Found an excellent explanation of the difference between D1S and D3S bulbs, short summary:
1. D1S has mercury in it, D3S does not.
2. D3S bulb requires less power to turn on.
2. D1S and D3S have the same base design (the "S" refers to the base "notching" configuration).
4. You CANNOT replace a D3S bulb with a D1S bulb! They will physically fit, but the bulb, ballast or both will be damaged when you turn it on. There is also a slight difference in electrical plug configuration between these two bulbs.
The link to this information:
http://www.shiflighting.com/support/...d1s-d3cd3rd3s/