Since the first primitive "virtual reality" systems were created in the 1990's, Humans have advanced hugely in their ability to recreate the sights and sounds of a real environments within an artificial setting. The early VR environments could by no means be called realistic, but by the mid twenty first century computers had advanced to the point where VR systems had gone into common use both in entertainment and many other more serious applications. VR technology was virtually abandoned in the aftermath of World War III, and no serious efforts to pursue simulated environments was made again until near the end of the twenty first century.
The major stumbling block to Virtual Reality as it existed at this time was physical - no matter how good the computer became at projecting images, sounds, and suchlike to the user, he or she was not actually in a real environment. Although body suits capable of simulating tactile impressions had come into use by 2120, these where never considered a serious substitute for actually handling real physical objects.
What was needed was a way to physically recreate an environment which the user could then interact with freely. This did not become possible until the invention of the replicator unit in 2315; based on transporter technology, the replicator allowed actual objects to be created in an instant and deleted as needed.
The first "holochambers" emerged in 2328; they used a small room equipped with a set of holographic projectors which could generate a realistic image of an outdoors scene onto the walls and ceiling. A replicator would then materialize objects within the room to go with the image - plants and trees, for example. The users where then free to pick up and use the objects without having to wear any kind of projection equipment themselves.
Early holochambers suffered from several limitations; a careless user could easily walk into a wall, for example, and if several users where in one chamber then they could only be as far away from each other as the size of the chamber allowed. The major limitation was in the creation of characters within the holochamber; although reasonably realistic images of people and animals could be projected, users could not physically touch these characters in any way.
More recent models have largely overcome these problems; a modern holochamber projects a forcefield across the floor of the chamber, and should a user walk towards the wall this field begins to act as a 'treadmill' to keep the person stationary; the computer automatically moves the replicated objects within the holochamber and adjusts the holographic projections to simulate the movement the user should experience. Replicated objects reaching the wall are dematerialized, while images of objects reaching the space within the chamber are replicated for real.1
The second hurdle was overcome by 'internal partitioning' of the chamber. Should two people enter a holochamber and walk in directly opposite directions, they would previously only be able to go so far before reaching the walls. While the 'treadmill' effect can convince a user that the environment is passing them, it cannot make the users continue to move further away from each other and so the illusion would be broken.
In modern holochambers, the computer would sense that this was about to happen and throw up an internal divide; halfway across the holochamber the computer would throw up a hologram showing each user an image of the other, continuing to move further away - essentially this process creates two miniature holochambers within one. Should the users head back towards each other the computer would reverse the process, merging the two into one again. A modern holochamber is capable of sub-dividing into many separate environments, allowing groups of people to wander around independently of each other.
Perhaps the most impressive advance in holochamber technology has been the advent of 'holomatter'. This is solid matter created within the holochamber energy grid and manipulated by highly articulated computer driven tractor beams; although early efforts where crude, modern holochambers can use holomatter to create and animate totally realistic characters within the chamber.
The basic mechanism behind the holochamber is the omni-directional holo-diode (OHD). The OHD is a small unit (several hundred million per square metre in modern holochambers) which is capable of projecting both full colour stereoscopic images and three dimensional forcefields. The OHD's are circuit printed onto large sheets, which are then subdivided into tiles of 0.61 square metres. A typical starfleet Holodeck wall consists of twelve sub processing layers totalling 3.5 mm thickness, diffusion bonded to a lightweight cooling tile. The panel is controlled by an optical data network similar to that used for standard panel displays. Dedicated subsections of the main computer system drive the holodeck, and it is the memory and speed of these computers which determines the number and complexity of the holodeck programmes available.1
Although modern holochambers are often touted as being just as good as the real thing, in practice there are still limitations. Even the best holochamber can only subdivide into a maximum of twelve separate environments, and many holochamber programmes are not complex enough to make full use of the holochambers technical capabilities. Perhaps the biggest limitation is in the holomatter itself; this is only stable within the energy grid, and looses cohesion almost instantly if removed from the holochamber.2
Holochambers come in various sizes and types; the federation is reputed to have the best models, with Earth boasting some of the largest known holochambers. Starfleet 'Holodecks' are probably the most technically sophisticated, while the Ferengi are known for having some of the most advanced and creative entertainment software.
