This part of ISO 16526 ÀÛÜspecifies a constancy check of a X-ray system where mainly the X-ray voltage is checked ÀÛDÜÀÛQÜÀÛGÜÀÛÜÀÛDÜÀÛOÜÀÛVÜÀÛRÜÀÛÜthe tube current and the constitution of the target which can be changing due to ageing of the tube. The thick filter method is based on a measurement of the dose rate behind a defined thick filter using defined distances between the X-ray tube, the filter and the measuring device. This method is very sensitive to changes of the voltage, but it does not provide an absolute value for the X-ray tube voltage. Therefore, a reference value is needed and, it is recommended to find this reference, for example, within the acceptance test of the system. The thick filter method is a rather simple technique and may be applied by the operator of an X-ray system to perform regularly a constancy check of the system. The method can also be applied for consistency checks after changing components which may affect the X-ray tube voltage. This method can be applied for all types of X-ray systems, i. e. for constant potential, half wave and impulse wave generators with a tube current larger than 1 mA.