The optical collimator makes the reflex sight possible. The collimator directs a stream of particles from a source into a parallel stream. The collimator can be refractive or reflective. An image of the reticle, generated by the collimator and illuminated by a fiber-optic or radioactive light source or LED, is projected to infinity. This image is directed onto a dichroic mirror or beam splitter which uses a special reflective coating so as to reflect only the exact colour of the reticle whilst allowing all other colours to pass through. This produces a perfect image of the target with the aiming mark projected to infinity.
Holographic reflex sights instead use a holographic image of the aiming mark or reticle produced 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 configuration of the sight is largely a result of the position of the collimator. If it is mounted beneath you can have a sight that is less bulky, with the sighting screen essentially a mini head up display. This configuration can have the drawback of putting the sight line high up - this suits AR15 rifles or others with a straight-through stock configuration, but works poorly with other configurations. These configuration can also be less sturdy. Side mounting makes tube-construction possible. Tubes make for immensely rugged sights and are inherently ambidextrous as you can simply rotate the sight 90 degrees in its rings to switch the adjusters from azimuth to elevation.
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.
Red dot sights almost always use internal adjustment. The sight is mounted firmly to the weapon and the internal optics move to shift the point of impact. These are calibrated and, in nearly all models feature audible/tactile clicks. The calibrations vary from manufacturer to manufacturer and model to model. The Aimpoint CompM4s (which recently became the latest US Army M68) for example has clicks that are 16mm at 100 meters or 1/2" at 80 yards whilst the USMC's Trijicon TA31RCO / AN/PVQ-31B ACOG uses clicks that move the point of impact 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.
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.
Holographic reflex sights instead use a holographic image of the aiming mark or reticle produced 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 configuration of the sight is largely a result of the position of the collimator. If it is mounted beneath you can have a sight that is less bulky, with the sighting screen essentially a mini head up display. This configuration can have the drawback of putting the sight line high up - this suits AR15 rifles or others with a straight-through stock configuration, but works poorly with other configurations. These configuration can also be less sturdy. Side mounting makes tube-construction possible. Tubes make for immensely rugged sights and are inherently ambidextrous as you can simply rotate the sight 90 degrees in its rings to switch the adjusters from azimuth to elevation.
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.
Red dot sights almost always use internal adjustment. The sight is mounted firmly to the weapon and the internal optics move to shift the point of impact. These are calibrated and, in nearly all models feature audible/tactile clicks. The calibrations vary from manufacturer to manufacturer and model to model. The Aimpoint CompM4s (which recently became the latest US Army M68) for example has clicks that are 16mm at 100 meters or 1/2" at 80 yards whilst the USMC's Trijicon TA31RCO / AN/PVQ-31B ACOG uses clicks that move the point of impact 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.
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.
About the Author:
Chris Pieterman is a gun enthusiast with 30 years experience of red dot sights. For more information on red dot sights please visit Combat Optics Reviews Dot Com which has more information on the Aimpoint CompM4s red dot sight.
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