# Augmented Reality and the Digital Twin: State-of-the-Art and Perspectives for Cybersecurity
## 3 Literature Review and State-of-the-Art
### Qualitative Results
#### 3.3.4 What?--AR Technologies in the Digital Twin Environment  
This section considers technologies in terms of possible technological solutions to realize an AR visualization of DT data. Besides the main characteristics of AR that need to be fulfilled by respective technology (cf. _Mobility_, _3D-space_, _Hands-free_, _Real-time_ defined in Section 2.1), there is a set of additional, more DT-specific features. These additional requirements need to be met by an AR technology, allowing it to display DT information effectively [6]:  
* **Continuous data flow:** The data flow needs to be bi-directional between the AR component and the DT [55]. A direct and synchronous interaction between users and the DT is only possible if this feature is addressed. * **Continuous tracking of physical and logical objects:** To ensure the ongoing and correct contextualization of DT data, both physical and logical objects need to be continuously tracked. Current placements of virtual objects need to be matched with user movement, as well as the possible movement of physical objects [15, 55]. * **Access to behavioral models:** If necessary, behavioral representation models for physical objects need to be accessible for logical objects. * **Adherence to physical constraints:** The representation and alignment of logical objects based on physical constraints need to be ensured. There are three different ways the 3D models of an AR visualization can be positioned in the user's view. This relative positioning can be realized based on markers, devices, or the surrounding physical room [34]. Analyzing the applied AR technologies in the identified literature reveals the dominance of Microsoft's HoloLens solution. A large proportion of authors use this HMD for their solutions in a wide variety of use cases [15, 34, 37, 40, 43, 44, 52, 55]. HMDs can fulfill all AR hardware criteria and, therefore, are the preferred technology to realize respective applications [15]. Additionally, the HoloLens is currently one of the few commercially available HMDs, mainly explaining its dominance. The HoloLens has been proven to be a very efficient and accessible solution for interacting in real-time with a DT [17]. However, studies reveal some constraints, such as wearing and viewing comfort, which should be further improved. Additionally, the first generation of HoloLens regularly faces significant issues with retrieving real-time data and ensuring a continuous data flow, thus violating two core criteria in the context of AR for the DT. Since the initial study by [PERSON] and [PERSON] is conducted with the first commercially available version of the HoloLens [17], it might be interesting to see how the HoloLens 2 performs in a similar experiment as it is promoted to be much more ergonomic and secure. Besides most businesses using the Microsoft HoloLens, an increasing amount of AR-DT combinations is implemented with other technologies, especially in the public sector. For example, Hand-held displays are primarily applied in training and education contexts [33, 49]. Commonly, in these use cases, smartphones or tablets are used as AR HHDs. Although they do not fulfill the third criterion for AR hardware, which is the possibility for hands-free interaction [15], they are still ubiquitous and accessible to almost everyone. This makes them a perfect trade-off solution for education and training, where many people use AR technology simultaneously. Offering the often more affordable HHDs in this context is reasonable. Other authors implement the AR-based visualization of DT data using standard displays and user interfaces [5, 35, 53]. In these applications, pre-captured models of reality or webcam streams are augmented with virtual information. Standard displays were more popular in the earlier days of combining AR with the DT, but their use has decreased with the maturing of technologies, such as the HoloLens. The potential and feasibility of applying AR technology for visualizing DT information are more and more examined empirically.