Since guns were invented, shooters struggled with shooting both quickly and accurately. Initially, the problem was getting multiple shots off quickly and reliably, but rifles developed toward the middle of the 20th century took care of that. However, there was still the problem of making a sight that will let you acquire your aim quickly in low light when traditional iron sights are almost invisible.
As is usually the case, a lot of the appropriate technologies were first tried on larger weapons where cost and size are less of a concern. Eventually, some of these were miniaturized enough to be used on small arms. One such technology is an occluded eye gunsight (OEG), with the Singlepoint being the first commercially viable example.
Singlepoint’s moment of fame came during the Vietnam War when it was used by U.S. troops in the Son Tay prison camp raid in 1970. Arguably, the success of Singlepoint gave rise to the popularity of a somewhat similar Armson OEG that is still made today, the rise of Trijicon’s Bindon Aiming Concept (BAC) and to the enormous variety of red-dot sights that have largely replaced OEGs in common use.
The OEG concept takes advantage of the peculiarities of our binocular vision. A human brain takes image data from both eyes and seamlessly superimposes it. Generally, most of it comes from our dominant eye, with the other eye providing some additional detail and field of view.
An OEG tricks our vision into doing something a little different. Occluded means “blocked” or “obstructed.” You cannot see through an OEG. All it does is project a bright aiming point to your dominant eye that looks like it is fairly far away so your eye can still be focused on a distant object.
However, if both of your eyes are open and your other eye is focused on the target, the brain will superimpose the bright red dot from the dominant eye onto the image of the target from the nondominant eye. The important thing is to not try to focus on the dot. This method works for high-speed engagements where you keep your focus on the target. If you try to focus on the dot, it all falls apart.
Not For Everyone
OEGs do not work for everyone. Some people do not have very good binocular vision (many are born that way, others have had some sort of an eye injury). This also does not work for people who are cross-eye dominant (i.e., right handed with left-eye dominance) if they shoot with the dominant side.
There are other problems with OEGs. The first one is parallax. It’s due to the distance between the eyes, so there is an offset between the point of aim and point of impact. Another is that the sight-in is individual. An occluded sight setup for one person is unlikely to be properly sighted in for another.
Aside from these issues, OEGs are extremely fast and work great for center-mass hits at close range. For me, 25 to 30 yards is the practical limit for reasonably accurate OEG use, but I know of people who are successful with them out to around 100 yards.
The aiming dot in the Singlepoint is generated via a fiber optic collector, but some have also been equipped with tritium vials for nighttime use. It is not clear if the ones used in Son Tay raid had tritium, but I would not be surprised if they were.
Both Singlepoint and Armson OEGs do not rely on batteries which, historically, has been one of their strengths. However, modern reflex sights with batteries that last for years (and some that are solar powered) largely superseded OEGs in common use and for a good reason.
Besides, to use a red-dot sight as an OEG, all you have to do is cover over the front of the sight. Still, the idea of shooting quickly with both eyes open started with occluded gun sights and continued with red dot and holographic sights we use today.
Interestingly, Trijicon got its start thanks to this technology. They were the importer of the South African-made Armson O.E.G.s between 1980 and 2000. On top of that, the Bindon Aiming Concept, popularized with the original 4x32mm ACOG, sort of works based on the same principles as OEG: when moving, your dominant eye, looking through the magnified ACOG, cannot make heads or tales out of the image, but the bright illuminated reticle comes through clear as day. That gets superimposed onto the image your brain gets from the other eye.
However, when you stop moving, your dominant eye takes over and you see the magnified image. If you flip the objective cover on, this works fine with any scope or red dot as long as the aiming point is bright enough.
That is one of the reasons I tend to prefer a single illuminated dot in any riflescope that is intended for general purpose use. In a pinch, I can close the objective cap and have a perfectly viable OEG for shooting at distances where magnification gets in the way.