A device for the treatment and prevention of prematurity retinopathy in the case of the.

摘要

Eddy current based method and apparatus uses a repetitive square wave signal applied to an excitation coil and operates in a pulse echo mode by gating the detected field signal at synchronised intervals to generate one or more sequences of like gated slices obtained from respective detector positions on a circular scan path. Suitable time gating to exclude surface and near-surface reflections overcomes surface clutter and flaws are detected by examination of the sequence of slices for characteristic changes in level. Both method and apparatus have special application for crack investigation at rivet fasteners or similar, e.g. in airframes, and have ability to cope with ferrous fasteners in non-ferrous material. Excitation coil and magnetic detector are combined in a probe and the latter is driven around a circular scan path. A centring display is derived by examination of the near-surface reflection signal to determine the position and level of the peak slice level on the scan path sequence of slices.