## 3 Results and Discussion
### Answering RQ3 to RQ5
Monitors and large screens are also com  
Figure 13: Number of articles about AR display devices in manufacturing over years 2010–2021. monly selected for developing a human-centered smart system to support assembly tasks or quality assurance activities [30; 147; 164], provide assembly training assistance tools [166] or real-time data for cyber-physical machine tools monitoring [200], etc. Monitors and large screens are popular devices that are available in any manufacturing or shop-floor context. By utilizing them for developing AR solutions, the cost aspect can be satisfied. However, these systems usually require an external camera system or webcam for capturing real-world images to support the tracking and registration modules of AR applications. Nevertheless, portability is a limitation of using these display devices. Regarding Spatial AR (SAR) display with a projector, this is a favorite display method applied in spot-welding by utilizing the advantages of direct display digital information onto the work piece to enhance workers' concentration, thus reducing the process errors [201]. The system is considered as SAR when the projection is directly displayed onto the physical object. In another scenario of spot-welding inspection, SAR was applied to directly indicate the welding spots for the operators to check during the quality control process of the welding spots, which helped in reducing the inspection process time [209]. For these applications that support spot-welding relevant processes, one important note is the correct rendering for the readability of the indicator text. Besides that, text legibility is also essential, which was comprehensively investigated in a study to enhance the quality and effectiveness of applying SAR in industrial applications [75]. The projector is also popularly used in assisting the assembly process. A projection-based AR system was proposed to monitor and provide instruction for the operator during the assembly process in [161], or to provide the picking information together with assembly data for the operation as in [185]. Then, in a higher-level conceptual system, real-time instructions for assembly using SAR were comprehensively studied and demonstrated in [63]. Finally, during implementation at a real working environment in a factory, projected AR was utilized together with data digitalization to support the setting up of a die cutters process, which resulted in effective cost saving and processing error reduction [198]. It is crucial to note that the capabilities of the above display hardware are changing rapidly, but they both have their advantages and drawbacks, which are detailed in Table 13.  
There is a small percentage of articles--16 articles, or 8% of the total--applying the multimodal displaying technique, and only 1 article, or 0.5% of the total, completely used another technique, which was haptic AR to support assembly process [11]. Multimodal display provides the immersive experience for more than one human sense, which can be mixed between visual displaying and audio to enrich the capabilities of AR applications in industry 4.0 [122] or effectively support and attract the awareness of the worker during the mechanical assembly process [164]. A combination of haptic feedback and visual information is commonly utilized in self-aware worker-centered intelligent manufacturing systems [166] or in applications that require lots of bare-hand interactions with physical objects such as assembly or maintenance [90; 127; 171]. By having a general view about how displaying devices are utilized in manufacturing through the above analysis, a conclusion regarding display device selection for AR assisting in the quality field can be made. By considering the specific working conditions/requirements such as human working environment, movements, flexibility, etc., as well as the advantages/disadvantages of each display technology, the appropriate display devices can be evaluated and selected for each specific AR-assisted quality application.