When looking at D2 HID bulbs, it can be tricky to determine which bulb you need. Do I need a D2S, D2C, or D2R? The difference between D2S and D2R is that D2S HID bulbs are designed to be used in a projector headlight application and D2R is made for a reflector application. D2R has a painted shield on it as to where the D2S doesn’t. The D2R HID bulbs have a painted section on the glass because otherwise there would be too much glare in a reflector housing, it acts as a type of deflector to only let the HID light shine in some directions, it also helps to reduce the amount of yellow light that is seen in the light output. The manufacturer of these D2 series HID bulbs has certain notches cut out of the base of each bulb making it impossible to accidentally install a D2R bulb into a D2S housing, and vice versa. The keyways only accept the proper bulb.
A D2C HID bulb on the other hand is the universal version of this bulb. It has the notches cut out to fit either a D2R or D2S HID housing:
When looking at HID bulbs, one important feature to look for is how the glass is attached to the bulb. Higher quality HID bulbs have a metal structure around the base of the glass where it attaches to the base. This metal structure helps to keep the glass in place and to stay in better alignment. Any shifting of the glass bulb inside of the housing will effect the headlights beam pattern.
As you can see the GTR Lighting D2C HID bulb is a direct copy of the OEM HID bulb. One major difference in the GTR Lighting bulb versus the OEM bulb is that the GTR Lighting HID bulbs come in different colors. OEM bulbs are available in only one or two colors like 4,300K (whitish yellow). GTR Lighting HID bulbs are available in colors anywhere from 3,000K (golden) to 12,000K (purple).