Red dot sight
Encyclopedia
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...
reflector (or reflex) sight
Reflector sight
A reflector or reflex sight is a generally non-magnifying optical device that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view...
for firearm
Firearm
A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...
s that gives the user an aimpoint in the form of an illuminated red dot. A standard design uses a red light-emitting diode
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
(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
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
free). They are considered to be fast acquisition and easy to use gun sights
Sight (device)
A sight is a device used to assist aligning or aim weapons, surveying instruments, or other items by eye. Sights can be a simple set or system of markers that have to be aligned together as well as aligned with the target...
for target shooting, hunting, and in police and military applications.
Description
The typical configuration for a red dot sight is a tilted spherical mirror reflector with a red light-emitting diodeLight-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
(LED) at its off axis focus. The mirror has a partially silvered multilayer dielectric dichroic
Dichroic filter
A dichroic filter, thin-film filter, or interference filter is a very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characterized by the color of light that they...
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
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
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
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. 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
Solid state
Solid state may refer to:In science:* Solid-state chemistry* Solid-state physics* Solid-state laser* Solid matterIn electronics:* Solid state , circuits built of solid materials* Solid-state fan...
and consumes very little power, allowing battery powered sights to run for hundreds and even tens of thousands 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.
Like other reflector sights, the collimated
Collimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...
image of the red dot is only truly parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
free at infinity, with an error circle equal to the diameter of the collimating optics for any target at a finite distance. This is compensated for by keeping the dot in the middle of the optical window (sighting down the sight's optical axis
Optical axis
An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope.The optical axis is an imaginary line that defines the path along which light propagates through the system...
). 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 yards).
Sights may also use a more sophisticated optical system that compensates for off axis spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
, an error that can cause the dot position to diverge off the sight's optical axis
Optical axis
An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope.The optical axis is an imaginary line that defines the path along which light propagates through the system...
with change in eye position. The optics used is a type of mangin mirror
Mangin mirror
In optics, a Mangin mirror is a negative meniscus lens with the reflective surface on the rear side of the glass forming a curved mirror that reflects light without spherical aberration...
system, consisting of a meniscus lens corrector element
Meniscus corrector
A meniscus corrector is a negative meniscus lens that is used to correct spherical aberration in image-forming optical systems such as catadioptric telescopes...
combined with the semi-reflective mirror, sometimes referred to in advertising as a "two lens" or "double lens" system. Although these are referred to as "parallax free" sights, the system only keeps the aiming dot in alignment with the sight itself and does not compensate the inherent parallax errors induced by a collimated sight.
Red dot sights generally fall into two categories, full tube or open tube designs. Full tube sights look similar to a standard telescopic sight
Telescopic sight
A telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...
, with a cylindrical tube containing the optics. Full tube sights offer the option of filters, such as polarizing
Polarization
Polarization is a property of certain types of waves that describes the orientation of their oscillations. Electromagnetic waves, such as light, and gravitational waves exhibit polarization; acoustic waves in a gas or liquid do not have polarization because the direction of vibration and...
or haze filters and glare reducing sunshades. Since a reflector sight only really needs a single optical surface, the "reflector", the tube is not needed. These "open sights" 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
Night vision
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...
in low light conditions.
History
The idea of attaching a reflector (or reflex)Reflector sight
A reflector or reflex sight is a generally non-magnifying optical device that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view...
sight to a firearm has been around since the sights invention in 1900. Many different types of reflector sights specifically designed for firearms have been marketed over the past half century (see Reflector sight/History), some lit by batteries, some lit by ambient light, one, the Weaver Qwik-Point, even presented the viewer with a red aiming dot due to the red plastic “light pipe” used to collect ambient light. All had the reticle illumination drawback common with reflector sights small enough for a firearms: proper ambient lighting could not be depended on and incandescent light bulb
Incandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...
s could drain a battery in a few hours.
In 1975 the Swedish
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
optics company Aimpoint AB
Aimpoint AB
Aimpoint AB is a Swedish optics company manufacturing red dot sights for civilian, police and military use. The most popular model is the CompM2 , currently used by numerous armed forces around the world.- Company :...
marketed the first "electronic" red dot sight combining a reflecting curved mirror and a light-emitting diode, based on a design by Helsingborg
Helsingborg
Helsingborg is a city and the seat of Helsingborg Municipality, Skåne County, Sweden with 97,122 inhabitants in 2010. Helsingborg is the centre of an area in the Øresund region of about 320,000 inhabitants in north-west Scania, and is Sweden's closest point to Denmark, with the Danish city...
engineer John Arne Ingemund Ekstrand. The sight was called the "Aimpoint Electronic" and had a closed tube design that could be mounted similar to a telescopic sight. The LED could run for 1500 to 3000 hours on mercury batteries. Other manufacturers soon followed with over a dozen offering models today. Newer generation red dot sights were produced with lower power consumption LEDs and power saving electronics, allowing them to run for years without being turned off. In 2000 the US military introduce a red dot sight into field use, the Aimpoint CompM2
Aimpoint CompM2
The CompM2 is a battery powered non-magnifying red dot type of reflex sight for firearms manufactured by Aimpoint AB. In the U.S. military, it is designated M68 Close Combat Optic and is also known as M68 Aimpoint and is designed to meet United States military standards...
