[Effective prefiltration of roentgen tubes and its amount in aluminum equivalent units]

It is shown that the Al equivalent of the prefiltration of an x-ray tube assembly varies significantly with the tube voltage, and hence the usual quotation of only one value without further specifications cannot suffice for the definition of the Al equivalent. The knowledge of the tube voltage, with which the equivalent was determined, is also important. The equivalent relationships can then be indicated for the total tube voltage range which is of interest.