A holographic diode is the part of the holodeck responsible for projecting the holographic image to the users. The USS Enterprise-D's holodecks contained billions of these diodes.
In an alternate timeline where Montgomery Scott went back in time to rescue James T. Kirk before his absorption into the Nexus, Scott and Geordi La Forge altered the holodeck aboard the USS Enterprise-D to route the holographic imaging output to the ship's external deflector shield system, projecting a sensor image; the ship would not be able to project the visual image outside of the holodeck without lining the ship's hull with holographic diodes. (Star Trek novel: Engines of Destiny)