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This part of ISO 16526 specifies a constancy check of a X-ray system where mainly the X-ray voltage is checked and also the 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 defineddistances 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 tubevoltage. Therefore, a reference value is needed and, it is recommended to find this reference, for example, withinthe 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 toperform 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 tubevoltage.This method can be applied for all types of X-ray systems, i. e. for constant potential, half wave and impulse wavegenerators with a tube current larger than 1 mA.