# Augmented Reality and the Digital Twin: State-of-the-Art and Perspectives for Cybersecurity
## 3 Literature Review and State-of-the-Art
### Qualitative Results
#### 3.3.2 Where?--Main Application Areas  
The application areas in which AR is applied in combination with the DT overlap with the core DT areas. [PERSON] et al. summarize the DT focus areas under three topics (1) _Manufacturing_, (2) _Education_, and (3) _Cities_, _Transportation_, _and Energy_[4]. We mostly follow this classification to describe the different areas where AR and DT are jointly applied. However, we extend the topic _Education_ to _Education and Training_. We summarize work that does not fit under the previously mentioned topics under the _General_ category. Table 1 matches the identified literature with the respective application areas. Please note that several publications address more than one application area, which is why the overall count in Table 1 is higher than the 33 identified unique, relevant papers. **Manufacturing:** This application area is the most popular in the AR/DT combinations as it is the current primary use case for the DT itself. Due to the evolution of Industry 4.0, the use of CPS and, respectively, of DTs will increase drastically [5, 17]. To ensure a practical and convenient workflow in this context, workers are required to use portable wearable devices, voices, or gestures to interact with CPS [35]. For example, AR and DT are used to improve predictive maintenance. Thereby, the combination enables industrial processes to predict and, thus, detect anomalies or prevent imminent failures [36]. The resulting maintenance procedures are launched through the DT and can be carried out in real-time through workers using AR. Thus, the maintenance procedures selected within the DT are \begin{table}
\begin{tabular}{c c c} \hline \hline
**Application Area** & **References** & **Count** \\ \hline Manufacturing & [5, 15, 17, 31, 32, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48] & 19 \\ \hline Education and Training & [33, 42, 48, 49, 50, 51, 52, 53] & 8 \\ \hline Cities, Transportation, and Energy & [13, 35, 54, 55, 56, 57, 58] & 7 \\ \hline General & [4, 6, 34, 59] & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Identified literature for the main application areas. Some sources address multiple application areas. made available for workers who then can be guided by AR [36]. Besides the manufacturing workers, higher employees (i.e., factory managers or supervisors) can benefit from AR applications displaying DT data. [PERSON] et al., for example, present a Virtual Twin of a smart factory for managers, allowing full remote accessibility and control of the factory, as well as the respective processes [37]. [PERSON] et al. are seeing a development towards a Lean manufacturing approach based on DTs and AR, enabling various advantages [41]. Besides manufacturing itself, Digital Assembly is also a related research area where AR and DT are currently raising expectations for significant improvements [38, 42]. AR is used to realize virtual reality fusion, assembly operation navigation, assembly scene perception, and many others. It is meant to improve quality and efficiency through the interaction of virtual assembly objects and real assembly objects. The DT is used for the stimulation of behavior in the assembly process of physical entities. This provides the prediction of assembly performance and accuracy. DT technology enables assembly instructions in real-time through AR and still guarantees the overall assembly performance [38]. Assembly processes driven by the DT gain efficiency and flexibility and can provide multi-modal feedback through adding AR [39]. Thus, AR displays just-in-time information, creates a more natural interaction, and improves the user experience for assembly design, training, and guidance [32]. Digital assembly benefits from these approaches by increased quality, as well as reduced costs and cycle times, from initiation to execution [40]. **Education and Training:** In the context of education and training, AR is a widely accepted concept to improve learning effectiveness, motivation, and engagement [42, 49].