Digital image correlation is an excellent strain measurement technology. This figure shows two images taken using image correlation techniques on a piece of prestressing steel. The top image illustrates time zero with an applied stress of zero. The second photo below shows the specimen at 100 seconds, with an applied stress state of 1950 MPa. The speckle pattern seen allows a software to track movement of the bar as it deforms (strains) between different pictures. The software tracks tiny patches. Think of them loke a box. The software can then be used to tell the user how much these patches move between the images.