sentences:  When laser light is passed through the CGH, the zero-order diffracted beam experiences no wavefront modification.The wavefront of the first-order diffracted beam, however, is modified to match the desired shape of the test surface.In the illustrated Fizeau interferometer test setup, the zero-order diffracted beam is directed towards the spherical reference surface, and the first-order diffracted beam is directed towards the test surface in such a way that the two reflected beams combine to form interference fringes.