HID Xenon Bulb Designations Puzzle Solved
Many of our customers inquire as to the difference between d1s and d3s bulbs. It is clear that with the halogen bulbs, "H" designation for the size stands for, "Halogen," and the number after the "H" gives you some idea when that particular bulb was introduced to the market. For example, H1 was introduced in 1961 and H7 came about in 1970 and so on. But, what about the HID Xenon bulb and its designation?
We also have several HID bulb types.
HID Xenon bulb sizes start with "D," as in D1, D2, D3, D4 and so on. One would think that since it is a xenon bulb, It would start with the letter, "X." Well, not sure who decided, but "D" stands for "discharge," as in HID(High-Intense Discharge) Halogen bulbs are ordered in relative chronological order. However, the development of HID bulbs made the designation a bit muddier. BMW had the first nonreplaceable xenon lamp system and it was designated, D1. When the first replaceable xenon bulb was developed, it was designated "D2." However, a few years later the designation of D1 was not permitted to be used with the newer models, so D1 became available to be used for the replaceable versions. 
The above photos show the differences in HID Xenon bulbs. (D2 and D4 series) As you can see, if you have one type of HID bulb fitment, you cannot put any other types. For instance, "S" (For Projector lens) and "R" (Reflective lens) have a different base from each other so that S bulb cannot be installed to the R bulb or vice versa. On a side note, "S" stands for a shutter that is used to produce low beam by sliding in front of the bulb to create the low beam.
Besides all that, you can generally categorize HID Xenon bulbs to two types. D2 and D4 are bulbs only and need to be connected to the external ignitor that is typically attached to the HID ballast controller. D1 and D3 types have built-in ignitor on the base and connect directly into the HID ballast controller. Now, there are two other types that are on the market. D5S is a relatively newer type of HID bulb that Philips developed. The below photo is our version of D5S, "DAMA Kanju Ultimate Vision D5S." Compare to D1 and D3, D5S has a larger chrome base because the bulb has all three components, the bulb, ignitor, and the ballast. The nice part about D5S is that you eliminate two extra points of troubleshoots by having all three in one. D8S, developed by Osram, is a 25W HID bulb that has a smaller luminous flux than the 35W older version, but it does not require automatic beam adjustment and is a lower-cost solution for the mid-market customers. 
