A method and an apparatus for scanning a test object are introduced. A reference object is scanned to build a gain correction map including gain values for scanning points on a surface of the reference object. The test object is also scanned to obtain measurements for scanning points on a surface of the test object. Amplitudes of the measurements obtained for the scanning points on the surface of the test object are normalized using the gain values of the gain correction map. The apparatus has a probe mounted on a mechanical scanner, and a controller controlling the scanning and normalizing operations. The method and apparatus can be used to create an image of the test object for non-destructive testing.