Most of the papers reviewed describe all the componentsof the AR solutions; however, some papers describe morethan one AR application (and thus the hardware compo-nents are more numerous) whereas other papers (in particu-lar, technical papers, review papers or conceptual papers) donot always focus on the description of the full AR architec-ture. Consequently, the number of solutions described belowdoes not reflect the number of published studies.Table 8provides an overview of the technological solu-tions as a function of the paper type, whereasTable 9describestheevolutionofthesetechnologiesovertime.From these tables it is easy to see that“camera,”or its vari-ant“camera connected to a monitor,”are the most com-monly used solutions for capturing the scene of the externalenvironment; in that case, the monitor displays the scene tothe users with the relevant additional information. Anothertechnological solution frequently adopted for visualizationpurposes is the HMD. Compared with the first solution, theHMD is easier to transport and does not require cameras ormonitors to be installed in the production area. The use ofHMD solutions has slightly increased in time: since 2011,more than six papers per year make use of this solution.However, some authors have recently criticized the use ofHMD, especially for remote assistance purposes (Gurevichet al.,2015); as a matter of fact, the use of HMD could forcethe worker to limit his/her head movements in an attemptnot to make the view of the remote assistant unstable. Somestudies also reported that users can suffer from decreasedvisualacuity while lookingataphysicaltarget throughHMDs (Livingstonet al.,2005).In studies published since 2010, most of the technologicalsolutions for visualization make use of tablets, smartphonesor other mobile devices, e.g., ultra-mobile PCs. In this case,the scene is captured by the device camera and is immedi-atelyvisualizedwithsuperimposedinformationonthedevice display. Compared with HMD solutions, the use oftablets or smartphones is more socially accepted and has theadvantageofbeingeveneasiertotransport.However,mobile devices are hand-held when used, which can hinderthe operator when he/she has to carry out manual tasks(e.g., assembly or maintenance tasks). For this reason, bare-hand solutions have been proposed, starting from 2011.These solutions aim at developing a natural and intuitivehand-based interaction with virtual objects and have beenapplied in seven papers; moreover, as this is a relatively newinteraction technique, five papers have investigated bare-hand interaction from a technical point of view.In some of the papers reviewed, non-conventional ARsystems were developed. Jimeno-Morenillaet al. (2013) pro-vided an example of use of a non-conventional system, i.e.,an infrared emitter coupled with a pair of active glasses.
