Reflex sights use the optical collimator. The collimator aligns a stream of particles from a source into a parallel stream. The collimator employed can be of refractive or reflective configuration. The collimator takes the image of the reticle illuminated by a radioactive or fiber-optic light source or an LED and projects it to infinity. The image is projected onto a specially coated dichroic mirror or beam splitter which lets all other wavelengths pass 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 reticle projected on to it.
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 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 1X power as this is much better for both-eyes open operation and fast shooting. There is a trade off in reticle size, the larger the aiming mark the faster it can be acquired, but the more it obscures the target making long range engagement difficult. Magnifying reflex sights are less common but have achieved great popularity, particularly with the US armed forces, offering the combination of a bright illuminated aiming mark for close ranges and a traditional 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 strategy is typified by the Trijicon ACOG, bought in huge numbers by the USMC and US Army. 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. Another solution is to put a non magnifying red dot sight on top of the magnifying one – an approach adopted with success by Trijicon with their 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. This is particularly important in those sights intended for use with night vision. The adjustment can be automatic – either electronic or via a fiber optic light gatherer that draws in light from its surroundings. That can create problems – for example when shooting into brightly lit terrain from a darkened room or bunker. 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 sight itself often incorporates a polarizing filter, mounting a single adjustable polarizing filter can have the same effect. This could be problematic with polarized eyewear, but the leading makes all now choose polarizations that do not conflict with sunglasses.
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 ACOGs please visit Combat Optics Reviews Dot Com which has more information on the TA31RCO-M4CP ACOG.
categories: red dot sights,holographic sights,reflex sights,rifle sights,rifle scopes,telescopic sights,hunting,shooting,firearms,weapons,guns,military,war