Abstract: A projector lamp comprising a single ended metal halide discharge lamp in a generally spherical reflector has been found to exhibit turbulence when running. The resulting fluctuation of light especially when used in conjunction with the reflector to concentrate the light for studio or theatre applications provides an apparent movement which is extremely disturbing to the eye. It has been found the turbulent movement of gas around the electrodes in the lamp envelope can be regularised by the application of a magnetic field to the envelope. The turbulence is sensitive to conditions within the discharge arc tube and the strength and position of the magnetic field has to be variable to cater for this. The magnetic field is conveniently applied by an electro-magnet using lamp current.