Reflector sight
Encyclopedia
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
. This is achieved by placing an image, a cross hair reticle or other pattern, at the focus of a lens and bouncing the image off a slanted glass plate, or at the focus of a partially reflecting curved mirror
. The viewer looking through the glass plate or curved mirror reflector sees an image of the reticle in front of them that stays in alignment with the device the sight is attached to regardless of their eye position, removing most of the parallax
error found in simple sighting devices.
Since their invention in 1900, reflector sights have come to be used as gun sight
s on all kinds of weapons from small firearm
s to fighter aircraft
. They are also used in civilian applications such as optical telescope
pointing aids, camera viewfinder
s and as the base component in many types of head-up display
.
with a luminous or reflective overlay image or reticle at its focus, creating an optical collimator that produces a virtual image of that reticle. The image is reflected off some form of angled beam splitter
or the partially silvered collimating curved mirror itself so that the observer (looking through the beam splitter or mirror) will see the image at the focus of the collimating optics superimposed in the sight's field of view in focus at ranges up to infinity
. Since the optical collimator produces a reticle image made up of collimated light
, light that is nearly parallel, the light making up that image is theoretically perfectly parallel with the axis of the device or gun barrel
it is aligned with, i.e. with no parallax
at infinity. The collimated reticle image can also be seen at any eye position in the cylindrical volume of collimated light created by the sight behind the optical window. But this also means, for targets closer than infinity, sighting towards the edge of the optical window can make the reticle move in relation to the target since the observer is sighting down a parallel light bundle at the edge. Eye movement perpendicular to the devices optical axis
will make the reticle image move in exact relationship to eye position in the cylindrical column of light created by the collimating optics.
A common type (used in applications such as aircraft gun sights) uses a collimating lens and a beam splitter (Fig. 1 right). This type tends to be bulky since it requires at least two optical components, the lens and the beam splitter/glass plate. The reticle collimation optics are situated at 90° to the optical path making lighting difficult, usually needing additional electric illumination, condensing lenses, etc. A more compact type (Fig.2) replaces the lens/beam splitter configuration with a half silvered or dichroic
curved collimating mirror set at an angle that performs both tasks of focusing and combining the image of an offset reticle. This type works well as a small arms sight. It is also possible to place the reticle between the viewer and the curved mirror at the mirror’s focus (Fig. 3). The reticle itself is too close to the eye to be in focus but the curved mirror presents the viewer with an image of the reticle at infinity. This type was invented by Dutch optical engineer Lieuwe Van Albada in 1932 originally as a camera viewfinder and has also been used as a gunsight on WW2 bazooka
s.
Reflector sights are generally non-focusing sights, simply a projected reticle bounced off a beam splitter or curved mirror right into the users eye. This gives them the defining characteristics of not needing considerable experience and skill to use, as opposed to simple mechanical sights such as iron sights
. This also gives them the characteristic of being a better alternative to sighting devices based on optical telescope
s in the aspects of field of view and eye relief in that reflector sights actually have neither: depending on design constraints their field of view is the users naked eye field of view, the sight simply puts a reticle right in the middle of it, and their non-focusing collimated nature means they have no eye relief. There are reflector type sights that are telescopic
, either by design or via add on lens kits, but they re-introduce the problems of narrow field of view and limited eye relief. The primary drawback of reflector sight is that they need some way to illuminate the reticle to function. Reticles illuminated by ambient light are hard to use in low light situations, and sights with electrical illumination stop functioning all together if that system fails.
optical designer
and telescope maker Sir Howard Grubb
in patent No.12108. Grubb conceived of his “Gun Sight for large and small Ordnance” as a better alternative to the difficult to use iron sight while avoiding the telescopic sight
's limited field of view, greater apparent target speed, parallax errors, and the danger of keeping the eye against an eye stop. In the 1901 THE “SCIENTIFIC TRANSACTIONS OF THE ROYAL DUBLIN SOCIETY” He described his invention as:
It was noted soon after its invention that the sight could be a good alternative to iron sights and also had uses in surveying and measuring equipment. The reflector sight was first used on German fighter aircraft in 1918 and widely adopted on all kinds of fighter and bomber aircraft in the 1930s. By World War II the reflector sight was being used on many types of weapons besides aircraft, including anti-aircraft guns, naval guns, anti tank weapons, and many other weapon where the user needed the simplicity and quick target acquisition nature of the sight. Through their development in the 30’s and into WWII the sight was also being referred to in some applications by the abbreviation “reflex sight”.
