A holodeck was a type of full immersion virtual reality technology that was produced for theme parks. Essentially, it was a room that a person went to to create a simulated reality.
Early 3D displays were crude compared to later versions. The first 3D images used a machine called a stereoscope to allow people to see the image in 3D. When movie theaters started going three-dimensional, early versions projected two versions of the scene: one in red and one in blue. The 3D glasses had different color filters for each lens: one for red and one for blue. Later, polarizing filters allowed light waves traveling at a certain angle to pass through. Then, in 2010, the invention of lenticular lenses eliminated the need for glasses entirely. This allowed for a successful 3D television that could be easily enjoyed, but it was still crude. Then, things changed.
Holograms were not easy to create. However, by the 21st century, the technology had advanced to the point that it could be for 3D displays. The most well-known was the virtual retinal display which went directly to the eye to allow people to see holograms without anyone else seeing. For a bunch of people, holograms were created by projectors that projected holographic images to a spot where the movie or such was being shown. One step above was the volumetric display. Instead of pixels, the volumetric display used voxels (short for volumetric pixels). This made a 3D image more realistic. The volumetric display was the precursor to the holodeck.
Tech Level: 11
The holodeck was an idea that came out of Star Trek. The real holodeck, however, did not use force fields. Instead, it tapped into the brain-computer interfaces that were being implanted into a person's brain. People could go into full immersion virtual reality without a holodeck. This was for people who could afford to go to a theme park like Walt Disney World. The holodeck had a computer that interfaced with the brain-computer interfaces in the brains to activate a full immersion virtual reality that was the attraction or ride. Since all people in the holodeck were on the same ride, they could communicate with each other in the same virtual reality. One noted example was Spaceship Earth which was renovated to incorporate a holodeck in place of the original ride for a "time travel" experience that was more realistic than the original. It was a revolution for theme parks around the world.