© 2002 IEEE. A novel optical fibre based system has been developed which is capable of measuring the light output intensity of a high power microwave powered UV light source (MPUVL). Calibration of the sensor has shown excellent agreement with conventional semiconductor UV sensors. There are several industrial applications for this type of microwave plasma driven light source especially in the drinking water industry where it is necessary to kill potentially life-threatening bacteria. The results of this investigation have demonstrated the suitability of fibre sensors for monitoring and performance optimisation. of the MPUVL. The technique is especially suited for light source temperature control as the phosphor decay times are less than a second (typically 3 ins). This is particularly important where the wavelength needs to be determined for operation in the germicidal region (254 nm) or the ozone-generating region (185 nm). The results have demonstrated a maximum departure from linearity of 2% in when measuring the light output intensity from the MPUVL in the power operating region (50 to 200 W). Apart from its low cost compared to conventional UV spectroscopic techniques, the fibre sensor of this investigation has the added advantage of being able to operate in a hostile electromagnetic environment of high power microwave electric and magnetic fields.