, designated the “M68 Close Combat Optic”.
Reticles
Red dot sight reticles are measured in minutes of angleMinute of Angle
Minute of angle is the measurement of a ballistic round's deviation from its initial heading due to gravity and/or the effect of air resistance on velocity. Informally known as a "Bullet's Trajectory" or "the rainbow effect". Long range weapons must account for this effect because a fired round...
, or "MOA". MOA is a convenient measure for shooters using English unit
English unit
English units are the historical units of measurement used in England up to 1824, which evolved as a combination of the Anglo-Saxon and Roman systems of units...
s, 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
Ballistics
Ballistics is the science of mechanics that deals with the flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.A ballistic body is a body which is...
. One of the most common reticles used in red dot sights is a small dot, covering 5 MOA (1.5 mrad
Radian
Radian is the ratio between the length of an arc and its radius. The radian is the standard unit of angular measure, used in many areas of mathematics. The unit was formerly a SI supplementary unit, but this category was abolished in 1995 and the radian is now considered a SI derived unit...
). 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
Action shooting
Action shooting is a term that encompasses target shooting events where the shooter's score is based on both the accuracy and elapsed time. Disciplines covered by the term include:*United States Practical Shooting Association...
, 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.
Uses
Red dot sights place the target and the reticle on nearly the same optical plane, allowing a single point of focus. This makes them fast acquisition and easy to use sights, allowing the user to keep their attention on the field of view in front of them. They are common in speed shooting sports such as IPSCInternational Practical Shooting Confederation
The International Practical Shooting Confederation is a shooting sport based on the concept of practical shooting. Accuracy, power and speed are all required to achieve a maximum score.-Founding and organization:...
. Military units and police forces have also adopted them. Red dot sights are also popular among paintball
Paintball
Paintball is a sport in which players compete, in teams or individually, to eliminate opponents by tagging them with capsules containing water soluble dye and gelatin shell outside propelled from a device called a paintball marker . Paintballs have a non-toxic, biodegradable, water soluble...
and airsoft
Airsoft
Airsoft is a sport in which participants shoot round non-metallic pellets launched via replica firearms.Gameplay varies in style and composition but often range from short-term skirmishes, organized scenarios, military simulations, historical reenactments, to competition target shooting events...
players for similar reasons.
Because there is no magnification, the shooter need not worry about parallax or eye relief
Eye relief
-Eye Relief and Exit Pupil:The eye relief of a telescope, a microscope, or binoculars is the distance from the last surface of an eyepiece at which the eye can obtain the full viewing angle. If a viewer's eye is outside this distance, a reduced field of view will be obtained...
. The long eye relief makes red dot sights appropriate for firearms with heavy recoil
Recoil
Recoil is the backward momentum of a gun when it is discharged. In technical terms, the recoil caused by the gun exactly balances the forward momentum of the projectile and exhaust gasses, according to Newton's third law...
that might drive a conventional short eye relief telescopic sight into the shooter's eye. 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
M16 rifle
The M16 is the United States military designation for the AR-15 rifle adapted for both semi-automatic and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO...
, or on a rail system (typically a Picatinny rail
Picatinny rail
The Picatinny rail or MIL-STD-1913 rail or STANAG 2324 rail or a "Tactical Rail" is a bracket used on some firearms in order to provide a standardized mounting platform. A similar system is the Weaver rail mount.-Description:...
) on top of the rifle. This leaves plenty of room for night vision equipment to be used with the red dot sight.
Further reading
- Customize the Ruger 10/22: Comprehensive Do-It-Yourself Guide to Upgrading America's Favorite .22, by James E. House, Kathleen A. House, page 239 "Red Dot Sights"
- John Arne Ingemund Ekstrand’s U.S. Patent 3,942,901 for a “Optical sighting instrument with means for producing a sighting mark” filed December 7, 1973
See also
- Aimpoint CompM2Aimpoint CompM2The CompM2 is a battery powered non-magnifying red dot type of reflex sight for firearms manufactured by Aimpoint AB. In the U.S. military, it is designated M68 Close Combat Optic and is also known as M68 Aimpoint and is designed to meet United States military standards...
and ITL MARSITL MARSThe ITL MARS is a gun sight that combines two sighting devices, a reflex sight and a laser sight. It is designed and produced by ITL Optronics company, based in Israel. The laser may be either visible or infrared and can be activated as need via a pressure switch. It has been purchased by a number...
red dot sights - Glossary of firearms terminologyGlossary of firearms terminology-A:*accurize, accurizing: The process of altering a stock firearm to improve its accuracy.*action: The physical mechanism that manipulates cartridges and/or seals the breech. The term is also used to describe the method in which cartridges are loaded, locked, and extracted from the mechanism....
- Holographic weapon sightHolographic weapon sightA holographic weapon sight or holographic diffraction sight is a non-magnifying gun sight that allows the user to look through a glass optical window and see a reticle image superimposed at a distance on the field of view...
- Iron sightsIron sightsIron sights are a system of shaped alignment markers used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector sights...
- Laser sight, projects a red (or other-colored) aiming point onto a distant target
- Telescopic sightTelescopic sightA telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...