全视系统空间MTF测试系统设计

Design of a pattern for measuring spatial MTF in the human visual system

目的 :介绍一种用VB程序编制的快速、简便、有效的全视系统黑白和色觉空间调制传递函数 (MTF)测量系统设计方法。方法 :按照调制传递函数MTF定义和Weber定律 :采用矩形条纹作为视标 (矩形条纹由VB编程实现 )。对比度由计算机的像素灰度级别变化决定。将MTF的范围按对数等间隔分成 13档 ,然后按公式分别计算每一档对应的前景和背景的灰度值。空间频率SF的设计由视标宽度、视标和被测者距离所决定。结果和结论 :在计算机显示屏上显示出各种空间频率和各种对比度的矩形条纹 (并保持它们平均亮度不变 ) ,条纹分黑白、红、绿、蓝四种 ,作为测量全视系统黑白和色觉空间调制传递函数 (MTF)的视标。空间频率从 3cpd到 47.5cpd ,分 13档 ,按对数等份 ,沿X方向改变 ;MTF也分 13档 ,沿Y方向改变。用本视标初步检测了 2 0只正常眼 ,所得结果和文献报道相似 ,均呈带通型。但本研究应用方波条纹 ,因为其边缘能增强视知觉 ,使得视觉系统感到暗区更暗 ,亮区更亮 ,而测量的MTF应包括这种边缘增强效应。另外 ,在相同空间频率下 ,黑白全视系统MTF与绿色MTF相差不大 ,而红色MTF与绿色MTF、黑白MTF相比 ,MTF值降低 ;蓝色MTF曲线与上述三种比较 。

Objective:To introduce a pattern which can be used to measure spatial modulation transfer function(MTF)(white-black and color) of the human visual system easily,fast and reliably.Methods:Modulation transfer function(MTF) is defined as the ratio of the fourier transformation of the image to that of the object. When the object consists of sinusoidal stripes,this is the ratio of the modulation(contrast) of the image to that of the object. For the visual system,if there is a steady illumination of the retina,the detectable contrast of the image stays constant according to weber's law. Then the modulation transfer function MTF(under this condition it is also called as threshold MTF or contrast sensitivity function(CSF) is determined by the contrast of the object M o. MTF=K/M o. K is a constant. The contrast of the object M o is determined by the brightness of the stripe versus the brightness of the background. The brightness of the stripes and background is determined by levels of gray(0～255) in computer pixels. The minimum value of M o is 0.0039 when the gray level of the stripe is 128 and that of the background is 127,when the total range of gray is 255. The range of M o(then MTF) consists of 13 levels ranging from 0.0039 to 0.702,and each is separated by an equal logarithmic interval. The relative values of MTF range from 100 to 0.56(where the maximum value of MTF is set at 100). The brightness(gray level) of the stripes and the background of each contrast is calculated according to the assigned values of M o,with total brightness kept constant(the brightness of the stripe added to that of the background). The spatial frequencies of the stripes range from 3 cpd(cycle per degree) to 47.5 cpd,and are also designed as 13 levels,equally separated in logarithmic value. All stripes with various spatial frequencies and MTFs are displayed on a computer screen with the frequency of the stripe increasing along the x axis and MTF decreasing along the y axis,and used as the pattern for measuring the spatial MTF of the human visual system. The designs are programmed in VB program language and run on a pc computer.Results and Conclusion:We designed four such patterns black-white,green,red and blue. The MTFs ranged from 100 to 0.56 along the y axis,equally separated in logarithmic values. Spatial frequencies ranged from 3 to 47.5 cpd along the x axis,also equally separated in logarithmic values. For the sake of simplicity and as a principle prototype,rectangle stripes were used instead of sinusoidal ones. As a preliminary application,the MTFs of 20 normal eyes were measured with the pattern. The results were similar to formerly reported studies. All MTF curves had a bandpass form. There was little difference between the green MTF and black-white MTF. The red MTFwas lower compared to the green and black-white MTFs along the entire frequency range. There were large differences between blue and black-white MTFs. Since rectanglular stripes were used,the results may include an edge enhancement effect which sharpens the contrast between stripe and background at the border. Additional studies are needed to learn more about the details.