operating frequency in dependence upon the target lamp current the ballast 900 is operable to drive the lamp 108 such that mercury pumping is avoided however when the target lamp current changes the actual lamp current and thus the lamp intensity changes at a slower rate than in the previous embodiment since the operating frequency control loop i e the duty cycle error value e d is solely in control of the operating frequency fig1 is a flowchart of the software executed by the microprocessor of the ballast 900 to adaptively change the operating frequency f op according to the second embodiment of the present invention steps 1002 through 1012 are similar in function to steps 502 through 512 of fig6 a and 6b executed by the microprocessor 450 of the ballast 400 according to the first embodiment of the present invention the process of fig1 does not utilize either a base period or a correction factor to determine the operating period t op and the operating frequency f op if the accumulator has reached a predetermined positive level at step 1014 then the operating frequency f op is decreased by a predetermined increment e g preferably 314 hz at step 1016 and limited to a minimum operating frequency f min e g preferably about 45 khz at step 1018 alternatively if the accumulator has reached a predetermined negative level at step 1020 then the operating frequency f op is increased by the predetermined increment i e 314 hz at step 1022 and limited to a maximum operating frequency f max e g preferably about 70 khz at step 1024 if the accumulator has.