Red dot sight

A red dot sight is a common classification for a type of non-magnifying reflector (or reflex) sight for firearms, and other devices that require aiming, that gives the user an aimpoint in the form of an illuminated red dot. A standard design uses a red light-emitting diode (LED) at the focus of collimating optics which generates a dot style illuminated reticle that stays in alignment with the weapon the sight is attached to regardless of eye position (nearly parallax free). They are considered to be fast acquisition and easy to use gun sights for target shooting, hunting, and in police and military applications. Aside from firearm applications, they are also used on cameras and telescopes. On cameras they are used to photograph flying aircraft, birds in flight, and other distant, quickly moving subjects. Telescopes have a narrow field of view and therefore are often equipped with a secondary "finder scope" such as a red dot sight.

The typical configuration for a red dot sight is a tilted spherical mirror reflector with a red light-emitting diode (LED) at its off axis focus. The mirror has a partially silvered multilayer dielectric dichroic coating designed to reflect just the red spectrum allowing it to pass through most other light. The LED used is usually deep red 670 nanometre wavelength since they are very bright, are high contrast against a green scene, and work well with a dichroic coating since they are near one end of the visible spectrum. The size of the dot generated by the LED is controlled by an aperture hole in front of it made from metal or coated glass. The LED as a reticle is an innovation that greatly improves the reliability and general usefulness of the sight. There is no need for other optical elements to focus light behind a reticle. And the LED itself is solid state and consumes very little power, allowing battery powered sights to run for hundreds and even tens of thousands of hours. Using a "dot" shaped reticle also greatly simplifies the sight since the small diameter image does not require a sophisticated optical reflector to focus it. More complex reticle patterns such as cross hairs or concentric circles can be used but need more complex aberration free optics.

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