The speckle pattern from the subset window can be tracked with high accuracy by using the proposed INCC procedure . In contrast to a 1D line profile provided by the LTP and the NOM , SAM provides a 2D map of the slope in a single scan by dividing each speckle image into multiple subset windows and performing the pixel - wise analysis perpendicular to the scanning direction . Moreover , unlike the LTP and the NOM , SAM is able through the proposed data analysis procedure ( Mode 2 ) to test strongly curved mirrors by measuring the first derivative of the slope .