# Augmented Reality Applications in Industry 4.0 Environment
## 2 Related Work  
The term industrial revolution represents a concept that essentially changes our society and economy [25]. The fourth industrial revolution (I4.0) envisages the implementation of production systems based on the use of techniques that connect virtual and real worlds and completely transforms the entire industrial environment [26]. In such conditions, smart factories are systems in which information processing systems monitor physical processes, creates a virtual copy of the physical world and executes decentralized decisions based on self-organization mechanisms [27]. Each entity from the physical world has its digital twin in the digital world and so the entire production system can be simulated. These systems are characterized by context awareness, which allows people and machines to perform their tasks in an optimal manner [12]. AR is a rapidly evolving technology for creating a digital reality in which the user's physical environment is complemented with additional digital content, such as text messages, images, 3D virtual objects, videos, etc., in the user's field of vision [28; 29; 30]. In some cases, the sensation can also be experienced by touch, hearing and smell [31]. AR applications have already been developed for various social and business sectors such as education [32], tourism [33] and medicine [34]. Unlike virtual reality, augmented reality keeps the user in the real world and provides additional visual information. This technology is already used in many industrial environments to help address product lifecycle challenges, including planning, design, ergonomics assessment, maintenance, management and training [35]. In addition, the application of AR technology significantly contributes to increasing flexibility and productivity, avoiding recurrence of errors, increasing the health and safety of workers and thus facilitating the optimization of production processes [36]. A typical AR application consists of three layers, as shown in Figure 1[37]. The lower layer represents the real and material world. In the middle layer, digital information that is relevant and useful is added to the real world. Combining these two layers creates a third augmented world in which users are given access to digital content. AR applications combine real and virtual objects and provide real-time interaction between users and the augmented world and the geometric alignment of virtual and real objects [38]. During the design and implementation of AR applications, developers must pay attention to the following [39]:  
* Provide the user with all the necessary information, but keep it as simple as possible;
* Place the information in the right position;
* Do not restrict the user in performing his tasks;
* Equip the environment, not the user;
* Strive for intuitive natural interaction;
* Design for personalized feedback;
* Enable users to control speed;
* Add motivating quantified-self information. One of the most important things, not only in the field of AR but in any human machine collaboration, is the interaction between users and the devices [39]. It must be easy and simple, otherwise the users will not be able to perform their tasks on time and the number of errors can also increase. The role of the device is not to slow down the user, but to help him/her complete the task. AR can be experienced by using wearable and non-wearable devices [40] depending on whether the performer needs more flexibility and a free hand. Some of the smart glasses available come with touch-sensitive fields on the side and so users can communicate that manner. Furthermore, voice control in this case can be very useful because it helps free the user's hands to be completely while performing the task. A new method of hand tracking is presented in [41] where the interaction with a virtual AR object is realized manually.