Since the collimated
image produced by the sight is only truly parallax free at infinity, the sight has an error circle equal to the diameter of the collimating optics for any target at a finite distance. Depending on the eye position behind the sight and the closeness of the target this induces some aiming error. For larger targets at a distance (given the non-magnifying, quick target acquisitions nature of the sight) this aiming error is considered trivial. On small arms aimed at close targets this is compensated for by keeping the reticle in the middle of the optical window (sighting down its optical axis
). Some manufactures of small arms sights also make models with the optical collimator set at a finite distance. This gives the sight parallax
due to eye movement the size of the optical window at close range which diminishes to a minimal size at the set distance (somewhere around a desired target range of 25-50 yards).
Compared to standard telescopic sights, a reflector sight with no magnification can be held at any distance from the eye (don’t require a designed eye relief
), and at almost any angle, without distorting the image of the target or 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
. Since Reflector sights are not dependent on eye relief, they can theoretically be placed in any mechanically-convenient mounting position on a weapon.
Reflector sights as aircraft gun-sights have many advantages. The pilot/gunner need not position their head to align the sight line precisely as they did in two-point mechanical sights, head position is only limited to that determined by the optics in the collimator, mostly by the diameter of the collimator lens. The sight does not interfere with the overall view, particularly when the collimator light is turned off. Both eyes may be used simultaneously for sighting.
The optical nature of the reflector sight meant it was possible to feed other information into field of view, such as modifications of the aiming point due to deflection
determined by input from a gyroscope. 1939 saw the development by the British of the first of these gyro gunsight
s, reflector sights adjusted by gyroscope for the aircrafts speed and rate of turn, enabling the display of a lead-adjusted sighting reticle that lagged the actual "boresight" of the weapon(s), allowing the boresight to lead the target in a turn by the proper amount for an effective strike
As reflector sight designs advanced after World War II, giving the pilot more and more information, they eventually evolved into the head up display (HUD). The illuminated reticle was eventually replaced by a video screen at the focus of the collimating optics that not only gave a sighting point and information from a lead-finding computer and radar, but also various aircraft indicators (such as an artificial horizon, compass, altitude and airspeed indicators), facilitating the visual tracking of targets or the transition from instrument to visual methods during landings.
The mid to late 70s saw the introduction of what are usually referred to as "red dot sights
", a type that gives the user a simple bright red dot as an aiming point. The typical configuration for this sight is a compact curved mirror reflector design with a red Light-emitting diode
(LED) at its focus. Using an 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 mirror can use a dichroic
coating to reflect just the red spectrum allowing it to pass through most other light. 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.
Reflector sights for military firearms (usually referred to by the abbreviation “reflex sight”) took a long time to be adopted. The US House Committee on Armed Services noted as far back as 1975 on the suitability of the use of reflex sight for the M16 but the US military did not introduce a reflector sights until 2000 with the Aimpoint CompM2
red dot sight, designated the “M68 Close Combat Optic”
powered lights, fiber optic light collectors, and even tritium
capsules. Some sights are specifically designed to be visible when viewed through night vision devices. The color of a sight reticle is often red or amber for visibility against most backgrounds. Some sights use a chevron or triangular pattern instead, to aid precision aiming and range estimation, and still others provide selectable pattern.
Sights that use dot reticles are almost invariably measured in minutes of angle
, or "MOA". MOA is a convenient measure for shooters using English unit
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
. 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.
Most 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.
s, to help aim the telescope at the desired object. Albada type sights are used on "Point and shoot" type camera
s and on simple disposable cameras. They have also been used in nautical navigation devices and surveying equipment.
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"...
