The Reflex Sight

What is a Reflex Sight?

The term "reflex" refers to the fact that the aiming reticle is projected forward, from a point behind the objective lens, and is then reflected off the back of the objective lens assembly toward the shooter's eye. In an Aimpoint® sight regardless of where you position your eye, the reflection of the LED is always parallel with the sight’s optical axis, thanks to the design of the double lens and its light refraction property. The points of aim and impact always coincide. Allowing both-eyes-open shooting a reflex sight allows the viewer to see the primary target as well as the periphery. Reflex sights work well because aiming is simple. Aiming with both eyes open, you instinctively place the red dot on the target, and are ready to pull the trigger at the same instant. 

Aimpoint drawing of red dot sight without parallax, a reflex sight
Aimpoint drawing of red dot sight tube without parallax