Reflex sight

"reddot" redirects here. For RedDot, see Red Dot (disambiguation).

File:Tradok matriell modi 55160a.jpg

A Norwegian soldier holding an MP5 with an Aimpoint CompM2 red dot sight.

Reflex sights are optical or computing sights that reflect a reticle image (or images) onto a combining glass for superimposition on the target. Reflex sights are most commonly configured as non-magnifying firearm sights (such as the M68 red dot sight), but they are also used to aid targeting on other devices, such as telescopes and point-and-shoot digital cameras. Reflex sights should not be confused with laser sights, which actually project a point of light directly onto a target.

Operation

File:Red dot sight principle.png

A lens can be used to create a virtual image (top) of the red object. By collimating the image using a reflective (middle) or refractive (bottom) lens, the image can be projected to infinity.

Reflex 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 (e.g. a red dot) simultaneously. If no magnification is utilized, this gives the viewer a theoretically parallax-free image of the reticle, superimposed over the field of view at infinity.

A reflex sight with no magnification can be held at any distance from the eye (see eye relief), and at almost any angle, without distorting the image of the target or reticle, and without causing the reticle to "move" relative to the target. But parallax compensation is not perfect, and depending on the sight's design, the range to the target, and the magnitude of angle at which it is looked into, aiming error can be non-trivial due to parallax.

Magnified reflex sights suffer from parallax and fixed eye relief just like conventional telescopic sights. Nevertheless, many reflex sights are available with magnification. The U.S. military has widely-deployed the Advanced Combat Optical Gunsight (ACOG) telescopic reflex sight, but reflex sights are also readily found in conventional-looking rifle scope configurations, used for such activities as hunting and target shooting.

Because reflex sights provide an illuminated reticle, they are often used with both eyes open (the brain will tend to automatically superimpose the illuminated reticle image coming from the dominant eye onto the other eye's unobstructed view), giving the shooter normal depth perception and full field of view. This capability, along with the parallax compensation found in un-magnified devices, makes target acquisition very fast compared to standard telescopic sights and iron sights. Un-magnified reflex sights are particularly suitable for installation on a wide variety of weapons used for close-range engagement, e.g. pistols, submachine guns, and shotguns.

Reticles

A view through a Tasco ProPoint 2 5 MOA red dot sight (model PDP2ST), mounted on a Ruger 10/22 rifle. Made in Japan for Tasco, the ProPoint 2 model was one of the first models of red dot sight that became widely popular.

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.

Many reticle illumination and pattern options are available. Common light sources used in reflex sights include battery powered lights, fiber optic light collectors, and even tritium capsules. Some reflex sights are specifically designed to be visible when viewed through night vision devices. The color of a reflex sight reticle is often red or amber for visibility against most backgrounds, but green is becoming widely available as well. And reticle shapes aren't limited to dots; e.g. some reflex sights use a chevron or triangular pattern instead, to aid precision aiming and range estimation, and still others provide selectable patterns and holographic reticles.

Types of reflex sights

Some of the first optical reflex sights to appear were "occluded" red dot sights; that is, the image of the target was not visible through the sight. Looking into the sight with just one eye, the shooter saw only a red dot against a black background (often passively illuminated with ambient light gathered through fiber optics). With both eyes open, the image of the dot was superimposed by the brain onto the target. Most currently available reflex sights use a curved partially reflective glass surface as an objective lens. This lens allows light from the target to pass into the device, collimating it with the light of the reticle before it reaches the ocular lens, thus allowing the shooter the tactical flexibility of using either one- or two-eyed aiming.

Modern optical reflex sights designed for firearms and other uses fall into three housing-configuration categories: full tube, open, and small tube.

• Full tube sights look similar to standard telescopic sights, with a cylindrical tube containing the optics. Many full tube sights offer the option of interchangeable filters (such as polarizing or haze-reducing filters), glare-reducing sunshades, and conveniently protective "flip-up" lens covers.

• Open sights (also known as "mini reflex sights" and "mini red dots") take advantage of the fact that reflex units require only a single reflective surface, and are offered in configurations consisting of a single loop of optical material, allowing objective light to pass through while also providing the necessary reflective surface for collimating the reticle. While some argue that the open design gives the shooter a wider field of view, the actual circumscribed viewable range of the dot is not usually significantly larger than that of a typical full tube sight, and if two-eyed sighting is used, the overall field-of-view is identical to that of a full tube design. Open sights do often weigh less than full tube sights (since less construction material is required), which could be construed as a non-trivial tactical consideration. But due to their diminished profile, open sights don't usually accommodate filters and other accessory options typically supported by tube designs.

• Small tube sights attempt to combine the advantages of full tube sights (most notably the ability to accept filter and sunshade attachments), while maintaining the larger field-of-view occlusion claimed to be provided by open-style reflex sights.

Uses

Military organizations have adopted reflex sights for a variety of reasons, including durability, simplicity, quick target acquisition, and superior performance over iron sights in low-light conditions. The United States Army issued thousands of Aimpoint CompM2 units (designated M68 Close Combat Optic) to its troops in support of the War on Terrorism. Reflex sights are also widely used by civilian shooters, e.g. in speed shooting sports such as IPSC, and among paintball and airsoft players.

Most reflex sights have either active or passive adjustments for the reticle brightness, which help the shooter adapt to different lighting conditions. A very dim reticle will help prevent loss of night vision in low-light conditions, while a brighter reticle will display more clearly in full sunlight. Since non-magnified reflex sights aren't dependent on eye relief, they can theoretically be placed in any mechanically-convenient mounting position, such as the carrying handle of the M16 rifle, or on a rail system on top of the rifle. This provides configuration flexibility, and the ability to more easily accommodate other important tactical optics, such as night vision equipment or co-mounted magnifiers.

The potentially long eye relief distance of non-magnified reflex sights makes them useful on firearms with heavy recoil. For example, the short eye relief required by traditional telescopic sights can present the risk of eye and face injury as the scope travels back toward the shooter when a powerful weapon is discharged.

Reflex sights are also used on some telescopes, to help aim the telescope at the desired object. "Point and shoot" type digital cameras with electronic viewfinders are sometimes aided by reflex sights as well. Such cameras often suffer from "viewfinder blackout" if the screen is slow to refresh when taking continuous action shots of moving objects. This can result in the subject drifting out of frame undesirably. A reflex sight properly calibrated with the camera's viewfinder allows the photographer to track the moving object using the reflex sight to aim the camera, keeping the subject in view between each frame.

See also

• EOTech and ITL MARS sights
• Iron sights