In Star Trek: Voyager, the Doctor uses a mobile emitter to leave sickbay and appear elsewhere aboard the ship.
The Emergency Medical Hologram is originally restricted to areas equipped with holographic projectors, especially sickbay. In the episode “Future's End,” Voyager's crew acquires a 29th-century mobile emitter that allows the Doctor's holographic program to operate independently of fixed projection systems.
The emitter is a compact device worn on the Doctor's arm. It projects and sustains his holographic body while also carrying the processing and energy technology needed for mobility. This changes the Doctor from a location-bound medical program into a crew member who can travel with Voyager.
The mobile emitter is not a transporter and does not convert the Doctor into ordinary matter. He remains a hologram, although the emitter gives him unprecedented freedom of movement. Its future origin also makes it far more advanced than 24th-century holographic equipment.