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Reflex sights use refractive or reflective optical collimators to generate a collimated image of a luminous or reflective reticle. This collimated image is reflected off a dichroic mirror or beam splitter to allow the viewer to see the field of view and a reflection of the projected reticle (e.g. a red dot) simultaneously. If no magnification is utilized, this gives the viewer a theoretically parallax-free image of the reticle, superimposed over the field of view at infinity.
A reflex sight with no magnification can be held at any distance from the eye (see eye relief), and at almost any angle, without distorting the image of the target or reticle, and without causing the reticle to "move" relative to the target. But parallax compensation is not perfect, and depending on the sight's design, the range to the target, and the magnitude of angle at which it is looked into, aiming error can be non-trivial due to parallax.[2]
Magnified reflex sights suffer from parallax and fixed eye relief just like conventional telescopic sights. Nevertheless, many reflex sights are available with magnification. The U.S. military has widely-deployed the Advanced Combat Optical Gunsight (ACOG) telescopic reflex sight, but reflex sights are also readily found in conventional-looking rifle scope configurations, used for such activities as hunting and target shooting.
Because reflex sights provide an illuminated reticle, they are often used with both eyes open (the brain will tend to automatically superimpose the illuminated reticle image coming from the dominant eye onto the other eye's unobstructed view), giving the shooter normal depth perception and full field of view. This capability, along with the parallax compensation found in un-magnified devices, makes target acquisition very fast compared to standard telescopic sights and iron sights. Un-magnified reflex sights are particularly suitable for installation on a wide variety of weapons used for close-range engagement, e.g. pistols, submachine guns, and shotguns.

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Unlike traditional red dot sights, holographic weapon sights utilize holography to achieve an effectively infinite parallax free distance. Most standard red dot sights use a simpler semi-reflective surface which reflects a projected dot of light while allowing the user to see through the sight. When the eye is not aligned with the center of the standard reflex sight, a certain amount of parallax error can be introduced at longer ranges.[3] The method of holographic projection used by EOTech allows the parallax error to be constant at any range, and never more than the radius of the sight window.
One requirement of holographic projection is a laser. Lasers use more power than a LED of an equivalent brightness, reducing the amount of time a holographic sight can run on a single set of batteries. Another disadvantage to the laser-based sight is that lasers are more sensitive to temperature. To prevent loss of accuracy, thermal changes must be compensated for. A diffraction grating is used in the holographic sight; however, the operating temperature range is typically less than that of a simpler optical reflex sight.