This facility is designed to allow holographic programs and characters to be easily designed or modified. In 2376 Janeway used the Hololab to alter the Michael Sullivan character from the Fair Haven program in order to make him more attractive to her.3 B'Elanna Torres also used the Hololab to project the effects of various alterations to the DNA of her child in 2377.4
One of the more interesting applications is the Federation Duck Blind. This is a system designed to allow the covert study of primitive cultures. An observation facility is placed in close proximity to the area to be observed and a hologram of a natural feature - commonly a rockface - is projected over it. With this technology an entire outpost can be sited close to or even within a village or town without the residents ever being aware of it. A 4 Gigawatt duck blind was used on Mintaka III to study the proto-Vulcan culture there5, and another was used by the Federation/Son'a team on the Ba'ku planet to study the inhabitants there.6
The EMH was developed by Dr. Lewis Zimmerman7 to provide a backup to organic doctors on the latest generation of Starships. The system was an impressive achievement, containing medical knowledge from over 2,000 reference sources and the experience of 47 physicians8 totalling over five million medical treatments, plus contingency and adaptive programs to make full use of the knowledge.9 However, as the name implies the EMH was designed as an emergency system only and suffered from various limitations. It was designed to last for 1,500 hours at most, and use beyond this period could result in serious degradation to the program.10 They could only be using in an area specially equipped with holographic projectors; the EMH Mark 1s fitted to the Intrepid11 and Sovereign class12 ships could only operate in sickbay.11
By far the biggest problem with the EMH Mark 1, however, was their personality. The program's was abrupt to the point of rudeness, sarcastic, egotistical, and arrogant. Many people found the program unpleasant, and various unflattering nicknames circulated - "Emergency Medical Hotheads" was one favourite. Eventually Starfleet retired the Mark 1 EMH from medical service and used all 675 of them to work on waste transfer barges - something which was an intense disappointment to Dr. Zimmerman, especially as he had designed the Mark 1 to look like himself.13
The Mark 2 was designed to correct many of these deficiencies. The model was incorporated into the Prometheus class Starship; improvements included the ability to move throughout the ship and knowledge of all the latest medical techniques and equipment. The Mark 2 also had an improved personality and a different appearance.14
With the Mark 2 in service, Dr. Zimmerman proceeded to produce the Mark 3 and Mark 4 EMH.13
Once the EMH was in service, Dr. Zimmerman proceeded to develop the Long-term Medical Hologram. The LMH was intended to provide a permanent replacement for Doctors in areas where staffing levels were too low to allow a medical staff to be stationed. The physical appearance of the LMH was originally to be patterned after Dr. Julian Bashir of Deep Space Nine, but this idea was dropped because of the controversy generated when it was discovered that Dr. Bashir had been genetically engineered as a child.15
The holo-communications system was introduced to Starfleet in 2372. It was tested aboard the Defiant and Excelsior class starship16, a move intended to asses how the system performed on both one of the newest designs in Starfleet and one of the oldest. The system consisted of a small holographic scanner and projector; each end would transmit a real-time animated hologram of the user to the other end whilst projecting the signal it was receiving in turn. The system operated to Starfleet's usual high specification in terms of image quality; the net result was that at each end of the conversation the user would be able to talk to a hologram of their counterpart which was completely real-looking.16
Although the holo-communicator was also eventually used on Deep Space Nine15, the system did not find favour with Starfleet and has not been issued widely.17
Federation technology is capable of producing holograms for many purposes. One common use of the technology is to provide three dimensional displays of information and images to aid in presentations; the Galaxy class starship is well fitted in this fashion.18 Holograms are also used for recreational purposes; the holodeck is fitted to many Federation Starships, but on a smaller scale holographic generators are fitted to personal quarters to show recreational programs.19
The holo-suit is designed to allow personnel to pass un-noticed through populated areas; in essence, it is an invisibility suit. It works by projecting a holographic image of the background over the suit. Anybody looking at the suit sees the background instead, rendering the wearer undetectable to the naked eye. The version currently in use with the Federation relies on a central hub to maintain them.6
In 2373 Voyager was involved with a Temporal incidient in which the ship was thrown back in time to 1990s Earth. The crew encountered Henry Starling, an Earth resident who had acquired 29th century technology from a Federation timeship. Amongst Starling's posessions was the Mobile Emitter, a small device into which a hologram could be downloaded. Once the download was accomplished the device allowed the hologram complete mobility; the emitter included a power supply and computer system capable of supporting the holoprogram for long periods independantly of any other external holoprojector system.20 The emitter even allowed a hologram to be reprogrammed as needed, for instance allowing the external appearence to be modified as needed.21
Federation technology allows flat screens to project holographic imagery, giving the screen an apparent depth. This technology was applied to the main viewscreen of the Galaxy class.22