Low-pressure mercury lamps are commonly used for germicidal applications such as water and wastewater sterilisation. The germicidal effect is due to the emission of light at 254nm, which leads to the destruction of most waterborne bacteria. The Microwave Plasma UV Lamp (MPUVL) is a new technology for generating a high intensity UV light. A Fluorescent Optical Fibre based sensor is presented which is used for monitoring the output of a high power microwave UV light source and its control. This sensor is a fibre which has had its cladding removed and been coated with a phosphor doped polymer.