## 1 Introduction
### AR-Based Didactic Experiences in Museums  
AR is a technology that integrates virtual elements over real scenes in real time [27]. In this case, the real world is visible during the experience, but combined with virtual elements. AR does not replace the real world as VR does. Instead, it supplements reality with additional information that is integrated in the 3D real world. In this case, the technology can help to enhance a real-world experience with added contents, which can range from verbal descriptions of exhibits to images, 3D models, or visual animations [28]. AR experiences usually are based on portable devices, such as smartphones or tablets, in which the application is installed and the 3D content is shown on the device screen superimposed onto the real world. The high number of smartphone users makes it possible to reach more participants for AR visits and to personalize the experience according to each visitors' needs [29]. In fact, the most commonly used types of devices in museum experiences are mobile devices, such as smartphones and tablets [30]. However, there are some experiences that used AR head-mounted displays, such as the Hololens (Microsoft, Redmond, DC, USA), achieving a more immersive experience [31]. As happens with VR, AR has applications in many different fields. The application field most closely related to the purposes of the present work is education. It was observed that the use of AR in education is associated with several positive factors, such as learning improvements, more motivation, and an easier comprehension of abstract concepts [32], specially in technologically related fields [33]. It is important to highlight that AR technology can benefit from advances in other fields, such as user interfaces and gestural interactions [34]. For example, user's in-air gestures could be used to interact with the AR content in a natural way in different settings, including museums. Museum professionals, researchers, and educators are paying increased attention to AR. In this field, the technology is mainly applied to deliver additional information about the exhibition to the visitors, thus resulting in an enhanced experience [35; 36; 37]. It has a great capacity to increase engagement, especially in children [37], adding value to the learning experience. The added information can be just a 3D model, as happens in Augusta Romancia, a Romana archaeological site in an open-air museum, where AR is used to show a 3D model of the site superimposed onto information panels [38]. In other cases, the augmented content includes animations, such as moving Pleistocene Cretan animals, which are shown during the visit to a museum fossil room [39]. Animations and texts can also be combined, as happened in an AR prototype developed to enhance the visit to a famous Chinese art piece [40]. In general, museums combined different kinds of content (text, video, audio, 3D animations, and avatars), either to better explain the museum and its history [36] or to give additional information about the artifacts that are exhibited [41]. It was observed that, in this kind of experience, receiving new information using AR improves the visitors' learning process [42]. AR was also used to show the original state of the objects of an exhibition or a building, combining it with the current state that is directly visualized in the real world during the visit. This kind of experience is available in the Casa Batllo, where both the current and the original state of the building with its old furniture can be observed by using an AR application [43]. The Cathedral of Valencia also has an AR application that allows visitors to visualize the Baroque vault that was removed in 2006, and which was digitized with a 3D ground laser scanner before removal [44]. AR can also be used to guide museum visitors during the experience. It was observed that a mobile AR guide system enhances participants' learning effectiveness in comparison with other participants who followed audio instructions or no instructions at all [45].