I was given this piece of advice:
"There are 2 types of factory HID's. D2R and D2S. D2R bulbs are optimized for reflector housings, and D2S are optimized for projectors. However, D2R are fairly uncommon nowadays as they have been pretty universally replaced by D2S setups. Some examples of factory D2R cars are the original couple of Lexus, Acuras and Infinitis that had HID's as an option, or as standard equipment. The 2nd Gen TL, 1st gen G35, Euro and Japanese 6th gen Accord SIR-T/Type R come to mind, as does the original HID fitted LS400, GS300/400, and some Benz models.
However, D2R and halogen reflector assemblies are not the same.
Since the HID bulb puts out significantly more lumens at 35W than the halogen would at 55W, the reflectors must be specifically optimized to run D2R bulbs. Because of the greater amount of total light output, the HID reflectors spread the light over a larger and less focused area, leading to better overall illumination. If you put D2R bulbs in a halogen housing, then you tend to get glare and hot spots because the light is too intensely focused for that housing, which was the result of having to aim the light over a smaller area, and more intensely, resulting in the problems associated with HID plug and play retrofits. It is also why it is not quite as simple as an OEM manufacturer just swapping in HID bulbs, and is why HID is typically at least a couple hundred dollar option on cars that offer both. They have to change the reflector/projector in addition to the bulb system in order to comply with DOT regs."
