Omnidirectional holographic diodes, or OHDs, Are the primary component to Holodeck functions. The OHD is comprised of two types of microdevices that project a variety of special forcefields. The optical section, centered in the hexagonal shaped OHD, is responsible for emiting full-color stereoscopic images, while the forcefield emiters, around the edge of the diode manipulate focefields in three dimensions to create the ability to touch and feel the projected images. Each OHD measures 0.01 mm, covering the holodeck walls with a density of 400 per sq. cm., and recieve their energy from standard medium duty electro plasma taps.
The OHDs were suspected of malfunctioning in Sherlock Holmes Program 3A in 2369. (TNG: "Ship in a Bottle")