There are no “absolutes” in terms of performance and wherever possible, performance measures need to reflect conditions similar to those in the intended application. Unfortunately, measurements are frequently constrained by practical and economic considerations. For example, the most convenient radiation source for testing EDX systems is the Fe55 radioactive source which produces Mn Kα radiation at 5.9keV and can be used with an air path between source and detector. However, the fwhm resolution at 5.9keV is a rather poor measure of what the resolution will be for x-ray energies below 0.5 keV. Furthermore, “P/B”, the ratio of Mn Kα peak height to background level at IkeV, is only a crude indication of incomplete charge collection ICC, and does not reveal problems that may give rise to shift and broadening of the C Kα peak for example.