Great 1st post but dang I got a head ache now...lolI know this is a bit belated, but I thought I'd shed some light on the topic of holographic sights. It seems like a lot of people on the wide world of the internets are a bit confused as to the true differences between a red dot sight and a true holographic. I'm not a pro at all of this, but I have taken a few courses in lens/mirror/optical physics that ahve REALLY shed a lot of light on the topic. I hope it might help.
Warning, lots of words ahead. I'll try to keep it simple and/or interesting.
So the simpler one first:
Red Dot Sights.
Basically, with a Red Dot Sight you have a lens (usually glass, but some cheap ones are plastic), and a small red laser or LED dot is shined (projected) onto its surface. When you look through the sight, the red laser dot is seen, and usually very clearly. 'Parallax-free' red dot sights are a bit more complex, since they use a lens to'collimate' the light beam, but the concept is the same. Think of collimation of light waves as organizing them so that they are not scattering in all directions. They will all be heading the same direction, right next to each other, making a crisp, non-diffuse dot on the lens.
Pros: It's dead simple in principle. It's also relatively inexpensive to manufacture, and if it uses an LED, its battery life can be incredibly good, a la Aimpoint. They are also often very visible in terms of brightness and crispness.
Cons: If you're in the service and cannot afford to give any visual notification of your presence, a RDS might not be the way to go. Unless you use shielding methods, an onlooker might be able to see your red dot on the lens. Not a deal breaker for 99.9% of us, as well as MANY in the service though, so it's usually not to be worried over too much. They also tend to have smaller optics than holo sights. Were they to have larger viewing area, it would be more likely to introduce parallax to the user when aiming with both eyes open. They are also a bit more prone to wash-out of the reticle than holo sights. If it's bright enough outside, the subject area can be sending a lot of photons into the sight that can compete and mix with the dot being projected (photon interference), making it harder to notice. Lastly, and most importantly, red dot sights require you to be good at getting your head/eyes into the exact same location each time, much like normal iron sights on a shotgun. If you rotate the weapon right or left, and you don't follow it accordingly with your head/cheek/etc, it can drastically affect your accuracy. Try this experiment sometime in your local shop. Take a RDS, and pinch it with your fingers, on either side of the scope. Pinch it exactly on the sides of the lens element where the projected dot hits (usually the lens element closest to your eye if it has multiple lenses). Now look through the sight at a fixed point across the room. Line up the red dot with it. Now SLOWLY twist the sight just a little bit to the right or left, while still staring at that same point across the room. The red dot will stay on that point you are staring at, but the sight will be aimed at a different location. This illustrates the point that the red dot does NOT always mean that the bullet will go to that point. To counteract this, many end up putting on iron sights in a co-witness configuration with their RDS, to make sure that they have the weapon pointed where the red dot says it is pointed.
Now the point of most confusion:
Holographic sights, like EOTech are not as complex as many think. They don't create some crazy, laser-generated image of the entire viewing area. The shooter sees through the lens just as usual. The differences are under the hood. First, a laser beam is generated inside the unit, not outside. It is reflected to a curved collimation mirror to gather the light, and it eventually projects that onto a lens. That sounds like the RDS, doesn't it? Here is where the water gets deep, so I won't try to get too technical. In essence, the image you see of the reticle is 'virtualized' out into space far in front of you. It's not 'real' in an optical physics sense of the word, but it looks like it. It's a bit like when R2D2 projects the message from Princess Leia onto a coffee table. Sort of. An accurate 'fake' image of the reticle is being holographically projected, and it looks like it is sitting in space far out in front of you. It's not projecting a picture of everything you see through the lens, mind you. Just the reticle. It accomplishes this with the sight's beam splitting mirror, and subsequent convergent and divergent optics. In that process, the visible reticle is 'added' into the image you see, it just acts like it is far out in front of you. I can't get much further into this before a basis in optical physics is needed, but here are a few images that might help to get the idea: Attachment 64369Attachment 64370 Since the reticle appears to be projected out in front of you (some say around 50-70 meters or so), it acts much the same as if you were to project a laser bem sight onto your target. It's not doing that, but it allows you to move your head and eyes around while remaining on target. Try that RDS experiment with a holographic sight, and you will see the reticle move around as you rotate it right and left. Awesome, isn't it?! You don't have to be squared up perfectly with your sight every time you look through it! Generally, that means that where you see the red dot, it will shoot there. It's incredibly genius. +1 Science!
Pros: The largest was just illustrated above. The effect is that, when you move the sight independently of your eye location (say, to draw a bead on a target to the side of you very quickly for example), the superimposed reticle will change its location. The reticle will appear to 'move' around in the clear viewing lens, staying on the target. VERY cool stuff if you are in a fast CQB situation. Look through the lens, and wherever you see the reticle, it'll shoot, no matter if you are squared up with your sight or not. Holographic systems are also not 'real' projections, so onlookers won't see the red dot if you're being sneaky. The holographic image is also far less sensitive to being washed out, since it isn't a real projection of light hitting the same optic as ambient light. Strange, but cool. Holo sights also don't need coatings on them like RDS sights do. The lack of coatings allows unadulterated light to enter the lens, making it a more 'natural' image when you are aiming with both eyes open (as you very well should!).
Cons: Some people complained of polarized glasses cancelling out their reticles. The laser light that is used to create the reticle is indeed polarized, so if your polarized glasses are at an an angle that will reject photons coming at that angle, it will disappear. But this has been fixed for years now. They angle the polarized reticle light so that it is not on a common cancelling angle for sunglasses. Not a big issue anymore. Battery life also is far less than with many modern red dot sights. Lasers require a LOT more juice to run than LEDs. They are also expensive. You're very lucky if you can find a used one under $320 USD online.
People also complain regarding both sight types over what I call 'preference' issues. Some like a certain shape or brightness of reticle. Some like this button or that switch to turn it on or off. I won't get into those things, as they have already been covered ad nauseum.
Summary: Both are great. They really are. The holographic sights, in my humble opinion, are the cooler of the two by far, but then again I'm a whore for all things nerdy. Both are very good options, and will serve their owner very well. Try both of them out, and see what you like most. No sight is a 'good' sight if you can't use it properly. Anyway, this post is far too long, especially for my first post here. Hope it helped.