A red dot sight (also called a reflex sight or, for certain models, a reflex scope) is a non-magnifying firearm sight that uses an illuminated reticle, typically in the form of a red dot. It can also be found on telescopes and point and shoot digital cameras that have electronic viewfinders. Red dot sights should not be confused with laser sights, which project a small red dot on the target.
Red dot sights use refractive or reflective optical collimators to generate a collimated image of a luminous or reflective reticle. This collimated image is reflected off a dichroic mirror or beam splitter to allow the viewer to see the field of view and a reflection of the projected reticle (the red dot) simultaneously. This gives the viewer an image of the reticle superimposed over the field of view at infinity theoretically with no parallax that is aligned with the weapon to which it is attached. Because the reticle image is collimated, magnifying the image of the target is impractical, as it would make the sight too hard to hold steady and introduce parallax. The collimated image does have its advantages, however, as the scope can be placed at any distance from the eye without distorting the image of the target or reticle. This makes red dot sights suitable for use on pistols, submachine guns, rifles, or shotguns.
The most well-known feature of red dot sights is that some, but not all, compensate for the parallax created when the shooter's head moves in relation to the sight. In other words, the dot will point close to the actual point of projected impact even when viewed at an angle. This manifests the other advantage of red dot sights: because the angle of view has so little influence on aiming, they can be aimed rapidly. However, parallax compensation is not perfect. Depending on sight quality, the range to the target and the magnitude of angle at which it is looked into, aiming error can be significant. The lack of magnification is also an advantage in that both eyes can be left open, and the eye that sees the reticle image will automatically superimpose that image with the image from the other eye, giving the shooter normal depth perception and full field of view. This makes the red dot sight very fast and easy to use.
Sights that use dot reticles are almost invariably measured in minutes of angle, or "MOA". One of the most common reticles used in red dot sights is a small dot, covering 5 MOA (1.5 mrad), illuminated by a red LED, hence the common term "red dot sight". MOA is a convenient measure for shooters using English units, since 1 MOA subtends approximately 1.0472 inches at a distance of 100 yards (91.44 m). This is generally rounded to 1 inch at 100 yards, which makes MOA a handy unit to use in ballistics. The 5 MOA (1.5 mrad) dot is small enough not to obscure most targets, and large enough to quickly acquire a proper "sight picture". For many types of action shooting, a larger dot is preferred; 7 (2.0 mrad), 10 (2.9 mrad), 15 (4.4 mrad) or even 20 MOA (5.8 mrad) dots or rings are used; often these will be combined with horizontal and/or vertical lines to provide a level reference.
Common light sources used in red dot sights include battery powered lights, fiber optic light collectors, and even tritium capsules. The color of the dot is usually red or amber for visibility against most backgrounds. Some dots are also visible when viewed through night vision devices. Newer red dot sights may use a chevron or triangular light instead of a dot for precision in aiming and range estimation.
Modern red dot sights generally fall into two categories, full tube or open designs. Full tube sights look similar to a standard telescopic sight, with a cylindrical tube containing the optics. Since a red dot only really needs a single reflective surface, however, the tube is not needed. Many current designs consist of a flat base, with a single loop of material to support the reflective surface. While some argue that the open design gives the shooter a wider field of view, the actual viewable range of the dot is no larger than a full tube sight. The open sights are usually lighter than a full tube sight, since less material is required. More expensive full tube sights offer the option of filters, such as polarizing or haze filters, and glare reducing sunshades, which are not practical on open sights.
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. Since dot sights can be mounted at any distance from the shooter's eye with no issues of focus, military rifle mounts usually place the sight in any mechanically-convenient mounting position, such as the carrying handle of the M16 rifle, or on a rail system (typically a Picatinny rail) on top of the rifle. This leaves plenty of room for night vision equipment to be used with the red dot sight.
Like conventional scopes, red dot sights place the target and the reticle on nearly the same optical plane, allowing a single point of focus. Because there is no magnification, the shooter need not worry about parallax or eye relief. The long eye relief possible makes red dot sights appropriate for firearms with heavy recoil that might drive a conventional short eye relief scope into the shooter's eye; one example would be shotguns used for hunting turkey.
Red dot sights are also used on some telescopes, to help aim the telescope at the desired object. They can also be used as an aiming device on Point and Shoot type digital cameras that have electronic viewfinders. Such cameras suffer from viewfinder blackout when taking continuous action shots of moving objects, often causing the subject of the to be out of frame after a couple of frames. A red dot sight properly calibrated with the camera's viewfinder allows the photographer to sight the moving object using the red dot sight alone to aim the camera, hence gaining an uninterrupted view of the subject matter between each frame.