Two techniques are used to measure the effect of removing the filament power from a two-lamp, F-40, rapid-start fluorescent lamp system. The changes are measured for a standard CBM core-coil ballast and for a solid-state ballast operating the lamp at high frequency. There is a 4 to 6% increase in system efficacy when the filament power is removed. Removal of filament power also decreases filament temperature from 1000 °C below 700 °C in lamps operated at 60 Hz, and from above 600 °C to 300 °C in lamps operated at high frequency. The study shows that the arc current and anode fall also determine filament temperature.