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A twin-lens reflex camera (TLR) is a type of camera with two objective lenses of the same focal length. One of the lenses is the photographic objective or "taking lens" (the lens that takes the picture), while the other is used for the viewfinder system, [ 1 ] which is usually viewed from above at waist level.
Each eye has an LED light source (gold-color metal) on the side of the display lens, and a camera under the display lens. The second broad category uses some non-contact, optical method for measuring eye motion. Light, typically infrared, is reflected from the eye and sensed by a video camera or some other specially designed optical sensor.