Analysis of the optical quality by determining the modulation transfer function for anterior corneal surface in myopes

AIM: To describe the characteristics of modulation transfer function (MTF) of anterior corneal surface, and obtain the the normal reference range of MTF at different spatial frequencies and optical zones of the anterior corneal surface in myopes. METHODS: Four hundred eyes from 200 patients were examined under SIRIUS corneal topography system. Phoenis analysis software was applied to simulate the MTF curves of anterior corneal surface at vertical and horizontal meridians at the 3, 4, 5, 6, 7mm optical zones of cornea. The MTF values at spatial frequencies of 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 and 60 cycles/degree (c/d) were selected. RESULTS: The MTF curve of anterior corneal surface decreased rapidly from low to intermediate frequency (0-15cpd) at various optical zones of cornea, the value decreased to 0 slowly at higher frequency (>15cpd). With the increase of the optical zones of cornea, MTF curve decreased gradually. 3) In the range of 3 mm- 6 mm optical zones of the cornea, the MTF values measured at horizontal meridian were greater than the corresponding values at horizontal meridian of each spatial frequency, the difference was statistically significant (P<0.05). At 7 mm optical zones of cornea, the MTF values measured at horizontal meridian were less than the corresponding values at vertical meridian at 10-60 spatial frequencies (cpd), and the difference was statistically significant in 25, 30, 35, 40, 45, 50 cpd(P<0.05). CONCLUSION: MTF can be used to describe the imaging quality of optical systems at anterior corneal surface objectively in detail.

Modulation transfer function characteristic of uniform-doping transmission-mode GaAs/GaAlAs photocathode

The resolution characteristic can be obtained by the modulation transfer function （MTF） of a GaAs/GaA1As photocathode. After establishing the theoretical model of GaAs（100）-oriented atomic configuration and the formula for the ionized impurity scattering of the non-equilibrium carriers, this paper calculates the trajectories of photoelectrons in a photocathode. Thus the distribution of photoelectron spots on the emit-face is obtained, which is namely the point spread function. The MTF is obtained by Fourier transfer of the line spread function obtained from the point spread function. The MTF obtained from these calculations is shown to depend heavily on the electron diffusion length, and enhanced considerably by decreasing the electron diffusion length and increasing the doping concentration. Furthermore, the resolution is enhanced considerably by increasing the active-layer thickness, especially at high spatial frequencies. The best spatial resolution is 860 lp/mm, for the GaAs photocathode of doping concentration 1 ×10^19 cm 3 electron diffusion length 3.6 μm and the active-layer thickness 2 μm, under the 633-nm light irradiated. This research will contribute to the future improvement of the cathode＇s resolution for preparing a high performance GaAs photocathode, and improve the resolution of a low light level image intensifier.

Modulating J-V hysteresis of planar perovskite solar cells and mini-modules via work function engineering

Commercialization of perovskite solar cells(PSCs) requires the development of high-efficiency devices with none current density-voltage(J-V) hysteresis. Here, electron transport layers(ETLs) with gradual change in work function(WF) are successfully fabricated and employed as an ideal model to investigate the energy barriers, charge transfer and recombination kinetics at ETL/perovskite interface. The energy barrier for electron injection existing at ETL/perovskite is directly assessed by surface photovoltage microscopy, and the results demonstrate the tunable barriers have significant impact on the J-V hysteresis and performance of PSCs. By work function engineering of ETL, PSCs exhibit PCEs over 21% with negligible hysteresis. These results provide a critical understanding of the origin reason for hysteresis effect in planar PSCs, and clear reveal that the J-V hysteresis can be effectively suppressed by carefully tuning the interface features in PSCs. By extending this strategy to a modified formamidinium-cesium-rubidium(FA-Cs-Rb) perovskite system, the PCEs are further boosted to 24.18%. Moreover, 5 cm × 5 cm perovskite mini-modules are also fabricated with an impressive efficiency of 20.07%, demonstrating compatibility and effectiveness of our strategy on upscaled devices.

Fuzzy image restoration using modulation transfer function

The key difficulty of restoring a fuzzy image is to estimate its point spread function( PSF). In the paper,PSF is modelled based on modulation transfer function( MTF). The first step is calculating the image MTF. In the traditional slanted-edge method,a sub-block is always manually extracted from original image and its MTF will be viewed as the result of the whole image. However,handcraft extraction is inefficient and will lead to inaccurate results. Given this,an automatic MTF computation algorithm is proposed,which extracts and screens out all the effective sub-blocks and calculates their average MTF as the final result. Then,a two-dimensional MTF restoration model is constructed by multiplying the horizontal and vertical MTF,and it is combined with conventional image restoration methods to restore fuzzy image. Experimental results indicate the proposed method implementes a fast and accurate MTF computation and the MTF model improves the performance of conventional restoration methods significantly.

