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 force field 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.
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 meter in modern holochambers) which is capable of projecting both full color 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 meters. A typical Starfleet Holodeck wall consists of twelve sub processing layers totaling 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.
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.
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.