## 4 Augmented Reality Devices
### Spatial Augmented Reality  
Spatial Augmented Reality (SAR), also known as \"room-scale AR\" or \"tabletop AR\", uses projectors to display graphical information on physical objects. A defining feature of SAR is that the display elements are not tied to the users, as is the case with handheld or head-mounted devices. SAR can provide a more natural and intuitive interface as it aligns with the physical world [57]. The main sectors with implemented applications using these AR devices are education, art and design, medical training, industrial applications (SAR can be used in the manufacturing industry to guide workers through assembly processes), museums and exhibits, and entertainment. One of the main advantages of SAR is that it allows multiple users to interact with the AR content simultaneously, without the need for any special wearables or devices [66]. Some examples of specific devices of SAR technology are [57]:  
* **Interactive Projectors:** Projectors such as the Lightform LFC Kit and Sony's Xperia Touch can be used to create SAR experiences. These projectors can scan the environment and allow users to \"paint with light,\" projecting AR content onto any surface.
* **AR Sandboxes:** An AR Sandbox is a combination of a physical sandbox, a 3D camera (like a Microsoft Kinect), and a projector. The 3D camera tracks the topography of the sand, and the projector overlays a topographic map and water simulation onto the sand. This is often used in educational contexts to teach geography and geology.
* **Projection Mapping Software:** Software solutions like TouchDesigner, MadMapper, or HeavyM can be used with standard projectors to create complex projection mappings onto 3D surfaces, which can be used for SAR applications.
* **Laser Projectors:** Devices like the Bosch GLM 50 C, which is technically a laser distance measurement with Bluetooth, can be used for SAR applications. Its ability to measure distances with a high degree of precision makes it suitable for applications that require precise overlays.
* **Projection Keyboards:** Devices like the Celluon Epic Projection Keyboard project a virtual keyboard onto a flat surface. Users can \"type\" on this virtual keyboard, and the device uses infrared sensors to detect the position of the fingers.  
It is common that sometimes this type of AR device is overlapped with projection-based AR devices but they have differences between them. Both categories involve augmenting physical spaces with virtual information, they do so in different ways and can serve different purposes, but projection-based AR focuses on projecting light onto real-world surfaces, which may or may not allow for interaction, while spatial AR focuses on integrating virtual objects into the physical environment in a way that they appear to share the same 3D space, with a strong emphasis on multi-user interaction.  
The fundamental principle of SAR is that it augments the physical environment by adding virtual objects that seem to co-exist within the same space as real objects. This generally requires that the system possess a deep understanding of the physical environment's geometry and lighting conditions, which can be achieved using advanced depth sensors or computer vision techniques.