Radius of curvature measurement based on wavefront difference method by the point diffraction interferometer

• Non-contact method does not damage the optical surface.
• Measurement precision ΔR/R0ΔR/R0 is less than 5×10−4.
• The high-numerical-aperture spherical surfaces can be tested and the error source from the NA of test mirror is eliminated by our method.
• The radius of curvature of spherical mirrors and the surface figure can be measured in a higher precision simultaneously.

A method for measuring the radius of curvature with a pinhole point diffraction interferometer (PDI) is proposed. Using the wavefront difference method and the Gaussian imaging equation, the longitudinal displacement of the converging rays passing through a standard plane-parallel-plate sample in PDI interference cavity is determined. Based on this longitudinal displacement, a precise formula for radius of curvature calculation is deduced. An experimental system for radius of curvature measurements is set up to verify the principle. With this testing system, the radius of curvature of spherical mirrors and the surface figure can be measured in a higher precision simultaneously. Some sources of uncertainty in measurement are discussed based on detailed error analysis. The experimental results indicate that the measurement accuracy ΔR/R0ΔR/R0 is in the order of 10−4.