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
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
. This is achieved by placing an image, a cross hair reticle or other pattern, at the focus of a lens and bouncing the image off a slanted glass plate, or at the focus of a partially reflecting curved mirror
Curved mirror
A curved mirror is a mirror with a curved reflective surface, which may be either convex or concave . Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices...
. The viewer looking through the glass plate or curved mirror reflector sees an image of the reticle in front of them that stays in alignment with the device the sight is attached to regardless of their eye position, removing most of the 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"...
error found in simple sighting devices.
Since their invention in 1900, reflector sights have come to be used as gun sight
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...
s on all kinds of weapons from small 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 to fighter aircraft
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...
. They are also used in civilian applications such as optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
pointing aids, camera viewfinder
Viewfinder
In photography, a viewfinder is what the photographer looks through to compose, and in many cases to focus, the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main optical system. Viewfinders are used in many cameras of...
s and as the base component in many types of head-up display
Head-Up Display
A head-up display or heads-up display is any transparent display that presents data without requiring users to look away from their usual viewpoints...
.
Overview
Reflector sights work by using a lens or an image forming curved mirrorCurved mirror
A curved mirror is a mirror with a curved reflective surface, which may be either convex or concave . Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices...
with a luminous or reflective overlay image or reticle at its focus, creating an optical collimator that produces a virtual image of that reticle. The image is reflected off some form of angled beam splitter
Beam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...
or the partially silvered collimating curved mirror itself so that the observer (looking through the beam splitter or mirror) will see the image at the focus of the collimating optics superimposed in the sight's field of view in focus at ranges up to infinity
Infinity
Infinity is a concept in many fields, most predominantly mathematics and physics, that refers to a quantity without bound or end. People have developed various ideas throughout history about the nature of infinity...
. Since the optical collimator produces a reticle image made up of collimated light
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...
, light that is nearly parallel, the light making up that image is theoretically perfectly parallel with the axis of the device or gun barrel
Gun barrel
A gun barrel is the tube, usually metal, through which a controlled explosion or rapid expansion of gases are released in order to propel a projectile out of the end at a high velocity....
it is aligned with, i.e. with no 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"...
at infinity. The collimated reticle image can also be seen at any eye position in the cylindrical volume of collimated light created by the sight behind the optical window. But this also means, for targets closer than infinity, sighting towards the edge of the optical window can make the reticle move in relation to the target since the observer is sighting down a parallel light bundle at the edge. Eye movement perpendicular to the devices 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...
will make the reticle image move in exact relationship to eye position in the cylindrical column of light created by the collimating optics.
A common type (used in applications such as aircraft gun sights) uses a collimating lens and a beam splitter (Fig. 1 right). This type tends to be bulky since it requires at least two optical components, the lens and the beam splitter/glass plate. The reticle collimation optics are situated at 90° to the optical path making lighting difficult, usually needing additional electric illumination, condensing lenses, etc. A more compact type (Fig.2) replaces the lens/beam splitter configuration with a half silvered or 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...
curved collimating mirror set at an angle that performs both tasks of focusing and combining the image of an offset reticle. This type works well as a small arms sight. It is also possible to place the reticle between the viewer and the curved mirror at the mirror’s focus (Fig. 3). The reticle itself is too close to the eye to be in focus but the curved mirror presents the viewer with an image of the reticle at infinity. This type was invented by Dutch optical engineer Lieuwe Van Albada in 1932 originally as a camera viewfinder and has also been used as a gunsight on WW2 bazooka
Bazooka
Bazooka is the common name for a man-portable recoilless rocket antitank weapon, widely fielded by the U.S. Army. Also referred to as the "Stovepipe", the innovative bazooka was amongst the first-generation of rocket propelled anti-tank weapons used in infantry combat...
s.
Reflector sights are generally non-focusing sights, simply a projected reticle bounced off a beam splitter or curved mirror right into the users eye. This gives them the defining characteristics of not needing considerable experience and skill to use, as opposed to simple mechanical sights such as iron sights
Iron sights
Iron 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...
