No disrespect intended - but have you people, entirely, lost your minds? The brightest part of an HID is the Arc Tube, the bulb and the filament have nothing to do with it! The Metal Halide gas or the High Pressure Sodium gas is located in the Arc Tube and it is there, in the Arc Tube, that the "reaction" takes place.
The light proceeds to expand outwardly in all directions but it's shortest path is straight out the side of the bulb (at 90 degrees). Plus; as the angle increases, towards each end of the bulb, the relative thickness of "glass" that the light must penetrate increases.
The filament, or electrode, only serves to direct the electricity to the Arc Tube, And the external bulb is only there to enclose neutral gas around the Arc Tube so it wont burn up.