Red Dot Sight - Wikipedia

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 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.[2]

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. 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 crosshairs or concentric circles can be used but need more complex aberration free optics.

Like other reflector sights, the collimated image of the red dot is truly parallax free only at infinity, with an error circle equal to the diameter of the collimating optics for any target at a finite distance.[3] This is compensated for by keeping the dot in the middle of the optical window (sighting down the sight's optical axis).[4] Some manufacturers modify the focus of the LED/optical collimator combination, making models with the optical collimator set to focus the dot at a finite distance. These have a maximum amount of parallax due to eye movement, equal to the size of the optical window, at close range, diminishing to a minimal amount at the set distance (somewhere around a desired target range of 25–50 meters).[5]

Sights may also use a more sophisticated optical system that compensates for off axis spherical aberration, an error that can cause the dot position to diverge off the sight's optical axis with change in eye position. The optics used is a type of Mangin mirror system, consisting of a meniscus lens corrector element combined with the semi-reflective mirror, sometimes referred to in advertising as a "two lens" or "double lens" system.[6][7][8] Although these are referred to as "parallax free" sights, the system keeps the aiming dot in alignment only with the sight itself and does not compensate the inherent parallax errors induced by a collimated sight.[9][10]

Red dot sights generally fall into two categories, "tube" or "open" designs. "Tube sights" look similar to a standard telescopic sight, with a cylindrical tube containing the optics. Tube sights offer the option of fitted dust covers and the ability to add filters, such as polarizing or haze filters, and glare reducing sunshades. Since a reflector sight really needs only a single optical surface, the "reflector", the tube is not needed. This allows for non-tubed "open sights" that consist of a flat base, with a single loop of material to support the reflective optics.

Most red dot sights have either active or passive adjustments for the dot brightness, allowing a very bright dot for high visibility in bright conditions, and a very dim dot to prevent loss of night vision in low light conditions.