Design of double refractive pattern recognition system for Optical Low Pass Filter

摘要

Photo-electronic imaging system is a discrete imaging system, according to Nyquist sampling theorem, if the maximum spatial frequency is higher than Nyquist frequency, there is aliasing, and Morie fringe appears on image. The quality of image is receded and the trueness of color depressed. An optical low pass filter (OLPF) used in front of photo-electronic imaging sensor, can effectively limit the frequency spectrum width and critically satisfy Nyquist sampling condition. Thereby, the aliasing will be eliminated and the quality of the image will be improved. This paper analyzes the characteristics of frequency response of the OLPF and designs a novel system to measure the optical characteristic of the OLPF. According to the characteristic of birefringent crystal, a light spot will be separated by the OLPF into several light spots which will be processed by the computer. For the size of light point determined the limit of measurement accuracy of OLPF's thickness, laser source, which can obtain light point with 2um diameter is used here as a target light point. Magnified lens are used to improve the precision of the system. Other system used long working distance (WD) microscope objective. Instead, this novel system uses the standard 100x optical microscope objective (WD<0.2mm) as magnifying system. In this way, the cost of the system will be reduced in a great deal. The software of the system is also very powerful, in addition to the basic function image caption and scanning, it can automatically detect the number of light spots, distance and angles between light spots. The system can accurately measure the distance of point light at a high resolution of 0.1um, and the measurable thickness of OLPF is from 0.5 to 5mm.