some people add a front grill. it focuses out so you dont see it(just like you cant see your iron sights even though its right infront of your optics), protects from inpact and covers up the reflectivness of the lense.
"killFlash: Fitted this this model you'll see a killFlash adapter. This cunning device is basically a honeycomb wire mesh that eliminates potential reflections from the surface of the outer lense that could compromise the user's position.
"The patented geometry of the CC-A’s honeycomb shield maintains a clear, bright view through the sight and does not interfere with target acquisition." killFlash
The idea is simple - direct light can pass through, but somewhat like a polaroid lense, light from angles slightly off Centrex cannot get it, and therefore you will not get glare, as sunlight is effectively trapped behind the kill flash. In these two images following you can see the killFlash looking dead on, and then from a slight angle."
from arnies
the pic is from an aimpoint, but i have seen them on scopes also.