An ultraviolet lamp assembly and corresponding methods is operable to generate ultraviolet light for irradiating a substrate. The lamp assembly includes a magnetron, an electrodeless lamp for emitting ultraviolet light when excited by microwave radiation generated from the magnetron, and a power control circuit arrangement configured to control an output power of the microwave radiation generated by the magnetron corresponding to the intensity of ultraviolet light produced by the lamp. A first control loop of the power control circuit is configured to regulate an input current to the magnetron based upon an input current setting associated with a desired intensity of UV light output of the lamp and a second control loop coupled to the first control loop configured to adjust the input current setting used by the first control loop to regulate the input current to the magnetron based upon an input power to the magnetron, which is proportional to the intensity of UV light output from the lamp.