Methods for preventing blackening of a gas filled arc tube of a mercury vapour discharge lamp. Lamp voltage and slope, as well as lamp current and slope are analyzed to determine the state of mercury saturation of the gas in the arc tube. A first control signal is sent to alter the lamp power and a second control signal is sent to alter the level of cooling of the lamp according to the state of mercury saturation. Methods for driving a mercury vapour discharge lamp using a monitoring arrangement and analyzing unit to dynamically control the lamp power and the level of cooling of the lamp are described.