Optical non-destructive testing and evaluation techniques include Holography, Digital Shearography and Electronic Speckle Pattern Interferometry (ESPI). These methods are non-contacting, whole field inspection techniques and employ a single mode laser as the medium to produce the optical interference. Holography most comonly uses a photographic emulsion to record the information produced by the process, whereas Digital Shearography and ESPI use video cameras and computers to record and process the information.
The presence of a defect weakens the structural integrity of an object, which in turn locally influences the object's surface deformation when stressed. As all three methods of inspection can be used to monitor an objects' surface displacement in response to an applied stress, these optical interference techniques are suited for the detection of material and structural defects.