The Mobile emitter was a device that was from the 29th century.
This small palm sized autonomous machine was a self sustaining mobile holographic emitter that was created with the intention of remotely powering and enabling a hologram to function any distance away from any holoemitters. This piece of future technology was constructed of poly-deutonic alloy and generated photons as well as the forcefield necessary to allow a hologram to remain active in any environment. The nature of the device allowed its capabilities to be transferred from one Holographic program to another. The emitter was able to be placed either on the exterior "skin" of the hologram or was capable of being enveloped by the hologram instead thus hiding it from view. (VOY episode: "Renaissance Man")
A mobile emitter was created in the 29th century and was present on the Timeship USS Aeon which was accidentally catapulted into the late 20th century where it was one of the technologies plundered by Henry Starling. He used the emitter to give mobility to The Doctor after downloading his program from the USS Voyager that had accompanied the Aeon accidentally into the past. At the end of the event, Captain Braxton of the Aeon returned Voyager to its place in the timeline and allowed them to retain the usage of the mobile emitter. (VOY episode: "Future's End, Part II")
Data on the mobile emitter was included in the list of information present in Project Voyager. Through studying it, Fabian Stevens attempted to make a replica of the technology as part of a project on the USS da Vinci. (SCE eBook: Malefictorum) By 2381, the entirety of the emitter had been mapped on a molecular level though despite this being the case Starfleet lacked the means to fully replicate the technology. (TOS novel: Captain's Glory)
In 2382, Admiral Owen Paris of Starfleet Research and Development ordered the confiscation of the machine and for it to be taken to Galor IV for study. However, the Doctor filed a legal action to block the transfer of the emitter on the grounds that he was a sentient being and that his performance as well as quality of life were dependant on the device. The Judge Advocate General blocked the transfer of the technology until it had time to study the case and make a ruling on the matter.