Apparatus is provided for use in detecting the peak voltage applied to a radiation source operating at an unknown input voltage. The apparatus includes a set of radiation absorbing filters including a first filter which includes a first element that exhibits a known K-absorption edge and a second filter constructed of a second element so that the filters exhibit essentially the same radiation absorption characteristics below the K-absorption edge of the first filter. The filters are adapted to be positioned so as to be irradiated by the radiation source so that the radiation impinges upon a surface of each filter and is partially absorbed as it passes therethrough so as to exit therefrom as attenuated radiation. A detector, such as X-ray film or a pair of photodiodes, is positioned for receiving the attenuated radiation passed by the first and second filters and provides an output indication when the radiation passed by the filters is differently attenuated. This is indicative that the known K-absorption edge of the first filter has been exceeded, thereby providing an indication as to the magnitude of the voltage applied to the radiation source.