Patent ID: 7176632

Claim:
A high intensity discharge lamp comprising: a light transmissive lamp envelope having a wall defining an enclosed volume; at least one electrode assembly extending in a sealed fashion from the exterior of the lamp through the lamp envelope wall to be exposed at an inner end of the electrode assembly to the enclosed volume; a fill material enclosed in the enclosed volume, the fill material being excitable to light emission with the application of electric power; a fill gas enclosed in the enclosed volume, the fill gas having a cold fill pressure of p in Pascals; wherein the inner end of the electrode has an integrally formed body (head) having a surface defining a recess with a recess volume and an opening from the recess volume to the enclosed volume, further defining a least recess spanning dimension S measured across the recess opening and defining a recess depth of D where S is greater than the electron ionization mean free path, and less than twice the minimum cathode fall distance plus the negative glow distance, during the glow discharge phase of starting, for the chosen lamp fill gas composition and (cold) fill gas pressure.