The primary objective in calibrating ultrasonic transducers and measurement systems is to ensure the interchangeability of data. Thus, one wishes to create a situation in which experimental results, e.g. A-Scans or C-Scans taken at one time and place, can be directly compared in a quantitative sense to similar data taken at another time and place. The presently available tools are not sufficient to this task. As noted by Burley [1], there has been no single type of calibration standard that is suitable in all ultrasonic applications and inspection procedures. Instead, there is a wide range of reference standards that are used in different situations to calibrate transducers and measurement systems. Although these have served the community well in bounding the results which can be obtained by different systems, they do not ensure uniformity of results. A striking example for the case of eddy currents can be found in Fig. 4 of Ref. [2].