Abstract: A ballast circuit is suitable for driving a high intensity discharge lamp in at least two power modes including a nominal power mode for driving the high intensity discharge lamp at a nominal power and a reduced power dimming mode for driving the lamp at less than the nominal power. The lamp ballast circuit generates a first alternating current waveform during the nominal power mode and generates a background alternating current waveform in the reduced power mode which is perturbed at intervals by a high intensity pulse which helps to reduce the change in correlated color temperature which would otherwise occur in the dimming mode.

Abstract: A ballast circuit is suitable for driving a high intensity discharge lamp in at least two power modes including a nominal power mode for driving the high intensity discharge lamp at a nominal power and a reduced power dimming mode for driving the lamp at less than the nominal power. The lamp ballast circuit generates a first alternating current waveform during the nominal power mode and generates a background alternating current waveform in the reduced power mode which is perturbed at intervals by a high intensity pulse which helps to reduce the change in correlated color temperature which would otherwise occur in the dimming mode.

Abstract: A method and system to supply current to a high pressure lamp is provided, the method and system comprising an alternating lamp current waveform with a half cycle mean amplitude I1 and a half cycle time duration T, a current pulse waveform including at least one current pulse with a half cycle mean amplitude I2 and at least one current pulse occurring after the trailing edge of a half cycle of the alternating lamp current, the pulse having a half cycle duration of Tp. These waveforms are combined to generate a current waveform including a current pulse waveform, the current pulse waveform starting after a time delay Td from the trailing edge of a half cycle of the alternating lamp current waveform, the time duration Tp of the at least one current pulse lasting less than the remaining time before the leading edge of the second half cycle of the alternating lamp current waveform.

Abstract: Electrical discharge lamps containing mixed halides of superior performance are provided by the inclusion in the lamp of a volatile complex halide compound of tin and sodium. The halide may be added as a mixture of tin and sodium halide but the complex is formed in operation of the lamp and has a much higher volatility than the constituent halides. The preferred lamps contain 5 to 100 micromoles tin and 1 to 250 micromoles sodium halide per cubic centimeter, in a gas fill containing more than 50 micromoles per c.c. mercury.

Abstract: The inclusion of an alkali metal halide in the gas fill of a high pressure mercury and aluminium halide discharge lamp improves the color appearance of the lamp; notably by lowering the color temperature, while maintaining or even increasing the efficacy and stability of the discharge. Preferred contents are 10 to 30 mg cm.sup.-3 mercury, 5 to 100 .mu.mol cm.sup.-3 aluminium halide and 0.5 to 150 .mu.mol cm.sup.3 alkali metal halide, with the molar ratio of alkali metal halide to aluminium halide in the range 0.1 to 1.5 and the total number of aluminium and alkali metal atoms preferably greater than the total number of halogen atoms.