High-precision Measurement Method Of Large Aspheric Surface Based On Angle Scanning

An aspheric lens is a very important optical component, which is widely used in aerospace, aviation, shipbuilding, medical and other fields. Compared with the spherical lenses, an aspheric lens has various curvature radius, which means the aspheric surface is more flexible and has more application advantages. For example, the aspheric surface can simplify the structure of the optical system, reduce the weight of a system, and improve the resolution of a system as well as the image quality. However, the aspheric surface testing, particularly for the large surface, is always a big challenge in the field of optical measurement. For the small surface which has small asphericity, the interferometry measurement can help achieve the ideal measurement precision. However, the large or the super large aspheric surface cannot be measured by a general interferometer easily, and may be scratched by the contact measurement.Aiming to solve the measurement problem of a large aspheric surface, this paper has designed a kind of high-precision measurement system of large aspheric surface based on angle scanning. The basic principle of angle scanning measurement is to derive the variation of target surface curvature by measuring the variation of measurement position’s angle, and then to plot the target surface profile through numerical integration of the curvature values.The following research has been carried out in this paper.1. The structure design of large aspheric surface measurement system. The three-dimension measuring method for large aspheric surface whose aperture is below 200 mm is proposed and the structure design of large aspheric surface measurement system is completed. The types of hardware are determined after calculating the precision requirements of measurement system are obtained and then the 3-D CAD drawings of the aspheric surface measurement system are accomplished. The main process of aspheric surface measurement is analyzed and the measurement steps are formulated.2. Error analysis and compensation methods of large aspheric surface measurement. Different kinds of errors factors, including the random errors and system errors, in the measurement system are analyzed. The sensitive error factors which impact the accuracy of measurement seriously are analyzed through calculating their influence on aspheric surface measuring. A series of error compensation methods for sensitive errors are proposed, and then the measurement uncertainty of these factors after compensation is calculated. The propagation of measurement uncertainty of the sensitive factors is analyzed and the codes which are used to calculate the propagation of measurement uncertainty are written with MATLAB. Then the correctness of error model is verified by Monte Carlo Simulation.3. The method of measurement data processing for large aspheric surface. The 3-D measurement method which is achieved by the radial angle scanning for target surface is applied. A data processing method is proposed to unify all the 2-D measurement results by establishing a cylindrical coordinate, the 2-D measurement is expanded to 3-D measurement. Based on the “LOESS” algorithm, a type of non-parametric regression, a data processing method of analyzing the spatial points cloud is proposed. The reliability of the method for processing measurement results can be verified reliable after 50 times of the Monte Carlo Simulation and the RMS measurement precision of large aspheric surface achieves /60(about 10 nm, = 632.8nm), which meets the precision requirement perfectly.