Patent ID: 8193719

Claim:
A method for controlling dimmable electronic lighting ballasts using pulse density modulation, said method comprising the steps of: generating a first plurality of pulses operating at a first number of pulses per second during a filament preheating time period, wherein filaments of a fluorescent lamp are heated thereby, wherein the first number of pulses per second is above a series resonant frequency of a dimmable electronic lighting ballast and the fluorescent lamp, and does not ionize gas in the fluorescent lamp; generating a second plurality of pulses operating at a second number of pulses per second during a lamp-bright time period, wherein the second number of pulses per second is at substantially the series resonant frequency of a dimmable electronic lighting ballast and the fluorescent lamp, wherein the second number of pulses per second is less than the first number of pulses per second, and whereby the gas in the fluorescent lamp is ionized to produce substantially maximum light brightness therefrom when the second plurality of pulses is applied thereto; generating a third plurality of pulses operating at a third number of pulses per second during a lamp-dim time period, wherein the third number of pulses per second is above the series resonant frequency of the dimmable electronic lighting ballast and the fluorescent lamp, wherein the third number of pulses per second is greater than the second number of pulses per second and less than the first number of pulses per second, whereby the gas in the fluorescent lamp is ionized to produce a light brightness less than the maximum light brightness therefrom when the third plurality of pulses is applied thereto; generating the first plurality of pulses for a filament heating time period after the lamp-dim time period; and the lamp-bright, lamp-dim and filament heating time periods are within a lamp dimming frame time period that repeats during dimming of the fluorescent lamp.