Emergency Hologram

The Emergency Hologram (EH) originally called the Emergency Medical Holographic program (EMH) was a sophisticated hologram used on most Federation starships in the late 24th century and 25th century that was designed to provide assistance to medical, engineering and sometimes command staff in times of emergency. It was developed by Dr. Lewis Zimmerman on Jupiter Station and was formally known as EMH Program AK-1 Diagnostic and Surgical Subroutine Omega-323.

Mark I
The Mark I was based on their designer, Dr. Zimmerman, both physically and psychologically. This produced an arrogant and irritable hologram that became notorious for its less than gracious bedside manner, which saw some doctors try to avoid activating the program. 

The first vessel equipped with the Mark I was the USS Voyager (NCC-74656). The Doctor, as it was known on Voyager would also become the most notable Mark I due to its continuous operation over several years and it evolving from a purely short-term emergency protocol to a critical and important contributor to the crew of the USS Voyager during its years stranded in the Delta Quadrant. 

The Mark I would eventually be declared "defective" and replaced by newer models. The Mark Is were transferred to undertake menial tasks such as dilithium mining and scrubbing plasma ducts.

Mark II
The EMH Mark II was the designation for the second installment of the Emergency Medical Holographic program, which became standard equipment aboard Federation starships starting in 2374. The Mark II prided itself as superior to its predecessor, the Mark I, whose appearance and personality were modeled after project designer Lewis Zimmerman.

Mark III
In early-2375, Federation starships were equipped with the Mark III. He was designed to be more personable than its two predecessors.

The EMH Mark III had stringent security protocols in place due to it being in use during the Dominion War, although by 2376, the security protocols were seen as a hindrance to effective caregiving.

Mark IV
By 2376, the Mark IV of the EMH program had been developed.

Mark V
As of the creation of the Mark V in 2381, the EMH had been renamed the EH or Emergency Hologram as of the inclusion of the hologram being able to act as a doctor and a commanding officer after the return of the USS Voyager (NCC-74656) in 2378.

One major difference of the Mark Vs was their ability to be customly designed for each starship they were used on, allowing a Captain to decide upon their behavior and other important details to a limited degree that were not possible on the previous versions.

Mark VI
In 2402 a new project was started entitled the Mark 6 Project. This was a project designed to create the Emergency Hologram Mark VI to enable it to be an effective member of the crew. In addition because holograms now had free reign to explore starships and could even leave said ships if the place they went to had holographic technology it was decided the project should be started.

A key difference between the Mark V and the Mark VI was the ability for expanded role additions, in particular the main roles being a Doctor, an Engineer, and a member of the Command staff which in the most dire case would be acting Captain. One of the unique abilities per say for the Mark VI was for the uniform colour to change depending on the location on the ship the hologram was called to, something the Mark V couldn't always do. The previous limitation of customization was also increased. The prototype model was installed on the USS Pureshasu (NCC-86521).

Starfleet

 * USS Voyager (NCC-74656)
 * The Doctor
 * Backup EMH
 * USS Pureshasu (NCC-86521)
 * EH MK VI "Kazue"

Background Information

 * The EMH-Mark I was played by Robert Picardo.
 * The EMH-Mark II was played by Andy Dick.
 * The EMH-Mark VI was based upon the likeness of Maki Goto.