I just want to make sure I'm not making any mistakes in this short article, so critiques are welcome.
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Holographic weapons sights have become increasingly popular with shooters, as they give the shooter the ability to quickly lock in on target, even if the shooter's head position is not aligned with the sight.
With a conventional optical scope, a shooter must have his eye aligned with the scope in order to place the reticle over the target. If the shooter's head isn't perfectly aligned, the scope's reticle isn't pointing to the same spot on the target as the gun.
With a holographic ("holo") sight, the reticle is always on the spot on the target where the weapon is pointed. Move the gun left or right, and the reticle moves as well, but is still on the point of aim.
How is this possible?
Unlike the view you see through a conventional scope, what you are seeing through a holographic sight is not real. It's a reconstruction of the view. What's more, the reticle that you see is not actually in the sight, but is a projection of a reticle image.
Before you start scratching your head, think about watching a movie. The movie camera has recorded the the light reflected from the objects in a scene onto film. When the film is projected onto a movie screen, you see the scene that was originally recorded.
The process of holography involves reconstructing the light waves that are reflected from an object. What we commonly call "light" is a spectrum of waves. The holographic sight encodes the wave patterns reflected from the view of the target area, and projects these wave patterns onto a clear window within the sight. The projected wave patterns are then illuminated by a laser, which reconstructs the wave patterns. The result is a three-dimensional image of the view of the target area.
Unlike a film, though, the holo sight is recording and then projecting the light waves in real time onto the clear window within the sight. As you move your sight up or down, or left or right, the sight is reconstructing the view instantly.
OK, you say, but how does the reticle move and stay on target?
The reticle is actually a laser beam that's being projected onto that same clear window within the sight. A reticle mask defines the shape of the reticle. The mask may be a dot, a triangle, a circle, or whatever other shape the manufacturer offers.
When the holographic sight is attached to the gun, the laser beam is projected along the same axis as the gun. Thus, when you move the muzzle of the gun to the left, the projected reticle image on the clear window moves to the left on the same axis. Up, down, left, right: the reticle image is always following the direction of the muzzle.
So, when you look through a holographic sight, you're actually seeing two things: the projected view of the target area, which changes as you move the sight; and the projected reticle, which moves along with the axis of the gun. The result of this combination is that, as long as you can see the view of the target area through the sight, and as long as you can see the reticle, you can get your gun on target.
The laser reticle is essentially performing the same function as a laser sight, except that the laser is projected onto the clear window, and not onto the target itself. This offers several advantages, one of which is that the laser beam is not visible to anyone but the shooter. Another advantage is that most holographic sights allow the user to vary the brightness level of the laser reticle, while the dots from laser sights may be hard to see on the target in bright sunlight.
With advances in technology, holographic sites have become more compact and more affordable, thus leading to their increased popularity with the shooting community.
