The lamp tube

The lamp tube
OSRAM invented the first HID system, namely D1 system.
D1 divides the ballast into two parts:
Rectifier, the bigger one
Booster, the smaller one
D1 makes the booster as the base of the bulb, so the lamp tube and the tie-in would be bigger. And then D1 is developed to D2 system, namely the D2S and D2R.
D2 system combines rectifier and booster as ballast, so now the genuine HID system is D2S and D2R.