VR originated in the second half of the 1960s with the head-mounted display (HMD) as the first device that provided immersive experiences with computer-generated imagery. An HMD houses two small display screens and an optical system that channels the images from the screens to the eyes, while a motion tracker continuously lets an image-generating computer adjust the scene to the user's current view. After extensive development at NASA and the Department of Defense, HMD technology became commercially available 20 years later in 1989. At that time, VR was coined to describe these immersive visual environments.