A ballast (11) includes a variable output DC converter source (12) and an inverter circuit (14) arranged to provide AC output voltage to an electro-luminescent (EL) lamp (16). A Q factor of EL lamp can characterize the age of the EL lamp. A series resonance-parallel loaded circuit and two voltage detector (18, 20) are constructed to measure the Q factor. A processor (17) controls the output of DC voltage source in accordance with a look-up table containing constant brightness algorithm based on the derived voltage and Q relationship.