. This also gives them the characteristic of being a better alternative to sighting devices based on optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
s in the aspects of field of view and eye relief in that reflector sights actually have neither: depending on design constraints their field of view is the users naked eye field of view, the sight simply puts a reticle right in the middle of it, and their non-focusing collimated nature means they have no eye relief. There are reflector type sights that are telescopic
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
, either by design or via add on lens kits, but they re-introduce the problems of narrow field of view and limited eye relief. The primary drawback of reflector sight is that they need some way to illuminate the reticle to function. Reticles illuminated by ambient light are hard to use in low light situations, and sights with electrical illumination stop functioning all together if that system fails.
History
The idea of a reflector sight originated in 1900 with IrishIrish people
The Irish people are an ethnic group who originate in Ireland, an island in northwestern Europe. Ireland has been populated for around 9,000 years , with the Irish people's earliest ancestors recorded having legends of being descended from groups such as the Nemedians, Fomorians, Fir Bolg, Tuatha...
optical designer
Optical engineering
Optical engineering is the field of study that focuses on applications of optics. Optical engineers design components of optical instruments such as lenses, microscopes, telescopes, and other equipment that utilizes the properties of light. Other devices include optical sensors and measurement...
and telescope maker Sir Howard Grubb
Howard Grubb
Sir Howard Grubb FRS was an optical designer from Dublin, Ireland. He was head of a family firm that made large optical telescopes, telescope drive controls, and other optical instruments...
in patent No.12108. Grubb conceived of his “Gun Sight for large and small Ordnance” as a better alternative to the difficult to use iron sight while avoiding the 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...
's limited field of view, greater apparent target speed, parallax errors, and the danger of keeping the eye against an eye stop. In the 1901 THE “SCIENTIFIC TRANSACTIONS OF THE ROYAL DUBLIN SOCIETY” He described his invention as:
- It would be possible to conceive an arrangement by which a fine beam of light like that from a search light would be projected from a gun in the direction of its axis and so adjusted as to correspond with the line of fire so that wherever the beam of light impinged upon an object the shot would hit. This arrangement would be of course equally impracticable for obvious reasons but it is instanced to show that a beam of light has the necessary qualifications for our purposes.
- Now the sight which forms the subject of this Paper attains a similar result not by projecting an actual spot of light or an image on the object but by projecting what is called in optical language a virtual image upon it.
It was noted soon after its invention that the sight could be a good alternative to iron sights and also had uses in surveying and measuring equipment. The reflector sight was first used on German fighter aircraft in 1918 and widely adopted on all kinds of fighter and bomber aircraft in the 1930s. By World War II the reflector sight was being used on many types of weapons besides aircraft, including anti-aircraft guns, naval guns, anti tank weapons, and many other weapon where the user needed the simplicity and quick target acquisition nature of the sight. Through their development in the 30’s and into WWII the sight was also being referred to in some applications by the abbreviation “reflex sight”.
Weapon sights
Reflector sights were invented as an improved gun-sight and since their invention they have been adapted to many types of weapons. When used with different types of guns, reflector sights are considered an improvement over simple iron sights (sights composed of two spaced metal aiming points that have to be aligned). Iron sights take considerable experience and skill in the user who has to hold a proper eye position and simultaneously focus on the rear sight, the front sight, a target at different distances, and align all three planes of focus to achieve a hit. The reflector sight’s single parallax free virtual image in focus with the target removes this aiming problem, helping poor, average, and good shooters alike.Since 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 produced by the sight is only truly parallax free at infinity, the sight has an error circle equal to the diameter of the collimating optics for any target at a finite distance. Depending on the eye position behind the sight and the closeness of the target this induces some aiming error. For larger targets at a distance (given the non-magnifying, quick target acquisitions nature of the sight) this aiming error is considered trivial. On small arms aimed at close targets this is compensated for by keeping the reticle in the middle of the optical window (sighting down its 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 manufactures of small arms sights also make models with the optical collimator set at a finite distance. This gives the sight 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"...
due to eye movement the size of the optical window at close range which diminishes to a minimal size at the set distance (somewhere around a desired target range of 25-50 yards).
Compared to standard telescopic sights, a reflector sight with no magnification can be held at any distance from the eye (don’t require a designed 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...
