High intensity discharge lights are used in vehicle headlamps and provide a glow generated via an electric arc, as opposed to filaments. HID headlamp D2 system bulbs, such as the D2R and D2S, vary in a number of ways.
D2S -- discharge second generation shielded -- bulbs are specifically designed for projector headlamps. Projection headlamps contain a shade positioned between the lens and reflector. Light projection from the top edge of the shade provides the low beam cut off, and facilitates a high beam of light without glare. D2R -- discharge second generation reflection -- bulbs, on the other hand, are used in reflector headlamps. Reflector headlamps contain a light source, such as an arc, positioned near the focus of a parabolic reflector, which helps reflect the glare caused by vehicle headlights in oncoming traffic.
D2S and D2R bulbs feature differences at the bulb base for fitment reasons. The two fitment notches on the D2S are spaced nearly twice as far apart to fit into the projector headlamp configuration. D2R notches are spaced more closely to fit into reflector headlamps.
D2R bulbs are painted with a black stripe around the sides of the bulb capsule in order to reflect glare. D2S HID bulbs are unpainted.
