Prism - Residual stress measurement based on hole-drilling and ESPI Prism is an advanced hole-drilling system that dramatically improves the ability to make quick and accurate residual stress measurements. Just like the conventional strain gage hole-drilling systems, the instrument measures changes in the part surface caused by the hole drilling and determines the previously existing residual stresses. However, the often time-consuming application of strain gages is avoided and a large area around the hole is analyzed, not averages from just a few areas. Prism measures surface distortion using electronic speckle pattern interferometry (ESPI). Only minimal sample preparation is required. The update of the coefficient data set in 2015 has significantly improved the quality of the stress results. This also increases the relative depth from which reasonable results can be expected.