Originally Posted by KAX
how exactly are projector housings different in terms of whether or not they are made for HIDs or halogens?
The dispersion properties of the light from HID bulbs is different from halogen bulbs, because the light is generated by different means. In addition the amount of light emitted by the HID bulb is much greater. Therefore the optical design of the housing must be different to properly accomodate the type of bulb being used.