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.
The closest to OEM we can get without paying dealership pricing are the Philips line of D1S, D1R, D2S, D4S etc… which run around $50-60/bulb. However, off brand aftermarket manufactures offer HID bulbs at a fraction of the price (e.g. HID warehouse , RCP , and HYB with comparable reviews – HYB also makes knockoff projectors BTW), though (in general) they won’t last as long or have as consistent of a color temperature.
Since all of these bulbs have the same base dimensions and hold the arc in the same location, aftermarket manufactures have cut costs by reducing the number of different models to produce. The result, sometimes called the D*C type (though this isn’t a real specification), universally fits multiple project key 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. But there is still the voltage difference – even though they might fit, D1/2 bulbs must not be used with D3/4 ballasts or vise versa!
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.
These are designed to allow aftermarket ballasts to be used after the OEM ballasts fail, or when projectors are retrofitted. These have the same downside as re-based HID bulbs: using them requires drilling into the headlight assembly to pass through the wires.
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):
This removes the key entirely, so you are free to use whatever bulb/ballast combination makes you happy.