A test cassette employing a modification of the Ardran-Crooks attenuation technique for measuring peak tube potential (and half-value layer) is described. The technique employs a radiographic intensifying screen which is exposed to a hardened x-ray beam. Copper penetrameters are used to determine the attenuator thickness which reduces the light output from the screen to that transmitted by an optical attenuator over an adjacent area of the screen. The test cassette was calibrated by using x-ray machines whose high voltages were measured electrically. The test cassette has a measurement precision of +/- 1 kVp and an accuracy of +/- 3 kVp or better in the range 50-130 kVp for x-ray generators with the same voltage waveform.