VR systems, most configurations fall into three main categories and each category can be ranked by the sense of immersion, or degree of presence it provides. Immersion or presence can be regarded as how powerfully the attention of the user is focused on the task in hand.

Immersion presence is generally believed to be the product of several parameters including level of interactivity, image complexity, stereoscopic view, field of regard and the update rate of the display.

For example, providing a stereoscopic rather than monoscopic view of the virtual environment will increase the sense of immersion experienced by the user. It must be stressed that no one parameter is effective in isolation and the level of immersion achieved is due to the complex interaction of the many factors involved. Non-Immersive (Desktop) Systems

Non-immersive systems, as the name suggests, are the least immersive implementation of VR techniques. Using the desktop system, the virtual environment is viewed through a portal or window by utilising a standard high resolution monitor.

Interaction with the virtual environment can occur by conventional means such as keyboards, mice and trackballs or may be enhanced by using 3D interaction devices such as a SpaceBallä; or DataGloveä; . The non-immersive system has advantages in that they do not require the highest level of graphics performance, no special hardware and can be implemented on high specification PC clones.

This means that these systems can be regarded as the lowest cost VR solution which can be used for many applications. However, this low cost means that these systems will always be outperformed by more sophisticated implementations, provide almost no sense of immersion and are limited to a certain extent by current 2D interaction devices.
Additionally, these systems are of little use where the perception of scale is an important factor.

However, one would expect to see an increase in the popularity of such systems for VR use in the near future. This is due to the fact that Virtual Reality Modelling Reality Language (VRML) is expected to be adopted as a de-facto standard for the transfer of 3D model data and virtual worlds via the internet. The advantage of VRML for the PC desktop user is that this software runs relatively well on a PC, which is not always the case for many proprietary VR authoring tools. Furthermore, many commercial VR software suppliers are now incorporating VRML capability into their software and exploring the commercial possibilities of desktop VR in general.

Semi-Immersive Projection Systems

Semi-immersive systems are a relatively new implementation of VR technology and borrow considerably from technologies developed in the flight simulation field.

A semi-immersive system will comprise of a relatively high performance graphics computing system which can be coupled with either:

• A large screen monitor
• A large screen projector system
• Multiple television projection systems

Using a wide field of view, these systems increase the feeling of immersion or presence experienced by the user. However, the quality of the projected image is an important consideration. It is important to calibrate the geometry of the projected image to the shape of the screen to prevent distortions and the resolution will determine the quality of textures, colours, the ability of define shapes and the ability of the user to read text on-screen. The resolutions of projection systems range from 1000 - 3000 lines but to achieve the highest levels it may be necessary to use multiple projection systems which are more expensive. Semi-immersive systems therefore provide a greater sense of presence than non-immersive systems and also a greater appreciation of scale.

In addition, images can be provided that are of a far greater resolution than HMDs and this implementation provides the ability to share the virtual experience. This may have a considerable benefit in educational applications as it allows simultaneous experience of the VE which is not available with head-mounted immersive systems. Additionally, stereographic imaging can be achieved, using some type of shuttered glasses in synchronisation with the graphics system.

Fully Immersive Head-Mounted Display Systems

The most direct experience of virtual environments is provided by fully immersive VR systems. These systems are probably the most widely known VR implementation where the user either wears an HMD or uses some form of head-coupled display such as a Binocular Omni-Orientation Monitor or BOOM (Bolas, 1994).