Circuit and method for improved dimming of gas discharge lamps

A control system for providing electrical power from a source to at least one gas discharge lamp, comprising:

• a) means for providing to said at least one lamp an ac component of current at a first frequency;
• and
• b) means for simultaneously providing to said at least one lamp an ac component of current at a second frequency, substantially lower than said first frequency;
• whereby a composite current waveform is established in said at least one lamp which substantially eliminates the occurrence of visible striations.

A control system for op rating a gas discharge lamp at light levels below about 40% of nominal light output, said control system having a maximum output impedance characteristic of at least about 5,000 ohms and comprising:

• a) a high frequency source having a relatively low output impedance;
• b) at least one passive impedance element connected in series between said source and said lamp;
• c) current sensing means for sensing an actual current in said lamp;
• and
• d) amplifier means responsive to said current sensing means for increasing the effective output impedance characteristic of said control system beyond that of said passive impedance element alone.

A control system for operating a gas discharge lamp at light levels below about 40% of nominal light output, said control system having a maximum output impedance characteristic of at least about 5,000 ohms and comprising:

• a) a high frequency source having a relatively low output impedance;
• b) at least one passive impedance element connected in series between said source and at least one lamp;
• c) means for sensing an actual current in said lamp and producing a first signal indicative of said actual lamp current;
• d) means for comparing said first signal to a reference value and producing a second signal indicative of the difference between said first signal and said reference value;
• and
• e) means responsive to said second signal for adjusting said high frequency source such that the difference between said first signal and said reference value tends towards zero.

A method of substantially eliminating the occurrence of visible striations in a gas discharge lamp, comprising the steps of:

• a) providing to said lamp ac current to strike and maintain an electric discharge therethrough;
• and
• b) simultaneously providing to said lamp a dc component of current;
• whereby a composite current waveform is established in the lamp which substantially eliminates the occurrence of visible striations.

A method of substantially eliminating the occurrence of visible striations in a gas discharge lamp comprising the steps of:

• a) generating an ac current at a first frequency;
• b) generating an ac current at a second frequency substantially lower than said first frequency;
• c) adding together said first and second frequency ac currents to obtain a composite ac current;
• and
• d) providing said composite ac current to said gas discharge lamp to substantially eliminate the occurrence of visible striations.