Abstract: A calibration apparatus for the calibration of patient dose measurement and control systems of the character used in diagnostic radiology. The calibration apparatus includes a novel ion chamber that is of rigid construction. The ion chamber has a radiation input face that incorporates an optical alignment target or reticule having markings generally corresponding to a range of field sizes and shapes. A series of radio-opaque targets are embedded beneath the surface of the optical target to facilitate alignment using a radiation image.