Applications of holography and speckle photography to the measurement of displacement and strain

Holographic interferometry was used to investigate the behaviour of a plastic pipe joint when subjected to internal pressure. By a combination of ‘live’ and double-exposure techniques the variation of bending moment along an axial surface generator of the joint can be evaluated. To obtain the surface strain, an interferometric technique using speckle photography was used. An important practical factor is the method of mounting the object. For the measurement of much larger surface movements, such as occur when a cantilever is loaded, simple speckle photography, with subsequent point-by-point analysis of the recorded image, is used.

Stresses developed in optical film coatings

The development of stress in evaporated dielectric and metal films, used as optical coatings, has been investigated experimentally by observing the bending of a thin silica strip as it becomes coated. The strip forms one mirror of a laser interferometer mounted within the coating chamber, giving high measurement sensitivity. Stress data on a number of materials commonly used in coating are presented, and the behavior of multilayer coatings investigated. Analysis of the effect of film stress upon the figure of optical surfaces is given, and the influence of stress upon the mechanical stability of multilayers is discussed.