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How A Red Dot Sight Works
Red dot sights employ an optical collimator. Collimators align rays of light from a source in parallel. Refracting and reflecting collimators can be found. The collimator generates an image of a dot or reticle illuminated by an LED, radioactive or fiber optic light source and projects it to infinity. The image is projected onto a specially coated dichroic mirror or beam splitter which lets all other wavelengths through to let the firer see the target, but reflects the colour employed by the reticle. The result is a perfect image of the target with the aiming mark apparently superimposed on it.
Holographic reflex sights use a laser hologram to project the image of the reticle into the target plane. This has the advantage that the entire reticle is free from parallax and not just its centre. This avoids parallax problems caused by variations in eye position relative to the aiming window. This makes the sight all but immune to all but total obscuration or removal of the aiming window as the reticle can be seen if any part of the window is visible. The main disadvantage of the holographic sight is its laser diode's greater power consumption than an LED - battery lives being measured in hundreds rather than tens of thousands of hours.
The shape of the sight is largely a result of the position of the collimator. The bottom mounted collimator makes for a less bulky sight with a small protruding screen. This can result in a sight line well above the axis of the bore which works well with Stoner designed rifles but not as well with most others. It can also result in a less robust sight as the protruding display can be vulnerable. Side mounted collimators enable a tube to be employed to contain the workings of the sight. Tubes are inherently immensely strong and result in immensely robust sights with adjusters that can be switched from right to left hand operation by simply turning the sight in its mount.
Reflex sights tend to be 1X magnification as this is best for close quarters, both eyes open shooting. The reticle or red dot size is dictated by the application - a large dot will be easy to acquire and place on target, but it will also obscure much of the target and thus be hard to center accurately, especially at longer ranges. However, magnifying red dot sights exist which take advantage of reflex operation to combine a large illuminated aiming point for fast target engagement with a traditional ballistically calibrated ladder reticle for long range precision. A bright illuminated aiming mark is used for close ranged shooting, backed by a traditional reticle for longer ranged engagements. This philosophy is exemplified by the Trijicon ACOG, adopted as standard across the US armed forces. The 1X sights can often be instantly turned into scopes and back again by the addition of flip to side magnifiers, making them an extremely versatile combination. The ubiquitous ACOG can attain similar versatility with the mounting of peep battlesights and/or a small rear mounted red dot sight on their housings.
Adjustments are typically internal, the sight remaining in a fixed position whilst the reticle is moved up and down with adjusters. The adjustments are calibrated and almost always take the form of audible or tactile clicks. The calibrations vary from manufacturer to manufacturer and model to model. The CompM4s from Aimpoint (the most recent incarnation of the US Army M68) for example employs clicks that are 16mm at 100 meters or 1/2" at 80 yards whilst the US Marines' AN/PVQ-31B / TA31RCO has clicks that move the point of impact 0.33" at 100 yards.
Amost all reflex sights feature variable brightness levels for the aiming mark. Some have night vision compatible ranges of settings. Some units will adjust automatically, either employing sensors or a fiber-optic light gatherer that pulls in light from its surroundings. The latter system can cause problems when firing from bunkers or rooms that are poorly illuminated into brightly lit terrain. Polarising filters are employed on some sights to adjust the brightness of the image - rotating two polarised filters will gradually reduce the image to complete black-out. As the dichroic mirror or beam splitter is itself often polarised, fitting one such filter can have a similar effect. In the past this caused problems with protective eyewear which was also polarised, but the leading manufacturers now use polarities chosen so as not to obscure the target when used with polarised glasses.
It is now commonplace to 'co-witness' iron sights through non magnifying reflex sights. It is unnecessary to align the red dot to sit on top of the foresight or anywhere else - it only matters that both systems are zeroed on the target. A popular method is to have the iron sights in the bottom 1:3 of the sight picture and manufacturers like Eotech are starting to make sights with optional risers to enable this; for example their 557.AR223.
Copyright Chris Pieterman 2010
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