Patent ID: 7372216

Claim:
A ballast for controlling brightness of an electro-luminescent (EL) lamp, the ballast comprising: a converter circuit that converts line AC voltage to a DC input voltage, an inverter circuit that inverts the DC input voltage to an AC outPut voltage, the ballast being constructed and arranged to provide the AC output voltage to the EL lamp, an input voltage detector constructed and arranged to measure the DC input voltage (Vin), an output voltage detector constructed and arranged to measure the AC output voltage (Vout), and a processor constructed and arranged to adjust Vin based on measured values of Vout and Vin to thereby adiust brightness of the EL lamp, wherein a look-up table is associated with the processor, the look-up table including a quality factor Q, where Q is related to Vout/Vin, and Q relates to an age of the EL lamp.