# Evaluation of HMDs by QFD for Augmented Reality Applications in the Maxillofacial Surgery Domain
## 2 Materials and Methods
### Desk Research on Augmented Reality Technologies  
Mixed reality (MR) was firstly defined by [PERSON] and [PERSON] as any point somewhere along the \"virtuality continuum\" which connects real environments to virtual ones (Figure 1). Among the many MR technologies emerged over the years, probably the best known is AR, which refers to all cases in which the display of an otherwise real environment is augmented by means of virtual (computer graphic) objects [33].  
In the early 1990s, the concept of AR was invented by Boeing: a technology used to \"augment\" the visual field of the user with information necessary in the execution of the current task [34]. The first immersive system, \"Virtual Fixtures\", was created by the USAF. In 1998, there was the first mainstream application by Sportsvision that broadcasted the first live NFL game using a graphic system to visualise a virtual 1 st & Ten line. Recently, the unveiling of the Google Glass device in 2014 gave the possibility of immersive experiences with Google apps; in 2016, Microsoft HoloLens started to be sold as more advanced AR device but with an important price to be an everyday accessory; in the same year, the app developer Niantic launched Pokemon Go, a smartphone AR georeferenced game that hit 1 billion downloads in March 2019. Devices are necessary to use AR, and HMD are a particular subset of MR-related technologies that involve the merging of real and virtual worlds. Inserting virtual objects into real scenes requires a process of registration or correct alignment of the virtual world with the real one. The objects in the real and virtual worlds must be properly aligned with respect to each other, or the two worlds' coherence will be compromised. Without accurate registration, AR cannot be accepted in many applications [35]. Registration methods can be divided into two types: sensor-based registration and computer-vision-based registration [36; 37; 38]. Following a form factor categorisation, AR devices can be broadly divided into four types:  
Figure 1: The virtuality continuum described by [PERSON] and [PERSON].  
Head-up displays (HUD) are mostly used on vehicles (e.g., airplanes and cars) to give additional info without taking the driver's eyes off the road. This kind of device has a fixed transparent screen, in the line of sight of the pilot/driver, where information is projected.
2. Holographic displays use light diffraction to generate 3D objects in the real space, or to give a depth to displayed images without using 3D glasses or other tools.
3. Smart glasses or wearable devices may have many aspects, ranging from glasses-type devices to more futuristic visors. Visors are also known as head-mounted displays (HMD), and they include two families: optical see-through and video see-through devices. This classification derives from the technological solution adopted to show the real-world image to the user: an optical device views reality directly through transparent displays while a video device shows the reality through camera images.
4. Handheld devices are tools such as smartphones or tablets where AR is realised as video see-through by capturing the reality with the camera of the devices.  
This article is focused on HMD because, using wearable devices, the surgeons may retrieve the information without looking away, while having their hands free to operate. This article aims to evaluate the readiness of commercial HMD to be used in a medical application, so knowing the market offer was necessary. Currently, it is not possible to trace a neat line between commercial product families because many features are common to all devices; for this reason, the devices evaluated have been grouped by their commercial purpose into industrial, professional, and entertainment devices. A functional classification was useful to have a deeper understanding of devices capabilities, avoiding subsequent trivial recommendations. The general traits of each group have been outlined in the next paragraphs.