Characteristics Of The Optical Low-pass Filter

Optical low-pass filter (OLPF), birefringent crystal is adopted in most case, has been wide-applying in opto-electric imaging system.In this thesis, Opto-electric imaging system is a discrete imaging system, Sampling of photoelectric imaging system can infinitely replicate the scene spectrum in spectrum space, And If the sampling intervals are too large to satisfy the Nyquist criterion, the replicas overlap, and there is aliasing. Morie fringe appear in image. The quality of image receded and the trueness of color depressed. An optical low-pass filter (OLPF) used in front of opto-electric imaging sensor, can effectively limit the scene spectrum width and critically satisfy Nyquist sampling condition, so the aliasing can be eliminated and quality of the image reconstructed is improved.Optical system is a space-filter by Fourier Optics. Optical transfer function (OTF) can evaluate the frequency characteristics of a optical system or device. Analytical method of optic transfer function is employed to OLPF in the paper. By this way, the amplitude frequency response characteristics of OLPF in one dimension and two dimensions are presented. MATLAB is used to simulate the OTF of different structure OLPF. The result of analysis shows that the cut-off frequency of filters at one of two dimensions is based on the thickness of birefringent crystal which normal direction of axial plane is vertical to the dimension. For multi-plate OLPF, an improving design with a birefringent crystal which normal direction of axial plane is at some angle, enhance the convergent response of filter at high space frequency but do not change the cut-off frequency.Based on those results of analysis, a method of design OLPF which is suited to special Opto-electric imaging sensor, is presented. According to the method, parameters of birefringent crystal can be obtained. An example is provided.A measurement system is developed to study the optical characteristic in further. Two sets of microscope were reformed. According to characteristic of birefrigent crystal, the separate images of a point light source passing through OLPF is captured and processed by computer. From the situation and data of processed image, the thickness of crystal and amplitude frequency response characteristics of OLPF would be obtained. The measurement accuracy of thickness is 0.005mm. Measurement system can not only satisfy the study of OLPF, but also be used to measure the qualityof OLPF in real-time with some improvement.