反射式瞄具

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反射式瞄具（reflector sight或reflex sight）是一類用半反射性的玻璃展示標線圖像的光學瞄具，其運作原理是恰好位於透鏡或曲面鏡的焦點/焦平面的圖案在觀察者眼中都像是位於遠處目標上，且沒有視差。

反射式瞄具發明於1900年，起初主要設計用來為航空器上的武器瞄準使用。最早的類似嘗試源於第一次世界大戰末期，但直到第二次世界大戰才得到普及。除了航空機關槍上外，反射式瞄具還廣泛使用於防空機炮、反坦克炮和其它需要應對快速移動目標的武器系統上。二戰結束後，隨著超視距作戰和飛彈在空戰中取代了依靠掃射的狗斗和俯衝式轟炸，反射式瞄具僅在輕兵器上還有使用，其中使用發光二極體成像的紅點鏡因其小型化便於快速瞄準更是廣泛用於各種特種作戰和射擊運動。除了武器瞄準以外，反射式瞄具的工作原理還廣泛應用在抬頭顯示器、光學望遠鏡、測量學儀器和照相機的取景器上。

## 設計

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Reflector sights work by using a lens or an image-forming curved mirror 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 device's 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. 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 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 is most often seen as the red dot type used on small arms. It is also possible to place the reticle between the viewer and the curved mirror at the mirror's focus. 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 was also used as a gunsight on World War II bazookas: the US M9 and M9A1 "Bazooka" featured the D7161556 folding "Reflecting Sight Assembly".

The viewing portion of a reflector sight does not use any refractive optical elements, it is simply a projected reticle bounced off a beam splitter or curved mirror right into the users eye. This gives it the defining characteristics of not needing considerable experience and skill to use, as opposed to simple mechanical sights such as iron sights. A reflector sight also does not have the field of view and eye relief problems of sights based on optical telescopes: depending on design constraints their field of view is the user's naked eye field of view, and their non-focusing collimated nature means they do not have the optical telescopes constraint of eye relief. Reflector sights can be combined with telescopes, usually by placing the telescope directly behind the sight so it can view the projected reticle creating a telescopic sight, but this re-introduces 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 altogether if that system fails.

## 歷史

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The idea of a reflector sight originated in 1900 with Irish optical designer and telescope maker 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 weapons where the user needed the simplicity and quick target acquisition nature of the sight. Through its development in the 1930s and into World War II the sight was also being referred to in some applications by the abbreviation "reflex sight".

## 武器瞄具

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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 focus exclusively on the front sight, keeping it centered on the (unfocused) rear sight, while keeping the whole centered on a target at different distances, requiring alignment of 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 expert shooters alike.

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 manufacturers 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 25–50yd).

Compared to standard telescopic sights, a reflector sight can be held at any distance from the eye (does not 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.

### 航空器

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反射式瞄具最早在1918年就應用在航空機關槍上。柏林的光學器材商Optische Anstalt Oigee利用格拉布的專利研發出了兩款用45度玻璃反射光纖的瞄具，其中一款被用在了信天翁D.Va雙翼飛機和福克Dr.I三翼飛機的試飛上，雖然引起了軍方的一些興趣，但因第一次世界大戰很快結束，所以同規格的反射式瞄具直到1930年代才被用在戰鬥機和轟炸機上。最先在實戰中使用反射式瞄具的是法國，隨後其他各國空軍才開始效仿。除了用於戰鬥機狗斗以外，反射式瞄具還被用在轟炸機的防禦機槍和投彈瞄準上。

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 gunsights, reflector sights adjusted by gyroscope for the aircraft's 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.

### 輕武器

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The idea of attaching a reflector sight to a firearm has been around since its invention in 1900. Soon after World War II, models appeared for rifles and shotguns including the Nydar shotgun sight (1945), which used a curved semi-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 Center Insta-Sight. Both were beam-splitter 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.

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The mid- to late 1970s 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 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, passing through most other light; and 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 as reflex sights) 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 rifle, but the US military did not widely introduce reflector sights until the early 2000s with the Aimpoint CompM2 red dot sight, designated the "M68 Close Combat Optic".

## 標線種類

Many reticle illumination and pattern options are available. Common light sources used in firearm reflector sights include battery 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 patterns.

Sights that use dot reticles are almost invariably measured in minutes of arc, sometimes called "minutes of angle" or "moa". Moa is a convenient measure for shooters using Imperial or US customary units, since 1 moa subtends approximately 1in at a distance of 100yd, which makes moa a convenient unit to use in ballistics calculations. A 5 moa (1.5 milliradian) 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 has traditionally been preferred; 7, 10, 15 or even 20 moa (2, 3, 4.5 or 6 mil) have been 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.

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 sights, with a cylindrical tube containing the optics. Many 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 the reflector sight's only optical element, the optical window, does not need any housing at all. This configuration consists 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.

## 其它應用

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Reflector sights have been used over the years in nautical navigation devices and surveying equipment. Albada type sights were used on early large format cameras, "Point and shoot" type cameras, and on simple disposable cameras.

These sights are also used on astronomical telescopes as finderscopes, to help aim the telescope at the desired object. There are many commercial models, the first of which was the Telrad, invented by amateur astronomer Steve Kufeld in the late 1970s. Others are now available from companies such as Apogee, Celestron, Photon, Rigel, and Televue.

Reflector sights are also used in the entertainment industry in productions such as live theater on "Follow Spot" spotlights. Sights such as Telrad's adapted for use and the purpose built Spot Dot allow the spotlight operator to aim the light without turning it on.

## 外部連結

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- Article on the WWII Maxon M45 machine gun mount with section on the Navy Mark 9 reflector sight
- May-June, 2007 CBS Interactive Business Network article: Seeing red: illuminated reticle sights