A 3D projector is a device that can project an image that creates the illusion of spatial depth. Instead of a flat image, the viewer perceives objects as if they are protruding from the screen or are in the background. The principle is based on imitating natural human vision, known as stereoscopic vision. Each eye perceives reality from a slightly different angle and the brain combines the two images into a single spatial perception. To achieve this effect, the projector projects two slightly offset images onto the screen in quick succession - one for the left eye and one for the right. To ensure that each eye sees only "its" image, special 3D glasses are required that synchronise with the projector. There are two main types of technology. The first is active, where the glasses contain electronics and their lenses alternately darken in a flash, letting only the right image into each eye. The second technology is a passive system that uses polarized light and simpler glasses without electronics. Of course, to watch, you need to have source material, such as a movie or game, that is in 3D format.