Reticle Red

How Red Dot Sights Work

The optical collimator makes the reflex sight possible. This takes the light from a source and aligns it in parallel The collimator can be refractive or reflective 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 dichroic mirror or beam splitter coated so as to reflect only light of the colour of the reticle and to let all other light pass through. This results in a perfect image of the target with the aiming mark projected to infinity.

Holographic reflex sights substitute 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 has the benefit that the entire reticle is free from parallax and not just its centre. This prevents parallax problems that can make shooting from different positions problematic. Another advantage is that the entire aiming mark is visible evan if the window is partially obscured or even shattered. However, laser diodes drain batteries far faster than LEDs – more than a hundred times faster in some cases, meaning that holographic sights are designed to turn off automatically – typically after four or eight hours.

The configuration the sight is largely dependent on the position of the collimator. 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 machine guns with an inline configuration, but less advantageous with others. It can also make for a somewhat less rugged sight. Side mounting makes tube-construction possible. Tubes are inherently immensely strong and make for immensely robust sights with adjusters that can be switched from right to left hand operation by simply turning the sight in its  rings.

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, with 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 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 CompM4s from Aimpoint (the most recent incarnation of the US Army M68) for example uses clicks that are 16mm at 100 meters or 1/2″ at 80 yards whilst the USMC’s AN/PVQ-31B has clicks that move the point of impact 0.33″ at 100 yards.

Amost all red dots sights feature variable brightness levels for the aiming mark. Night vision compatible sights have a wider range of adjustments to suit this role. Adjustment is set automatically in some models, either electronically or by the amount of light the fiber-optic light gatherer can take in from the environment. The latter system can be problematic when shooting from bunkers or rooms that are poorly illuminated into brightly lit terrain. A polarising filter can be employed on some models to reduce the brightness of the image – rotating two polarised lenses can graduate the image from complete brightness to complete black-out. As the dichroic mirror or beam splitter is itself often polarised, fitting one such filter can have the same effect. This could be problematic with polarised glasses, but the leading makes all now select polarisations that do not conflict with sunglasses.

It is now commonplace to ‘co-witness’ iron sights through non magnifying red dot sights. It is important to realise that it of no consequence where the reflex aiming mark is relative to the foresight post as long as both are zeroed to hit the target. Some shooters attempt to shim the sight to get the red dot to sit directly above the foresight, but this is completely pointless. 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 permit this; for example their Model 557.

Copyright Chris Pieterman 2010

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