles\", was invented by Ivan Sutherland [13]. The way humans interact with industrial AR today is influenced by this invention. However, the term Augmented Reality was first officially formulated years later in 1992 by Thomas Caudell, who was a Boeing researcher. He implemented a heads-up display (HUD) application to demonstrate his idea of designing and prototyping an application to support the manual manufacturing process [14]. In 1996, immersive reality was classified by different levels of immersive experience, which depends on the type of dominant content--reality information or virtual information--and was introduced into the Reality-Virtuality Continuum (RV continuum) by Milgram et al. [15] as shown in Figure 2.  
One year later, Azuma defined the three main technical characteristics of AR based on its technology, which are combining real and virtual objects, interacting with real/virtual objects in real-time and registering (aligning) virtual objects with real objects [16]. Technically, a general AR system is constructed of software built on a selection of four fundamental hardware components: a processing unit, a tracking device, a display device and an input device. The processing unit creates augmentation models, controls devices' connections and adjusts the position of superimposed information into the real world with respect to the pose and position of the user by employing the information coming from a tracking device. The tracking device is used to track the exact position and orientation of  
Figure 2: Reality-Virtuality Continuum adapted from [15]. the user to align/register the augmentations accurately to the desired positions. This device usually consists of at least one element of image capture (a Charge-Coupled Device CCD, stereo, or depth-sensing camera Kinect) [17]. Regarding the tracking technology of AR, depending on the selected tracking devices and tracking methods, it can be classified into three groups: computer vision-based tracking (CV-based tracking), sensor-based tracking and hybrid tracking. The input device is used to obtain the stimulation of the environment or users to trigger the augmentation functionalities. However, the input device is optional because there are some built-in input methods integrated with display devices, especially HMD and HHD. In some cases, the activating elements (images, GPS positions, sensor values, markers, etc.) are pre-defined, thus an input device is not essential in those cases. The current input techniques for HMD are hand-tracking, head/eye-gaze and voice. The processing data are visualized onto the display device via a user interface (UI) enhancing two-way communication between the user and the system. The current display devices can be classified into two groups: in situ display (desktop monitor, projection-based augmentation, spatial augmentation, etc.) and mobile display (hand-held device HHD, head-mounted device HMD).