Research On Digital Knife-edge Technique To Measure The Large Aperture Aspheric Optical Element

With the development of machining craft for aspheric optical elements, aspheric optical elements have been widely used in aviation, spaceflight, astronomical telescope, military and life. Optical testing and machining are always the two problems of optical industry. With the development of modern machining craft of high precision computer controlled digital machine tool, machining precision of aspheric optical element has been improved continuously. In order to fit the status of machining technology of modern aspheric element, researchers had investigated many new aspheric element testing method and technology.Measurement methods, such as interferometer, profilometer, computer holograph, and Hartmann methods, each has its own deficiency, have not solved the testing question of aspheric element thoroughly. Based on the analyzing of different measurement method of aspheric element, digital knife-edge tester for measurement of large diameter aspheric optical element is brought forward in the paper.As it has the advantage of high sensitivity and intuits, simplicity, low cost and better applicability, knife-edge is very suitable for shape testing of large diameter and long focal length optical elements. Digital knife-edge has the advantage of traditional knife-edge, and combined the computer and digital image processing technology to quantify the measurement result of knife-edge. It is one of the developing aspects for aspheric optical elements.Digital knife-edge measurement methods and technology for measurement of aspheric element is studied in this paper. Through theory analysis and experiment work, following problems are solved in this paper.①According to the knife-edge testing principle, digital knife-edge apparatus was built. Optical design, structure and software design of digital knife-edge apparatus were studied and introduced in the paper, including the designing of the light and light source arrangement, designing of knife-edge and three dimensions moving stage, choosing CCD and analyzing of its character, and designing of software.②Digital knife-edge testing theory is researched in the paper. Mathematics rule of change of gray value when knife-edge is moving to cut the light spot is established in the paper. And mathematical model is also established according to this rule. Function approach, least square fitting and subsection data fitting methods are used respectively to fit the gray data at every pixel in CCD, so that to get the dark threshold corresponding knife-edge position. According to the position, we can get the bias of collimated light from the ideal focus position induced by the defect of the optical element to be measured. That is the kernel of digital knife-edge apparatus to quantify the shape of optical elements. Subsection data fitting method brought forward in the paper has the advantage of little amount of computation and high precision, which reduces the complexity of calculate and time of digital knife-edge apparatus software.③Programmed with VC++ software quantification software, the software of the digital knife-edge apparatus has the ability to control the knife-edge cutting process, image sampling, processing and saving. In the end, according to the variation rule of the gray value of every pixel to analyze, fit and calculate the data, we can retrieve the shape of the optical element be measured, give the PV and RMS value, show the two and three dimensions shape plot. Except that, the software also has the function of image replay, which can replay the image when knife-edge cutting the light spot. So we can observe the shape of optical element more intuitively.④Using digital knife-edge, the spherical and parabolic optic elements with large aperture and long focal length have been measured and the results are given. The measurement results are compared with the results of interferometer, the consistency is about 0.01μm. which proves the correctness and validity of digital knife-edge apparatus researched in the paper.⑤The measurement uncertainty of digital knife-edge apparatus is analyzed in detail. methods for improving the measurement precision are given in the paper.According the research work of this paper, a set of digital knife-edge apparatus was developed, which not only has the advantage of traditional knife-edge but also combining the content of quantification and calculation, so the research work of this paper is valuable bith in theory and practice.