The Hologram Generator is a large and elaborate machine used to project an illusion to trick the Headless Horseman.
As part of their plan to capture the Headless Horseman once and for all, the Ghostbusters concluded the only way to trap the ghost would be to deceive it in to driving over a body of water. Egon Spengler set up the Hologram Generator below the Queensboro Bridge. The machine created a hologram that made the bridge look like just another street. Peter Venkman served as the bait and successfully lured it onto the bridge. The Horseman was immobilized and trapped by Kate.