What’s the difference between D1C/D1R/D1S and D3C/D3R/D3S?

Starting the late 1990's, automobile manufactures like Ford, Audi and Mercedes Benz have been using xenon high intensity discharge headlights as an option for consumers in their new vehicle purchases.  The most common OEM HID bulb of the time being the D1 series bulb.

First came the D1R (for reflector housings), which was followed by the D1S (for shielded housings a.k.a. projectors).  The difference between the two types being that the "R" type bulbs have a painted section that was designed to block the light output to a specific part of the housing to help create a safe output pattern while with the "S" type bulb there is nothing on the bulb itself as the shield is positioned inside the projector housing.

Then the D2 series bulbs entered the market with us seeing the D2R and the D2S, with "R" and "S" meaning the same thing as with the D1 series bulbs.

Followed afterwards by the D3 and D4 OE bulbs, with D1 and D3 in the same family, and D2 and D4 respectively as well. 
The D1 series and D3 series bulbs have a built-in igniter base
The D2 and D4 have their igniter built inside of their accompanied ballast.
The biggest difference between the D1/D2 and D3/D4 series bulbs is that the new D3 and D4 bulbs do not use mercury and require less energy to turn on.  You can easily tell between the new No-Mercury series bulb and the old style by looking at the wire harness connection part where you will see it is a green color.

Many vehicles currently are using the D3 type bulb, including the new Ford Mustang, Ford Flex and all new Audi vehicles like the A3, A4, Q5 and S5.  When shopping for OEM style D1S, D1R, D1C, D2S, D2R, D2C, D3S, D3R, D3C, D4S, D4R, D4C HID bulbs, you should know the difference between the S, R and C identifiers.  Manufacturers wanted to make it easy for consumers to put the correct bulb inside their factory HID headlight housing, so they built the bulbs with little notches in different placements allowing them to only fit the correct housing type, but otherwise they are identical.

As you can see, the D1C / D2C / D3C / D4C bulbs are all designed to replace either the S or R type bulbs because they have all of the notches for any application.  This is a great benefit in that less inventory is needed to stock one bulb per style instead of 2 or 3 different ones, resulting in the cost of the bulb being less than an OEM product.

Some more things to note:

  1. A great option for replacing any OEM style bulb is the “C” type based-bulbs.
  2. You can't replace a D3 bulb with a D1 bulb. Although they will physically fit, either the bulb, ballast or both will be damaged when you turn it on.
  3. You can't replace a D4 bulb with a D2 bulb. Although they will physically fit, either the bulb, ballast or both will be damaged when you turn it on.
  4. D1 and D2 xenon bulbs use Mercury. D3 and D4 xenon bulbs are Mercury-free.
  5. Philips and other OEM producers of xenon lighting in cars DO NOT offer anything besides 4,300K bulbs. All bulbs listed as Philips or Sylvania in other color temperatures are fakes!