Abstract: The invention relates to non-destructive testing for flaws in the surfaces of, particularly, billets, strip or sheet. There have been many systems developed for the detection of flaws in the surface of a workpiece such as visual techniques employing ferromagnetic materials in a fine suspension including an ultra violet dye, and techniques employing the eddy current effect, both of which suffer in that they are unsatisfactory when a workpiece is moving or when a workpiece is hot. Attempts have been made to employ micro-waves but such attempts have essentially required a very close spacing between the detector-receiver and the surface being scanned again making such equipment unsuitable for use with a moving workpiece or when a workpiece is hot.