There are three types of VR systems namely: non-immersive systems, semi-immersive projection systems, and fully immersive head-mounted displays. The non-immersive systems are the least immersive among VR techniques because it only utilizes a portal window to enable to view the virtual environment. Keyboards, mice, and trackballs are used to interact in the virtual environment. The semi-immersive projection systems provide a greater sense of presence compared to the non-immersive system because it uses large screen monitor, large screen projector system or multiple television projection systems which enhance the feeling of immersion of the user (Costello, 1997).
But among the three types of VR systems, the fully immersive head-mounted systems provides the most direct experience of virtual environment (Bolas,1994). The first device that provided immersive experiences with computer-generated imagery was the head-mounted display (HMD) which was invented in the mid-1960s. It has an optical system that channels images from a screen to the eyes and a motion tracker that continuously lets an image-generating computer adjust the scene to the user's current view. After the extensive development of two NASA Ames scientists, Fisher and McGreevy, HMD technology was produced and commercially available in 1989 (Costello, 1997).
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