Abstract: Apparatus and a method for operating high intensity discharge (HID) lamps at a predetermined substantially constant luminous flux output. A power-time characteristic is developed using a test lamp of similar type and rating to the HID lamps. This characteristic defines the power required at different times, which may be regularly spaced intervals of for example 20 hours, during the operating life of the HID lamps to operate the lamps at the predetermined luminous flux output. The power time characteristic is then “played back” in real time via a microprocessor based master controller for the HID lamps. The communications to the HID lamps may be wireless or hard wired. The result is that the HID lamps are operated at substantially constant lumens resulting in significant energy savings and improved lamp performance.

Abstract: A controller 5 for a high intensity discharge lamp 4 includes a primary ballast for providing a primary current to the lamp 4, and a current injector which in the case shown comprises a control ballast 8 and a switch 9 (e.g. a triac) controlled by a control unit 12. The current injector injects a secondary current into the lamp 4 that is less than the primary current and can be varied by the control unit 12, e.g. by varying the amount of time within an a.c. cycle that the control ballast 8 is switched into the circuit. The controller 5 allows the lamp 4 to be controlled, for example, to operate at a constant power (variable light output) or at a constant light output (variable power).