Apparatus for efficiently exciting an electrodeless lamp to produce visible light. A source of microwave energy is coupled to a cylindrical cavity which encloses an electrodeless lamp. The cylindrical cavity includes a sidewall and end wall which is made from a metallic mesh which passes light produced from the electrodeless lamp. The electric field intensity within the cylindrical cavity is increased in the region above the lamp center. The increased electric field intensity produces a more uniform temperature across the bulb service, increasing the rate of plasma heating of gas molecules in the lamp.