Aspheric surfaces are generally tested using profilometry or interferometry. But profilometry causes damage to the surface and takes long time to measure large scale. These problems are solved by interferometry which has also high resolution. Therefore interferometry is the best choice to test the large surface. However, the interferometry has one problem. If the spatial frequency of the interferogram is higher than the detector resolution, the result is not reliable. In this case, null lens and computer generated hologram (CGH) are needed. These test methods have some drawbacks. The use of these compensating elements are very expensive and it takes long time to make them. Whenever the aspheric surface changes, it also needs new null optics. To solve this problem, we present zonal scanning method that avoids the use of compensating elements. This method is based on a Fizeau interferometer and a nanopositioning motorized stage. Zonal scanning method consists of varying the distance of an aspheric surface from the focus of an interferometer reference sphere with a nanopositioning motorized stage. As the aspheric surface mounted on the stage is moved along the optical axis, the radius of curvature of the reference wavefront which matches the aspheric surface changes. When one of reference wavefronts having a different radius of curvature most fits the aspheric surface, we can measure that position. This procedure is iterated starting from the optical axis up to the end of the aspheric surface to obtain the surface error being tested. And then each result connects with a suitable algorithm. To obtain accurate results, we present an optimized sliding distance of each step and total sliding distance. We compared the experimental results of zonal scanning method with the null lens test.
