A reflex type optic generally uses an LED for the light source of the dot. This type of sight reflects light off a specialized mirror that reflects a narrow bandwidth visible to the shooter.
Imagine a person sitting in a living room next to a reading lamp at twilight. A window is nearby. One can see the reflection of the lamp in this window. If a person was outside, the seated reader could put the reflection of the reading lamp on that person's forehead and cause that reflection to follow the outside person around.
The reflex-type sight works on the same principle, only the reflective surface travels with the shooter and the outside person cannot see the lighted living room. Aimpoint uses an LED to power its red dot in its COMP M2, called the M68 CCO when used by the U.S. military, including Special Forces. The 300,000 or so currently in service is a testimony to the reliability of the Aimpoint system.
A holographic sight coincides a visible laser hologram with the "real world." Essentially, it records a 3D image and reconstructs it on the target plane. To the user, the holographic image is painted on the target, parallax free. Through the EOtech Holographic sight, one literally sees the floating reticle on the target. The 3D quality is quickly evident when the shooter shifts the position of his head. As long as the target is "painted" within the sighted range of the system, it can be engaged.
This kind of sighting defies the imagination. The concept of "wavefront reconstruction process" is a product of scientists making science fiction into reality. They combined the technology into a compact waterproof package and made it run on common batteries.
Where would the holographic sight be most advantageous? Answer: In a target-rich environment. The user can maintain a sight picture on the most immediate threat and monitor incoming threats. An EOtech-equipped soldier on the battlefield can rapidly sort and engage targets. An officer sweeping for an armed suspect can maneuver and see the sight at the same time.
How long do the batteries last? Some low-power optical devices can be left on for months without a battery change. They are not expensive to use and can be "press checked" thousands of times without a failure. Trijicon uses TriPower platforms which incorporate fiber optics, tritium and battery power in the same unit while the Trijicon Reflex optic is dual illuminated. Aimpoint reports about 10,000 hours on the daytime setting.
Still, they should never be used without a co-witnessing sight system. The units may be shock resistant, but batteries do not come with the same warranty. Carry extra batteries and mount iron sights. GG&G has designed flip-up sights that are out of the way until needed and maintain their zero. They also have designed a cantilever mount for the Aimpoint Comp M2 ring for an extra battery.
If the user has flipped up the front and rear sight, the dot or aiming point of the optic will align with the front sight. That is, a rifle sighted in with iron sights will coincide with the aiming point of the optic. For red dot optics like the Aimpoint Comp M2, the dot should look like it is sitting on the front sight post.
This is called co-witnessing, an indication the optic is sighted correctly. Iron sights should coincide for the same distance and conditions. When adjusting the optic and sights for co-witnessing, don't worry about centering the dot (crosshairs, triangle) in the optic, just align the sights with the dot. The red dot really doesn't have to be centered in the optic — it just has to be visible. Co-witnessing works best when the flip-up sights use the bottom third of the optic. This ensures that the optic has an iron sight back up or confirmation when needed.
Shooters used to conventional sights will need to train themselves to keep both eyes open when shooting. Use reactive targets like steel speed plates or falling head plates that deliver a loud reward when hit. Set up at least six reactive targets per shot string and mow them down. Practice flowing from one target to the next without allowing the eyes to leave the sighting axis. Change the distance between targets and reshoot the plates.