You may have seen references to various D-Series HID (xenon) bulbs in the context of projector swaps, D1S, D1R, D2S, D2R, D3S, D3R, D4S, D4R, sometimes even D2C and D2H. So what’s the difference?
D-Series Naming System
D-Series bulbs are named using 3 characters, like “D1S”.
D - "Discharge"
N - 1,2,3,4 is the type
T - S is for projector, R is for reflector
To illustrate this
1. Ignitor: integrated (D1*, D3*) vs. separate external (D2*, D4*)
Igniter built-in: D1, D3
No integrated ignitor: D2, D4
2. Voltage: mercury (D1*, D2*) 85V vs. mercury-free (D3*, D4*) 42V
3. D*S is used in projector vs. D*R is used in reflector headlight with shield on envelope glass
What if you are given D1R and D3R or D2S and D4S without label? You can’t see if there is mercury inside (and thus the voltage). It is possible to differentiate using the notches on the bulb.
Again, all of these bulbs have the arc in the same place, so to reduce the number of different models to produce aftermarket manufactures have developed the D*C type, which universally fits multiple notches. For D1/D3, a removable ignitor adapter is available.
Aftermarket manufactures don’t want to make a lot of different models. D2C that can replace both D2R and D2S. D4C for D2S, D2R, D4S, D4R and D2C. D1S with 5 notches won’t be hard to understand.
All of this makes it simpler for production, but much more complicated for consumers… And most sellers even don’t know about the real reason or the difference.
C-type bulbs with integrated ignitors can be differentiated by the keying on the ignitor connector
Finally, the last aftermarket type is sometimes called “D2H” which includes AMP connectors for aftermarket ballasts. It is always 85V / Mercury.
AL Gen2 Projector “de-key” Modification
You can easily modify AL Gen 2 projectors to fit any of the D base bulbs by bending the key tab out of the way (aka dekey the projector):