Armament-related equipment

Collimator
Is a device which make lines that pass trough it in general more aligned to specific direction. For example redirect or filter light rays so the output ones would in average have lower angles from center of the beam.

So when an operator is looking trough such lens of a Collimator sight with a reticle located in their focus, he sees the reticle image at location on the lens, that has direction to the eye more parallel to direction of the lens and therefore the sight.

Reflex sight
Reflect a light from their light source (/ LED/-s), in the operator's direction. Typically use ~0.6mA of Electric Current.

Holographic sight
Have it's light source illuminate a projection that was printed on a lens. Typically use <200mA of Electric Current.

Scope
Is a sight that use a system of lenses or other devices to observe objects magnified and/or, with and/or by, other properties.