Anplar fields of view filling at least the region directly viewable on the optical axis of the eyes -about
90 - are necessary to support the illusion of being immersed in another space, be it computer generated or the
product of remote cameras. Any greater view, up to 270° laterally, enhances the illusion. Stereopsis, and
completing the 360° visual sphere by head motion, round out the illusion; they require the wearing of a
head-mounted-display (HMD). This paper describes the optical viewing system used in almost all existing HMD
systems, and makes the case that, compared to video monitors, these HMD systems provide a qualitatively
different kind of access to remote or computer generated reality -a difference that, even in the present crude
state of the art, is striking enough to herald a new order of computer interfacing and real time telepresence.
A caveat is presented: that if the spatial presentation is false - not "orthospace" - then the charm and power
of the illusion, and the utility of the system, are sharply diminished.