Comparison of the retinal modulation transfer function between amblyopes successfully corrected and normal subjects

AIM:To analyze the retinal modulation transfer function between amblyopes whose visual acuity was corrected to 5.0 and normal subjects at the same age. METHODS: RM-800 used to detect contrast sensitivity was adopted to measure MTF of 96 amblyopes (96 eyes) whose visual acuity was corrected to 5.0 and 80 normal controls (80 eyes) at the same age under six interference fringes (IVA=0.06, 0.1, 0.2, 0.4, 0.6, 0.8). RESULTS: The functional values of amblyopes were significantly lower than those of normal subjects in every fringe (P<0.01), especially in medium and high frequency. CONCLUSION: For amblyopes, MTF was still abnormal after stopping the treatments.

A new modulation transfer function for ocean wave spectra retrieval from X-band marine radar imagery

When imaging ocean surface waves by X-band marine radar, the radar backscatter from the sea surface is modulated by the long surface gravity waves. The modulation transfer function （MTF） comprises tilt, hydrodynamic, and shadowing modulations. A conventional linear MTF was derived using HH-polarized radar observations under conditions of deep water. In this study, we propose a new quadratic polynomial MTF based on W-polarized radar measurements taken from heterogeneous nearshore wave fields. This new MTF is obtained using a radar-observed image spectrum and in situ buoy-measured wave frequency spectrum. We validate the MTF by comparing peak and mean wave periods retrieved from X-band marine radar image sequences with those measured by the buoy. It is shown that the retrieval accuracies of peak and mean wave periods of the new MTF are better than the conventional MTF. The results also show that the bias and root mean square errors of the peak and mean wave periods of the new MTF are 0.05 and 0.88 s, and 0.32 and 0.53 s, respectively, while those of the conventional MTF are 0.61 and 0.98 s, and 1.39 and 1.48 s, respectively. Moreover, it is also shown that the retrieval results are insensitive to the coefficients in the proposed MTF.

Solving the atmospheric scattering optical transfer function using the multi-coupled single scattering method

The atmospheric scattering optical transfer function （OTF） is solved by applying the multi-coupled single scattering （MCSS） method to the three-dimensional radiative transfer equation （RTE） under the periodic ground condition. This approach is a direct hit to the atmospheric scattering OTF using the same original context of modulation transfer function （MTF） measurement, i.e., images of sinusoidal grating at different spatial frequencies. Both the amplitude and phase shift of the OTF at various zenith and azimuth angles can be obtained at an arbitrary spatial frequency.

基于MTF理论的遥感图像复原

由于遥感图像成像过程中受传感器性能下降、大气扰动等因素影响,致使图像质量退化。为了改善遥感图像质量,从图像质量退化原理分析,提出了一种基于调制传递函数(modu lation transfer function,MTF)理论的图像复原方法。该方法主要包括去噪和MTF拉伸两部分。针对遥感图像的特点,给出了一种有效的基于频域的去噪方法。为了能够调节频谱拉伸的程度,还给出了一种指数调节MTF曲线的方法。中巴(中国和巴西)卫星红外遥感图像的实验结果表明:该新方法有效地提高了图像的对比度(对于实验图像来说,较复原前图像对比度提高了7倍多,方差也由原来的8.77提高到17.37)、熵等图像要素,从而改善了图像的质量。

刀刃法在轨MTF测量性能分析

文章主要研究采用刀刃法测量在轨遥感器的MTF。刀刃法MTF测量准确程度如何,需要有一个定量的客观的评价,文章用仿真的方法对MTF测量精度进行了检验,对于噪声对MTF的影响进行了研究。

基于MTF和变分的全色与多光谱图像融合模型

Pan-sharpening将高分辨率图像全色(Panchromatic,Pan)波段的空间细节注入多光谱(Multispectral,MS)波段,以生成同时具有高光谱和高空间分辨率的多光谱图像.为改善融合效果,需要考虑多光谱和全色波段的调制传输函数(Modulation transfer function,MTF).本文提出了一个新的基于MTF和变分的Pan-sharpening模型.该模型的能量泛函包括两项,第1项为细节注入项,基于高通滤波器从Pan波段中提取细节信息并注入融合图像;第2项为光谱保真项,基于MTF设计多孔小波的低通滤波器以保持MS波段的多光谱信息.在Quick Bird、IKONOS和Geo Eye数据集上的融合结果表明,该模型可以生成同时具有高空间和高光谱质量的融合图像,融合效果优于AWLP、IHS BT、HPM-CC-PSF、NAWL、快速变分等算法.

图像三要素与MTF的分析研究

中低分辨率在轨运行卫星遥感器的调制传递函数（Modulation Transfer Function，MTF）的监测一直是研究的难题。现提出的方法解决了中低分辨率在轨运行卫星遥感器MTF监测的关键技术。考虑到卫星遥感器的最终产物是遥感图像，从图像信息理解的三要素出发，即灰度、纹理和边缘，分析图像三要素与遥感器MTF的关系。在分析过程中，应用主成分分析方法，不仅对多组原始数据进行了简化，而且对图像参数进行了降维。通过与其它方法的比较，进一步验证了使用主成分分析方法降维的有效性；同时实验结果亦表明了主成分分析方法的可靠性。最后建立了图像单个综合参数与MTF的关系模型，拟合结果说明，此模型能较好地表达图像参数与MTF之间的关系，这将为在轨运行条件下的建模提供十分重要的技术支持。