), and at almost any angle, without distorting the image of the target or 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
Depth perception
Depth perception is the visual ability to perceive the world in three dimensions and the distance of an object. Depth sensation is the ability to move accurately, or to respond consistently, based on the distances of objects in an environment....
and full field of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
. Since Reflector sights are not dependent on eye relief, they can theoretically be placed in any mechanically-convenient mounting position on a weapon.
Aircraft sights
The earliest record of the reflector sight being used with fighter aircraft was in 1918. The optical firm of Optische Antal Oigee of Berlin, working from the Grubb patents, developed 2 versions what came to be known as the Oigee Reflector Sight. Both used a 45 degree angle glass beam splitter and electrical illumination and were used to aim the plane's machine guns. One version was used in operational trials on the Albatros D.Va fighter biplane and Fokker Dr.1 fighter triplane. There was some interest in this sight after World War I but reflector sights in general were not widely adopted for fighter and bomber aircraft until the 1930s, first by the French, then by most other major airforces. These sights were not only used for aiming fighter aircraft, they were used with aircraft defensive guns and in bombsights.Reflector sights as aircraft gun-sights have many advantages. The pilot/gunner need not position their head to align the sight line precisely as they did in two-point mechanical sights, head position is only limited to that determined by the optics in the collimator, mostly by the diameter of the collimator lens. The sight does not interfere with the overall view, particularly when the collimator light is turned off. Both eyes may be used simultaneously for sighting.
Deflection (military)
Deflection is a technique used for effectively firing a ranged weapon at a moving target, that describes "leading the target"; that is, shooting ahead of a moving target so that the target and projectile will collide...
determined by input from a gyroscope. 1939 saw the development by the British of the first of these gyro gunsight
Gyro gunsight
A gyro gunsight is a modification of the non-magnifying reflector sight in which target lead and bullet drop are allowed for automatically, the sight incorporating a gyroscopic mechanism that computes the necessary deflections required to ensure a hit on the target...
s, reflector sights adjusted by gyroscope for the aircrafts speed and rate of turn, enabling the display of a lead-adjusted sighting reticle that lagged the actual "boresight" of the weapon(s), allowing the boresight to lead the target in a turn by the proper amount for an effective strike
As reflector sight designs advanced after World War II, giving the pilot more and more information, they eventually evolved into the head up display (HUD). The illuminated reticle was eventually replaced by a video screen at the focus of the collimating optics that not only gave a sighting point and information from a lead-finding computer and radar, but also various aircraft indicators (such as an artificial horizon, compass, altitude and airspeed indicators), facilitating the visual tracking of targets or the transition from instrument to visual methods during landings.
Firearm sights
The idea of attaching a reflector sight to a firearm has been around since the sight's invention in 1900. Soon after World War II models appeared for rifles and shotguns including the Nydar shotgun sight (1945), which used a curved simi-reflective mirror to reflect an ambient lit reticle, and the Giese electric gunsight (1947), which had a battery powered illuminated reticle. Later types included the Qwik-Point (1970) and the Thompson Insta-Sight. Both were a beam spliter type reflector sights that used ambient light: illuminating a green crosshair in the Insta-Sight, and a red plastic rod “light pipe” that produced a red aiming spot reticle in the Qwik-Point .Red dot sight
A red dot sight is a common classification for a type of non-magnifying reflector sight for firearms that gives the user an aimpoint in the form of an illuminated red dot...
", a type that gives the user a simple bright red dot as an aiming point. The typical configuration for this sight is a compact curved mirror reflector design with 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 its focus. Using an 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 mirror can use a 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 to reflect just the red spectrum allowing it to pass through most other light. 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 of hours.
Reflector sights for military firearms (usually referred to by the abbreviation “reflex sight”) took a long time to be adopted. The US House Committee on Armed Services noted as far back as 1975 on the suitability of the use of reflex sight for the M16 but the US military did not introduce a reflector sights until 2000 with 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...
red dot sight, designated the “M68 Close Combat Optic”
Reticle types
Many reticle illumination and pattern options are available. Common light sources used in firearm reflector sights include batteryBattery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
powered lights, fiber optic light collectors, and even tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
capsules. Some sights are specifically designed to be visible when viewed through night vision devices. The color of a sight reticle is often red or amber for visibility against most backgrounds. Some sights use a chevron or triangular pattern instead, to aid precision aiming and range estimation, and still others provide selectable pattern.
