Head-mounted displays (HMDs) are small displays or projection technology integrated into eyeglasses or mounted on a helmet or hat. Heads-up displays are a type of HMD that does not block the user’s vision, but superimposes the image on the user’s view of the real world. An emerging form of heads-up display is a retinal display that “paints” a picture directly on the sensitive part of the user’s retina. Although the image appears to be on a screen at the user’s ideal viewing distance, there is no actual screen in front of the user, just special optics (for example, modified eyeglasses) that reflect the image back into the eye. Other heads-up displays that are not worn by the user but are projected on a surface (for example, on a car or plane windshield) are not covered in this discussion. Some HMDs incorporate motion sensors to determine direction and movement (for example, to provide context-sensitive geographic information) or as the interface to an immersive virtual reality application.