The basic mechanism behind the Holodeck is the omnidirectional 
holo diode (OHD). The OHD comprises two types of microminiature device 
that projects a variety of special forcefields. The density of OHDs is 400 per 
cm®, only slightly less than the active visual matrix of a multilayer display 
panel, and powered by standard medium-duty electro plasma taps. Entire 
walls are covered with OHDs, manufactured in an inexpensive wide-roll 
circuit printing process. A typical Holodeck surface comprises twelve 
subprocessing layers totaling 3.5 mm, diffusion bonded to a lightweight 
structural cooling tile averaging 3.04 cm thick. The primary 
subprocessor/emitter materials include keiyurium, silicon animide, and 
superconducting DiBe<2>Cu 732. Each single OHD measures 0.01 mm. The 
optical data network mechanism by which OHDs are sent impulses is similar 
to that for smaller display screens, though complete walls are broken down 
into manageable high-speed segments, each 0.61 m®. Dedicated high-speed 
subsections of the starship main computers drive these room-sized displays.
	In addition to their ability to project full-color stereoscopic images, 
OHDs manipulate forcefields in three dimensions to allow Holodeck visitors 
to ÒfeelÓ objects that arenÕt really there. This tactile stimulation provides the 
proper feedback one might expect from a rock on the ground or a tree 
growing in a forest. The only limiting factors to the numbers and kinds of 
objects described by the computers are memory and time to record or 
calculate from scratch the originals of the desired objects, whether real or 
imagined, such as a Klein bottle.
	Other stimuli, such as sound, smell, and taste, are either simulated by 
more traditional methods, such as speakers or atomizers, or built into the 
created objects using replicator techniques.
	The optic version of an OHD emits a complete image of an overall 
environment based on its location in the installed surface panel. The visitor, 
however, sees only a tiny portion of any one OHD, in much the same manner 
as a flyÕs eye operating in reverse. As one moves about, the visible portions 
of the OHDs change, altering the view. The actual energy emissions are 
unlike direct visible EM projections, but rather polarized interference 
patterns. The image is reconstructed where the patterns intersect at the lens 
of the eye or other visual receptor.
	The forcefield version creates a tiny steerable forcefield. Its larger 
cousins are the more familiar tractor beams and navigational deflector. 
Under computer control, over a vast number of OHDs, the cumulative field 
effect is substantial. If the Holodeck is recreating, for example, a large mass 
of rock, the computer would first create the three-dimensional surface of the 
rock. This is accomplished by commanding certain OHDs to intersect their 
fields at the required polygon coordinates. If the field strength is tuned to 
provide the proper mineral hardnesses, the mass will feel like rock. A vast 
library of recorded real substances is available, and custom settings may be 
commanded for experimental purposes.
	The shaped forcefields and background imagery allow the visitor to 
experience volumes and distances apparently larger than the Holodeck room 
could physically accommodate. The environment can be scrolled to continue 
if desired, or set for bounding limits indicated by soft wall contacts and 
audible reminders of wall proximity.
	Within the USS Enterprise, crew members can visit four main 
Holodecks on Deck 11. In addition, a set of twenty smaller personal 
holographic simulator rooms are situated on Decks 12 and 33.
	In a working environment like a Federation starship, safety is of prime 
importance and is engineered into every system. Because the starship living 
environment is so highly controlled, the emotional release associated with 
encounters with limited real physical hazards has been shown to be of 
significant value in maintaining the psychological well-being of many crew 
members. Simulated high velocities and forces are normally created by 
sensory illusions. While safeguards against critical bodily harm are 
programmed into the computers, certain scenarios can result in unavoidable 
sprains and bruises, even for experienced users. Hazards posed by 
ÒdangerousÓ lifeforms can seem exceedingly real and will fulfill most 
requirements.  Æ