Minute 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...
. 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.
Most 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
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, while a brighter reticle will display more clearly in full sunlight.
Configurations
Modern optical reflector sights designed for firearms and other uses fall into two housing-configurations: tubed and open.- Tube sights look similar to standard 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...
s, with a cylindrical tube containing the optics. Many tube sights offer the option of interchangeable filters (such as polarizing or haze-reducing filtersUV filterUV filters are individual compounds or mixtures to prevent ultraviolet light from getting through. UV filters are used in sunscreens to protect skin or in photography to reduce the level of ultraviolet light that strikes the recording medium....
), 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 the reflector sight's only optical element, the optical window, doesn’t need any housing at all. This configuration consisting of a base with just the necessary reflective surface for collimating the reticle mounted on it. Due to their diminished profile, open sights do not usually accommodate filters and other accessory options typically supported by tube designs.
Other uses
Reflector sights are also used on some telescopeTelescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s, to help aim the telescope at the desired object. Albada type sights are used on "Point and shoot" type camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
s and on simple disposable cameras. They have also been used in nautical navigation devices and surveying equipment.
Similar types
- Collimator sightCollimator sightA collimator sight is a type of optical sight that allows the user looking into it to see an illuminated aiming point aligned with the device the sight is attached to regardless of eye position...
s (also called collimating or "occluded eye gunsight" (OEG)) are simply the optical collimator focusing a reticle without any optical window. The viewer cannot see through them and only sees an image of the reticle. They are used either with both eyes open while one looks into the sight, with one eye open and moving the head to alternately see the sight and then at the target, or using one eye to partially see the sight and target at the same time. The reticle is illuminated by an electric or a passive ambient light source. The Armson OEG and the Normark Corp. Singlepoint are two examples of commercially available ambient lit collimator sights .
- 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...
s are similar in layout to reflector sights but do not use a projected reticle system. Instead a representative reticle is recorded in three-dimensional spaceThree-dimensional spaceThree-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
onto holographicHolographyHolography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...
film at the time of manufacture. This image is part of the optical viewing window. The recorded hologram is illuminated by a collimated laserLaserA laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
built into the sight. The sight can be adjusted for range and windageWindageWindage is a force created on an object by friction when there is relative movement between air and the object.There are two causes of windage:# the object is moving and being slowed by resistance from the air...
by simple tilting or pivoting the optical window.
Further reading
- Journal of the Military Service Institution of the United States: Volume 37 - Page 552 -THE GRUBB GUN SIGHT
- The Mining engineer, Volume 23 By Institution of Mining Engineers (Great Britain). THE GRUBB SIGHT FOR SURVEYING INSTRUMENTS By Sir HOWARD GRUBB and HENRY DAVIS
- http://books.google.com/books?id=pIzpAAAAMAAJ&pg=PA353&lpg=PA353&dq=Transactions+of+the+Royal+Dublin+Society+March+20+1901&source=bl&ots=SoFiUSEcXx&sig=NG5Wek7LoxTHzY1hPxgw_Z076b0&hl=en&ei=wbGXTd-_MuGX0QGg0uz6Cw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBcQ6AEwAA#v=onepage&q=grubb&f=falseThe scientific transactions of the Royal Dublin Society By Royal Dublin Society - A NEW COLLIMATING TELESCOPE GUN SIGHT FOR LARGE AND SMALL ORDNANCE By SIR HOWARD GRUBB FRS Vice President Royal Dublin Society]
- 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. (early patent for a “red dot sight”)
External links
- Youtube video of a WWII US Navy Mark VIII reflector sight
- Youtube video of a 1942 Japanese type 98 reflector sight in action
- Article on the WWII Maxon M45 machine gun mount with section on the Navy Mark 9 reflector sight
- Youtube video of a Red Dot Sight on an Airsoft gun
- May-June, 2007 CBS Interactive Business Network article: Seeing red: illuminated reticle sights