**2.4 Precision measurement requirements for large radius of curvature**

In order to ensure similar performance for each laser line of National Ignition Facility (NIF), it is necessary to keep each surface radius of curvature identical. The interferometric method is generally used to measure the radius of curvature of a spherical optical surface. However, it is difficult to perform on surfaces with large radii exceeding a few meters with large aperture, because it requires the test part to be moved over a distance equal to its radius. It means the distance between the surface under test and the reference is very long so it increases the measurement uncertainty. The NIF developed a method, based on dual-focus zone plates to solve this problem (Wang, Q. et al., 2008). In contrast to the NIF, Chengdu Fine Optical Engineering Research Center in China had made the measurement of long radius of curvature on large aperture lens with pencil bean scanning LTP-MF (Ye & Yang, 2011) with very promised accuracy. The nano-accuracy measurement can increase the test accuracy of radii of curvature significantly.
