The basic concept of Shearography is to examine the test surface of the specimen with a shearography camera. The camera records one interferometric photo of the surface. This photo can be thought of as a unique footprint of this surface at this unloaded state, including surface roughness and shape.
We now stress this material with a small amount of load, for example with heat. The material wants to expand when heated up, and if it has weak spots, it will be allowed to expand more. At the loaded state one more interferometric photo is taken. Now we also have a interferometric footprint of the area at the deformed state.
To extract information about the difference between the two states, we subtract the two images and a shearogram is created.
This shearogram can be thought of as a topography of the surface, but only gradients (slopes) are measured, not absolute heights of the hills. Defects will be seen as “hills” popping out of the plane. You can quantify the size of the defects (in plane size) by measuring how large your hills are.
Speckle and Shearing
When a surface is illuminated with a coherent laser light source, a stochastic interference pattern is created. This “speckle” pattern is projected on a camera’s CCD chip. In contrast to ESPI (Electronic Speckle Pattern Interferometry) where the speckle is compared with a reference light path, Shearography uses a reference created by shearing the image of the test object to create a double image. This makes the method much less sensitive to external vibrations and noise, and you can measure without any need for a vibration damped table, making Shearography ideal for field usage.
The basic Shearography principle is to subtract two images (interferometric footprints) of a test object, before and after load. The intensity information from those images is then subtracted, and the surface deformation can be displayed due to the speckle information. Surface roughness will be neglected ad-hoc in the subtraction process.
Phase Shift Technology
To increase the sensitivity of the technique, a real-time phase shift process is used in the sensor. This uses a stepping mirror that shifts the reference beam and enhances the results with directional information included with the deformation.