An emitter array was a grouping of devices generally found on starships, starbases, and devices which may have a number of purposes, including in tractor beams, deflectors, force fields, shield generators, and transporters.
Geordi La Forge once increased the tractor beam's efficiency by adding VISOR technology to the emitter array's process. (TNG: "The Masterpiece Society")
Miles O'Brien once re-routed an emitter array on Zayra IV that was infested with Talarian hook spiders. (TNG: "Realm of Fear")
In order to survive for an extended period of time in a transporter, Montgomery Scott connected the emitter array to the phase inducers, turned off the override and locked the pattern buffer into a continuous diagnostic cycle. (TNG: "Relics")
Beverly Crusher once set up a portable force field generator. It didn't work as intended becuase the emitter array was not set up to block the entire passageway. (TNG: "Chain of Command, Part I")