Systems and methods for monitoring and controlling the operation of extreme ultraviolet (EUV) sources used in semiconductor fabrication are disclosed. A method comprises providing a semiconductor fabrication apparatus having a light source that emits in-band and out-of-band radiation, taking a first out-of-band radiation measurement, taking a second out-of-band radiation measurement, and controlling the in-band radiation of the light source, at least in part, based upon a comparison of the first and second out-of-band measurements. An apparatus comprises a detector operable to detect out-of-band EUV radiation emitted by an EUV plasma source, a spectrometer coupled to the electromagnetic detector and operable to measure at least one out-of-band radiation parameter based upon the detected out-of-band EUV radiation, and a controller coupled to the spectrometer and operable to monitor and control the operation of the EUV plasma source based upon the out-of-band measurements.