# Augmented Reality and the Digital Twin: State-of-the-Art and Perspectives for Cybersecurity
## 2 Background
### Augmented Reality  
AR is a concept and, by now, a technology that augments real-world objects with additional digital information. This augmentation of reality is the main benefit of AR as it allows additional information to be added to a user's perception. AR moves the simple digital display of data towards a more immersive, contextualized experience in the real world. Examples of this technology can now be found in many areas. However, applications are always based on the principle that information is superimposed on users' reality with the corresponding end devices. Thus, users perceive both reality and the additional information. For example, navigation information is mapped directly onto the environment, or augmented assembly instructions directly highlight the next required component and placement.  
The concept of AR is very closely related to Virtual Reality (VR). Both terms describe some form of a perceptual combination of the virtual world and the real world. However, they are clearly differentiated by the Reality-Virtuality Continuum by [PERSON] et al. [10], depicted in Figure 1. In VR, a user's perception is entirely dominated by virtual, computer-generated information. Users are immersed in a virtual environment and do not perceive the real world around them. Thus, VR is located on the virtuality end of [PERSON]'s continuum. In contrast, AR is located on the other end of the continuum. It superimposes the virtual information into the real world, while users perceive a combination of both the virtual and the real world. This immersion is achieved through three main functional features (FF) of AR [11, 12]:* **FF 1:** Combining virtual and real-world in a single perceptual space.
* **FF 2:** Allowing users to interact with this perceptual space in real-time.
* **FF 3:** Registering virtual and real objects in a 3D space accurately.  
Initial ideas and concepts for AR have been around for more than five decades. In 1968, [PERSON] created the first Head-Mounted Display (HMD) to let users explore simple wire-frame drawings [2]. AR's most common understanding in today's world is the virtual, transparent visualization of information superimposed on the real world [13]. Enhancing the real world with additional computer-generated information is not limited to visual perception but can span other sensory modalities, such as auditory, somatosensory, or haptic [14]. However, these approaches are less common.  
There are various hardware solutions for realizing AR. They are categorized into five main types [15], including Head-Mounted Displays (HMD), Hand-Held Displays (HHD), projectors, 2D smart glasses, and monitors. An example of common HHDs is any modern smartphone. Several respective AR applications for smartphones have been trending and thriving in the last years [16]. The most advanced HMD is the HoloLens built by Microsoft [17]. In addition to these examples, many other technical approaches exist. A device can be explicitly characterized as AR hardware based on the following four criteria:  
* **Mobility:** Allow users to be mobile and not fixed to a specific space.
* **3D-space:** Spatially register the 3D-space in which it is used.
* **Hands-free:** Allows a hands-free operation of an interaction with the hardware.
* **Real-time:** Transmit data in real-time.  
AR technology has been explored and applied for many different use cases throughout recent years and is considered as one of the key enabling technologies for Industry 4.0 [15, 18]. Another vital key technology for the industry's future is the Digital Twin, which we introduce throughout the following section.