Virtual reality
Around the same time, Douglas Engelbart used computer screens both as input and output devices. In 1968, Ivan Sutherland, with the help of many students including Bob Sproull, created what was widely considered to be the first head-mounted display (HMD) system for use in immersive simulation applications. It was primitive both in terms of user interface and realism, and the HMD to be worn by the user was so heavy that it had to be suspended from the ceiling. The graphics comprising the virtual environment were simple wire-frame model rooms. The formidable appearance of the device inspired its name, The Sword of Damocles.
Mixed reality
In 1994 Paul Milgram and Fumio Kishino defined a mixed reality as "...anywhere between the extrema of the "virtuality continuum"" (VC), where the virtuality continuum extends from the completely real through to the completely virtual environment with augmented reality and augmented virtuality ranging between. The first fully immersive mixed reality system was the Virtual Fixtures platform developed at US Air Force, Armstrong Labs in 1992 by Louis Rosenberg to enable human users to control robots in real-world environments that included real physical objects and 3D virtual overlays called "fixtures" that were added enhance human performance of manipulation tasks. Published studies showed that by introducing virtual objects into the real world, significant performance increases could be achieved by human operators.