Spatial Frequency Domain Technique: An Approach for Analyzing the Scanning Laser Acoustic Microscope Interferogram Images

An acoustic interferogram of equal phase wavefronts is produced with the scanning laser acoustic microscope. The interfero-gram contains information about the ultrasonic velocity distribution of the specimen under investigation. Through appropriate signal processing, the information can be transformed into a one-dimensional velocity map. This has been done quite successfully in the spatial domain, where the specimen is relatively homogeneous. For heterogeneous specimens, the spatial domain technique often fails. A new technique has been developed to reduce the interferograms to a one-dimensional velocity map. This approach operates in the spatial frequency domain and appears to be more reliable for heterogeneous specimens than the spatial domain approach. The theory and implementation of the spatial frequency domain technique (SFDT) are reported, and the two techniques are compared with each other. SFDT utilizes a smaller pixel size than the spatial domain technique. In addition SFDT is faster and more noise tolerant. The extension of the SFDT to produce two-dimensional veloc-ity distribution images is also reported.