A red dot sight is a common classification for a non-magnifying reflector sight that provides an illuminated red dot to the user as a point of aim. A standard design uses a red light-emitting diode (LED) at the focus of collimating optics, which generates a dot-style illuminated reticle that stays in alignment with the firearm the sight is attached to, regardless of eye position.
View through Tasco ProPoint red dot sight (model PDP2ST) on a Ruger 10/22. Made in Japan for Tasco, the ProPoint 2 was one of the first red dot sight models to become widely popular.
Reflex sights have varying dot viewing angles; that is the maximum angle the operators eye can be offset from the center-axis of the sight whilst the dot still remains visible.
Left: Aimpoint Acro C2 reflex sight laying on its side. Right: Acro rail on a Picatinny riser.
A U.S. Marine looking through an ITL MARS combination red dot and laser sight mounted on his M16A4 MWS rifle during the Second Battle of Fallujah, 2004
A reflector sight or reflex sight is an optical sight that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view. These sights work on the simple optical principle that anything at the focus of a lens or curved mirror will appear to be sitting in front of the viewer at infinity. Reflector sights employ some form of "reflector" to allow the viewer to see the infinity image and the field of view at the same time, either by bouncing the image created by lens off a slanted glass plate, or by using a mostly clear curved glass reflector that images the reticle while the viewer looks through the reflector. Since the reticle is at infinity it stays in alignment with the device to which the sight is attached regardless of the viewer's eye position, removing most of the parallax and other sighting errors found in simple sighting devices.
Prototype of the Grubb reflector sight attached to a rifle
German paratrooper looks through the reflector sight of the Flakvisier 40 gunsight on a FlaK 38 anti-aircraft gun (1944), one of the more sophisticated sights at the time
Longitudinal cross-section of a basic reflector sight for pre-WWII German fighter planes (1937 German Revi C12/A)
HUD inside the cockpit of a fighter jet