Examination of materials by coherent light techniques

The diffraction and interference of coherent light from lasers can be used to record and analyze metallographic structures. Optical transforms, holography, optical correlation, and other coherent light methods are presently being applied to materials problems ranging from basic research to testing and inspection. This paper describes the basic concepts used in some of these methods and describes examples of their application. Holography is a particularly attractive method for measuring changes in the shapes of objects, especially because it allows interferometric measurements without special surface preparation. The use of holography for studies of crystal growth and electropolishing is described in some detail, and reconstructed holograms are shown. Holography and other coherent light techniques can often reveal the presence of defects or inhomogeneities even when quantitative evaluation of the optical output is difficult.