Interference fringe pattern of a laser beam reflected from the surface to be measured is constructed directly on the CCD image sensor, but the density of fringes is high in the region near the sides of the image figure. This makes image processing, especially the phase concatenating procedure, difficult and noise occurs easily as a result. When phase concatenation is carried out for reference surface and for the surface to be measured independently, further image processing becomes impossible. In this report, a new method for processing the brightness of interference fringes is shown. Using planar test pieces with simple form deviation, significant improvement in the results of the proposed data processing method is shown.