In Star Trek: Voyager, what device lets the Doctor project outside sickbay?

In Star Trek: Voyager, the Doctor uses a mobile emitter to project outside sickbay.

The Emergency Medical Hologram was originally restricted to Voyager’s sickbay and other areas covered by the ship’s holographic systems. In the episode “Future’s End,” the Doctor acquires a mobile emitter from a future version of Voyager. The device allows his holographic program to operate away from fixed emitters.

The emitter is small enough to wear on the Doctor’s arm. It contains the hardware needed to sustain his projection, making him more mobile and allowing him to visit planets, shuttles, and other locations. This changes his role from a stationary medical program into a crew member who can participate directly in missions.

The mobile emitter is not a transporter. It does not move a physical body; it enables the Doctor’s holographic form to exist beyond the ship’s ordinary projection range.