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A friendly reminder regarding spoilers! As always, the expanded Trek universe is in a period of major upheaval with the continuations of Strange New Worlds and Prodigy, the advent of new eras in gaming with the Star Trek Adventures RPG and Star Trek Online, as well as other eventful releases such as Section 31, the ongoing IDW Star Trek comic and spin-off Star Trek: Defiant. Therefore, please be courteous to other users who may not be aware of current developments by using the {{spoiler}}, {{spoilers}} OR {{majorspoiler}} tags when adding new information from sources less than six months old (even if it is minor info). Also, please do not include details in the summary bar when editing pages and do not anticipate making additions relating to sources not yet in release. THANK YOU

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Memory Beta, non-canon Star Trek Wiki

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)