How Does A Red Dot Sight Work?
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 used. An image of the reticle, generated by the collimator and illuminated by a radioactive or fiber-optic light source or LED, is projected to infinity. 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 literally on the target.
Holographic reflex sights use a laser hologram to project the image of the reticle into the target plane. This results in an aiming mark that is optically centered in its entirety. This in turn avoids parallax problems caused by variations in eye position relative to the aiming window. It also means partial obstruction of the aiming window will not prevent the shooter seeing the reticle in its entirety. However, laser diodes exhaust 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 location of the collimator. If it is mounted beneath you end up with a sight that is less bulky, with the sighting screen taking the form of a mini head up display. This configuration can have the drawback of putting the sight line way above the bore – this works well with AR15 rifles or others with a straight-through stock configuration, but is disadvantageous with other configurations. These configuration can also be less rugged. Side mounted collimators enable a tube to be used to contain the workings of the sight. Tubes are inherently very 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.
Most reflex sights are non magnifying as this is highly advantageous for both-eyes open operation and rapid target engagement. There is a trade off in reticle size, the larger the aiming mark the more easily it can be acquired and placed on the target, but the more it obscures the target making long range accuracy problematic. 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, with a traditional reticle for longer ranged engagements. This philosophy is exemplified by the Trijicon ACOG, adopted as standard across the US armed forces. However 1x magnification or ‘unitary’ reflex sights can often be fitted with detachable or flip to side magnifiers to provide a very versatile combination. Another approach is to ‘piggy back’ a non magnifying red dot sight on top of the magnifying one – a strategy adopted by Trijicon in the latest ACOGs.
It is most common 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 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 Aimpoint CompM4s (the most recent incarnation of the US Army M68) employs 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 displace the MPI 0.33″ at 100 yards.
Brightness of the reticle can be adjusted to suit varying light conditions. Some have night vision compatible ranges of settings. 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 firing from bunkers or rooms that are not illuminated into brightly lit surroundings. A polarising filter is employed on some units to cut down glare in bright conditions – rotating two polarized lenses can adjust the image from complete brightness to complete black-out. As the dichroic mirror or beam splitter is itself often polarized, fitting one such filter can have the same effect. It was discovered that this could cause problems with sunglasses, goggles etc. which were also polarized, but the leading companies now use polarities chosen so as not to obscure the target when used with polarized glasses.
Shooters now typically ‘co-witness’ their iron sights with their non magnifying red dot sight. It is unnecessary 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 permit this; for example their Model 557.
Chris Pieterman is a gun enthusiast with 30 years experience of red dot sights. For more information on holographic weapon sights please visit Combat Optics Reviews Dot Com which has more information on the Aimpoint CompM4s red dot sight.
Processing your request, Please wait....
