The present invention relates to a test block for use with interferometric analysis apperatus which simulates defects of known dimension in a test object. The known defects created by cavities in the test block which is positioned within the interferometric analysis apparatus test chamber for analysis to determine whether the apparatus is properly calibrated. The cavities have a predetermined geometry and location within the test block which control flexure of a sheet of flexible material associated with the test block. The predetermined flexure can be correctly detected only by a properly calibrated interferometric device. The test block provides for a vent line to communicate with the cavities when the test block is placed inside a vacuum chamber associated with the interferometric analysis device to allow a pressure differential to create the forces for the predetermined fixture.