光学低通滤波器的特性研究

【作者】 李云川；

【导师】 林斌；

【作者基本信息】 浙江大学 ， 光学工程， 2006， 硕士

【摘要】 光学低通滤波器(Optical Low Pass Filter, OLPF),经常采用双折射晶体,并被广泛的运用于光电成像系统中。 本文中,光电成像系统是一种离散成像系统,光电成像传感器对图像的抽样过程会引起图像频谱在频谱空间上的多次复制,如果抽样过程不满足系统的奈奎斯特条件,相邻的频谱项就会产生混叠,在图像上产生莫尔条纹,影响成像质量和色彩真实性。OLPF通过前置滤波能够有效地限制图像的高频分量,满足系统的奈奎斯特条件,避免图像质量的下降。 在傅立叶光学中光学系统是一种空间的低通滤波器,光学传递函数能够评价光学系统或器件传递信息的频率特性。利用光学传递函数的方法分析了OLPF的光学传递函数。通过这种方法,得到了一维和二维OLPF的幅频响应特性,并用MATLAB模拟了几种不同结构OLPF的光学传递函数,通过比较和分析,提出OLPF二维方向上的截止频率点取决于光轴面法线方向垂直于该维方向的晶片厚度。对于多片式OLPF一种改进型的光学低通滤波器,其中光轴面法线和二维方向有夹角的晶片能够加强对高频部分的抑制作用,而不改变截止频率。 基于对OLPF的光学传递函数的分析,提出了选取晶片参数的方法,设计与光电图像传感器相适配OLPF。结合实际提供设计的例子。 为了进一步研究OLPF的光学特性,利用两套改造的显微镜组,研制了一套OLPF的测试系统。根据双折射晶体的光学特性采集点光源通过OLPF后所成的分离像点,通过软件设计进行图像处理,从采集到的光点像分布情况和数据可以获得组成OLPF各晶片的尺寸,同时还能得到OLPF的调制传递函数。对晶片的测量,精度可以达到0.005mm。整个测试系统不但可以满足对OLPF的特性研究,经过进一步改造还能对OLPF的生产质量进行实时检测。更多还原

【Abstract】 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.更多还原