Fox Headlights

I believe it is called a bulb shield. From what I have read it blocks unusable waste light that would otherwise cause glare (and backscatter in bad weather). The waste light (backscatter) cannot contribute usefully to the beam. Removing the bulb shield does not (cannot) improve the headlamp's performance in any way.

The 9004 bulbs (at least the Sylvania Silverstar's I have) are blacked/silvered out end of the bulb. I noodled around on the internet and read that the blacked out tip on the bulb is also typically present on 9003/HB2/H4, 9004/HB1, 9006/HB4, 9007/HB5, H7, H8, H11, and H13 bulbs. There were several comments that without the bulb shield built into the headlight housing the blacked out tip on the bulb does a half ass job of controlling stray light. If the reflector in the headlight housing is deep (not sure what deep is) then that helps with controlling the stray light coming out of the headlight assembly. Even then you can get problems with stray light bouncing around inside the lamp and eventually exiting. Basically it sounded like the optimum design is a deep reflector type headlight assembly that has the bulb shield with a blacked out tip bulb.

Not sure if this helped but hopefully its entertaining.

If you could pictures would help a lot but they may be difficult to take being they are inside the housing.