The optical collimator makes the red dot sight possible. This takes the light from a source and aligns it in parallel. Either refractive or reflective collimators are employed. An image of the reticle, generated by the collimator and illuminated by a radioactive or fiber-optic light source or LED, is projected into the distance. This image is directed onto a dichroic mirror or beam splitter covered with a special reflective coating so as to only reflect the specific colour of the reticle whilst permitting all other colours to pass through. This results in a perfect image of the target with the aiming mark seemingly on the target.
Holographic reflex sights instead use a holographic image of the aiming mark or reticle created by a laser diode. Holographic reflex sights use a laser to generate a holographic reticle or aiming mark which the collimator then projects to infinity. This results in an aiming mark that is optically centered in its entirety. This prevents parallax problems that can make shooting from different positions problematic. This makes the sight invulnerable 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. However, laser diodes drain batteries far faster than LEDs – more than 100X faster in some cases. Therefore, holographic sights are designed to turn off automatically – usually after four or eight hours (this is selectable in Eotech models).
The position of the collimator dictates the configuration of the sight. The bottom mounted collimator makes for a less bulky sight with a small protruding screen. This can put the sight line well above the bore – a perfect configuration with M4s or other rifles or support weapons with an inline configuration, but less advantageous with others. It can also result in a less robust sight as the protruding window can be vulnerable. Side mounted collimators lend themselves to tube construction, like a scope. Tubes are inherently very strong and make for immensely rugged 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. Magnifying reflex sights have become ubiquitous in the War on Terror where target identification is at a premium. A bright illuminated aiming mark is used for close ranged shooting, backed by a traditional ‘ladder’ reticle for more distant targets. This philosophy is exemplified by the Trijicon ACOG, the choice of the US and British armed forces. However 1x magnification or ‘unitary’ reflex sights can be boosted by with detachable or flip to side magnifiers to provide a very versatile combination. Trijicon have gone the opposite route by attaching peep battlesights and/or small red dot sights to the top of their most recent ACOGs.
It is almost universal for reflex sights to use internal adjustment. The sight is mounted firmly to the weapon and the optics move internally to move the point of impact. The adjusters are calibrated so that you know how far a graduation or click will move the point of impact. The calibrations vary from manufacturer to manufacturer and model to model. The Aimpoint CompM4s (the latest US Army M68) for example utilizes clicks that are 16mm at 100 meters or 1/2″ at 80 yards whilst the US Marines’ Trijicon TA31RCO / AN/PVQ-31B ACOG has clicks that adjust the point of impact 0.33″ at 100 yards.
The sights have to cope with different brightnesses of daylight and thus feature variable reticle brightness settings. Some have night vision compatible ranges of settings. The adjustment is sometimes automatic. This can be electronic or via a fiber optic light gatherer that captures light from its surroundings as on many ACOGs. The latter system can cause problems when shooting from structures that are poorly illuminated into brightly lit terrain. Polarizing filters are used on some sights to adjust the brightness of the image – rotating two polarized filters will gradually reduce the image to complete black-out. As the sight itself often incorporates a polarizing filter, mounting a single adjustable polarizing filter can have the same effect. In the past this caused problems with protective eyewear which was also polarized, but the leading manufacturers now use polarities selected so as not to obscure the target when used with polarized glasses.
It is now standard practice to ‘co-witness’ iron sights through non magnifying red dot sights. It is not necessary to align the aiming mark 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 facilitate this; for example their 557.AR223.
Chris Pieterman is a gun enthusiast with 30 years experience of red dot sights. For more information on holographic sights please visit Combat Optics Reviews Dot Com which has more information on the TA31RCO-M4CP ACOG.