Red dot sight

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 firearm the sight is attached to, regardless of eye position (nearly parallax free).

Telescopes have a narrow field of view and therefore are often equipped with a secondary "finder scope" such as a red dot sight to orient them.

The mirror has a partially silvered multilayer dielectric dichroic coating designed to reflect just the red spectrum allowing most other light to pass through it.

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.

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.

This allows for non-tubed "open sights" that consist of a flat base, with a single loop of material to support the reflective optics.

In 1975, the Swedish optics company Aimpoint AB marketed the first "electronic" red dot sight combining a reflecting curved mirror and a light-emitting diode, based on a design by Helsingborg engineer John Arne Ingemund Ekstrand.

In 2000, the U.S. military introduced a red dot sight into field use, the Aimpoint CompM2, designated the "M68 Close Combat Optic".

The 1.6 mrad (5 MOA) dot is small enough not to obscure most handgun targets, and large enough for most competition shooters to quickly acquire a proper sight picture.

When red dot sights started to appear at the practical shooting competition circuit in the 1990s, reticle sizes of up to 3, 4.5 or even 6 mrad (10, 15 or 20 MOA) were common in order to compensate for the lack of bright illumination.

Miniature red dot sights are becoming increasingly popular for use on pistols, both for competition and military applications.