On Star Trek Voyager the portable holographic imaging device is used by the EMH to access areas without holographic projectors. First used in the two-part episode "Future's End", the portable Holo-Emitter allowed the EMH to freely exist in the real word without the need for fixed Holo-Projectors.
Always wanted your own?
Up for sale is a fan-made one 1:1 scale replica of the EMH Holo-Emitter. Measuring approximately 1.5" x 2.25", the resin emitter is painted Star Fleet gray and finished with protective clear coat.
Thanks for looking.