Apparatus is provided for use in detecting the input voltage applied to a radiation source (10) operating at an unknown voltage within a given voltage range. The apparatus includes a set of radiation absorbing filters (F1, F2) including a first filter which includes a first chemical element and a second filter which includes a second chemical element. These elements are chosen so that the filters exhibit different radiation absorption characteristics within the given voltage range. The filters are adapted to be positioned so that the first and second filters are irradiated by the radiation source (10) with the radiation (18) impinging upon a surface of each filter and partially absorbed thereby as it passes therethrough so as to exit therefrom as attenuated radiation. A detector (20) receives the attenuated radiation passed by the first and second filters and provides first and second signals (I₁, I₂) having magnitudes which vary with the attenuated radiation respectively passed by the first and second filters. A ratio (34) is determined as to the magnitude of the first signal to that of the second signal with the magnitude of the ratio varying with that of the input voltage. At least one of the first and second elements (F1, F2) exhibits a known K absorption edge within the given voltage range. Consequently as the input voltage is increased to exceed the known K absorption edge that chemical element exhibits a greater attenuation characteristic to extend the useful range of the relationship of the magnitude of the ratio and the input voltage.