An electrodeless plasma lamp and a method of controlling operation of a plasma lamp are provided. The plasma lamp may a power source to provide radio frequency (RF) power and a lamp body to receive the RF power from a feed. The lamp body may comprise a dielectric material having a dielectric constant greater than 2 and bulb is provided that contains a fill that forms a plasma that emits light when at least a portion of the RF power is coupled to the fill. A light guide directs light from the bulb to a photosensor that is shielded from light output from a front side of the lamp body. The lamp includes a drive circuit to control operation of the lamp based on a level of light detected by the photosensor.