Lighting system and compact electric lighting unit

A lighting system having a high pressure gas discharge light source and an incandescent light source within a common sealed and evacuated bulb. A ballast unit fitted externally to the neck of the bulb includes in series with the incandescent light source an interrupter element and control circuitry therefor, and a choke coil in series with the gas discharge light source. The control circuitry represents an equivalence logic with two variables, wherein the output terminal of an equivalence circuit is connected to the control input of the interrupter element while its input terminal is connected to a voltage divider system connected across the gas discharge light source. The output terminal is energized only if either both or neither of the input terminals of the equivalence circuit are energized. The interrupter element is closed when the output terminal of the equivalence circuit is energized. The lighting system instantly emits light when switched on from the mains and maintains the illumination at an approximately constant level during the cool or cooling down periods of the gas discharge light source following its start, warm up, or re-start after switch off.