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reflector sight or reflex sight is an optical device that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view. These sights work on the simple optical principle that anything at the focus of a lens or curved mirror (such as an illuminated reticle) will look like it is sitting in front of the viewer at infinity. Reflector sights employ some sort of "reflector" to allow the viewer to see the infinity image and the field of view at the same time, either by bouncing the image created by lens off a slanted glass plate, or by using a mostly clear curved glass reflector that images the reticle while the viewer looks through the reflector. Since the reticle is at infinity it stays in alignment with the device the sight is attached to regardless of the viewer's eye position, removing most of the parallax and other sighting errors found in simple sighting devices.
Since their invention in 1900, reflector sights have come to be used as gun sights on all kinds of weapons. They were used on fighter aircraft, in a limited capacity in World War I, widely used in World War II, and still used as the base component in many types of modern head-up displays. They have been used in other types of (usually large) weapons as well, such as anti-aircraft gun sights, anti tank gun sights, and any other role where the operator had to engage fast moving targets over a wide field of view, and the sight itself could be supplied with sufficient electrical power to function. There was some limited use of the sight on small arms after World War II but it came into widespread use after the late 70s with the invention of the red dot sight, with a red light-emitting diode (LED) as its reticle, making a dependable sight with durability and extremely long illumination run time.
Reflector sights are also used in civilian applications such as sights on surveying equipment, optical telescope pointing aids, and camera viewfinders.