H.I.D. D1S lamps are the so called third generation design. They have the igniter electronics/housing integrated with the bulb eliminating the need for transporting high tension spikes (up to 20 KV) from the electronic ballast unit whereas in the D2S lamps, the igniter is part of the ballast. The result of this improvement is that EMI is greatly reduced and therefore the risk for radio frequency noise and interference to other avionics is minimized with the third generation D1S lamps. Also, ballast to lamp wire length can be up to 10 feet long (approx. 12″ is typical for D2S lamps).