Study Of MTF Measurement Technique Based On Image Analysis

Optical Transfer Function (OTF) is an objective, accurate, quantifiable image quality evaluation parameter, and it can be directly measured, which has been widely applied in optical designing, manufacturing, testing and information processing. In the OTF measuring technique on image analysis, OTF data of an optical system is calculated by analyzing images of a characteristic object. For the simple structure of its measuring system and without theory error, now it has been the trend of development of OTF measuring method. With OTF measuring demand of a big aperture and long focus optical system in the 863 practical project, OTF measurement technique on image analysis is deeply researched in this paper.The main researched contents and their achievements are as follows:(1) research on OTF measurement technique based on special object image analysing.In slit image analysis method, Adaptive Average Rows Processing Method (AARPM) is proposed, in which according to slit image tilt angle, a large number of average rows is used to suppress noise effect at low spatial frequency and a small number of rows is used to decrease the measuring error generated by convolution smoothness effect at high spatial frequency. It can overcome the shortcoming of the traditional algorithm, which uses a fixed number of average rows to process the whole frequency band to produce a bigger deviation at high frequency just for suppressing noise effect. According to the deep analysis of image energy distribution of tiny tilted slit, the auto-scan computer method of sub-pixel sampling space is established. The method uses the rows that energy distributions of slit image stagger one pixel exactly to determine the scale of sub-pixel segment. The practical test results prove that the algorithm is correct, effective and reliable.Pinhole has two dimension character, and can be used as object in OTF measurement system for getting two dimension OTF data one time without changing scan angle or slit direction. The main disadvantage of OTF measurement based onpinhole image analyzing is low image-plane energy and liable to being polluted by noise. The OTF measurement technique based on image analysis of the pinhole is researched deeply. Two-square-shape-difference window and adaptive window size are proposed for reducing dark background effect and computing sampling window size respectively.(2) Effects of measurement system parameter setting on measurement precision and corresponding correction method are researched. The effects of auxiliary optical system designed independently can be corrected by dividing out Geometry Aberration Modulation Transfer Function (GMMTF). The system parameters affecting the spatial frequency calibration are determined, and the measurement methods of these parameters are provided. A novel test method using rectangular grating is adopted to determine the system whole magnification, without changing the structure of the test system, and the spatial frequency transform is used to calculate its correspondent spatial frequency scaling. Through analysis of CCD imaging theory, the effect of CCD pixel size, pixel space, linearity, transfer efficiency, and dark current on OTF measurement precision is studied and its relative error correction method is established.(3) Aimed at the test requirement of a camera optical system in practical engineering, a set of OTF measurement equipment on the pinhole image analysis was developed. The OTF measurement equipment consists of modules and can give OTF measurement results of varies system by replacing a certain module. According to the practical index of the camera optical system, the OTF measurement equipment was calibrated by a 50mm focus plane-convex standard lens, and the noticeable problems in calibration process and their solving methods are given in detail. Compared experiments to national first class metrology station of China show that the measurement error of this OTF measurement equipment less than ± 3% within 1001p/mm spatial frequency. Combined with 20m-focus collimation, the engineering camera OTF data were measured, and the measurement results agree well with results measured indirectly by ZYGO interferometer.(4) Through the analysis of imaging mechanism of the fiber optic element,optical transfer function theoretic model of fiber optic element is established. Optical transfer functions of a set of optic fiber taper and optic fiber faceplates were measureed by pinhole image analysis, and the test results agree well with the theory analysis.