copy from other forum//
On the highest quality sights and red-dot scopes (open type and closed type red dot SIGHTS), the "reflector lense" is often coated with a colored polarizing film. This typically, at least on high quality optics, is not visible in the sight, but only from the outside.
Technically, the reflector lens is somewhat like a 2 way mirror, it lets the image of the target pass through the lens, but does not let the red-dot out.
On very low quality red dot scopes, the "red halo" visible in the tube is generally stray "light" from the collimator that has scattered inside the tube. It's a dead-give away of a poor quality scope. The ONLY projected light on the reflector should be the reticle dots.
//
入門版真鏡, sightmark/ holosun都唔錯