数字化X射线成像系统MTF的刀口测量法

讨论了数字化X射线成像系统调制传递函数(MTF)的刀口测量法.首先通过数值模拟方法研究了采样率和噪声对准确测量MTF的影响,真实测量中采用多条过采样刀口响应函数(ERF)取平均以压制噪声获取光滑的ERF曲线;然后用指数函数对所获ERF进行分段拟合以确保刀口边缘强度值变化的单调性,将由其获得的MTF值与线对卡结果进行比较,证明了该方法用于成像系统MTF测量的有效性.同时,通过实验分析介绍了刀口法材料、材料厚度、光源曝光电压及曝光时间的选择,最终在本系统测量中选择的刀口材料为1 mm厚铅板,曝光管电压为33 kV,曝光时间为10 s.

长波红外相机多次采样叠加对MTF的影响分析

根据长波红外相机积分时间短的特点,阐述了多次采样叠加技术的原理和应用.并在此基础上分析了采用多次采样叠加技术对系统MTF产生的影响.给出了不同采样系数和叠加次数对应的系统MTF变化.当采样系数固定时,探测器MTF随叠加次数的增加而减小,系统推扫方向MTF随叠加次数的增加而减小.

影像系统MTF算法实现及分析

调制传输函数可对线性影像系统的频率传输特性进行描述。利用调制传输函数计算空间分辨率与线对卡方法相比 ,虽然测量方法复杂 ,但是利用调制传输函数对系统性能进行评价会更客观、更全面。利用编写的调制传输函数计算程序对CT和MRI的频率传输特性进行计算 ,并与线对卡检测结果进行了比较和讨论。

一种优化的刃边法MTF在轨评估算法

调制传输函数MTF(Modulation Transfer Function)是评价光学传感器性能的一个重要的指标。本文对MTF在轨评估技术中的刃边法算法进行深入研究,通过分析对比当前主流算法在刃边法关键环节上的处理方法,对算法进行了优化,然后利用仿真方法对优化算法进行了精度分析,并与国际标准组织提供的MTF评估算法(ISO 12233)进行了比较。精度分析结果表明:在相同仿真条件下,该算法的计算精度优于ISO12233评估算法,适用于高分辨率光学载荷的MTF在轨评估。

航天相机MTF测试靶标研究

在采用矩形靶标测量航天相机系统MTF时,由于采样初始时刻靶标像与CCD像元间存在配准误差,因此给准确测量系统MTF带来很大困难。为了减小配准误差对测量的影响,提出了采用粗分与细分相结合的梯形靶标,并推导出在奈奎斯特频率下动态测量调制传函时,由采样初始时刻配准误差所引起的配准误差调制传递函数MTF计算公式,并按此公式分别得到了粗分精度和靶标斜率对测量相机系统MTF精度的影响。通过分析,采用梯形靶标可以有效减小配准误差对测量系统MTF的影响,提高测量精度。

细节信息容量与MTF相关分析

文章从信息容量的角度出发,对图像宏观质量做出评价,再通过研究图像的细节信息容量参数,对在宏观上质量相当的图像做出更进一步的评价,挑选出边缘清晰、纹理等细节信息丰富的图像。然后将该评价参数与Ealing仪测得的调制传递函数(MTF)值的变化趋势作曲线回归分析,实验结果表明,细节信息容量与数字成像系统的MTF值的变化有着很好的正相关关系,这对评价数字成像系统MTF变化情况具有较好的实际应用价值。

基于MTF辅助正焦判断的自动调焦搜索算法

针对传统爬山搜索算法受调焦评价函数局部极值的干扰而不能准确聚焦的问题,提出一种基于调制传递函数辅助正焦判断的自动调焦搜索算法.该算法对爬山法进行改进,在选择图像清晰度评价函数基础上,同时考虑图像的调制传递函数值,综合判断搜索方向;采用改进的刃边法,在自动完成刃边区域选择的基础上,可精确计算图像的调制传递函数值,并给出了改进算法的原理和实现.为了衡量改进算法的性能,验证调焦系统成像效果,分别考虑搜索算法对不同调焦初始位置以及环境变化影响,在经纬仪自动调焦系统平台上展开算法的调焦实验分析.实验结果表明改进的自动调焦搜索算法能有效地排除局部极值干扰,使系统准确、可靠的调焦.

凝视热成像系统MTF测试技术分析

概述了目前进行凝视热成像系统ＭＴＦ测试的几个主要方法，着重分析了其中的斜缝法和激光散斑法各自解决问题的工作原理、技术特点及技术关键，指出，对于凝视热成像系统的ＭＴＦ测试，这是两种比较值得研究和推广的技术．

利用MTF法分析Varian 23 EX锥形束CT图像空间分辨率

目的:研究在扫描范围内不同空间位置处锥形束CT图像的空间分辨率。方法:使用安装在Varian 23 EX直线加速器上的锥形束CT,重复扫描在不同位置处的Catphan 504模体。模体依次放置于加速器等中心、偏心3 cm、偏心6 cm、偏心12 cm以及不同Z轴位置。扫描采用标准头部模式,重建层厚为2.5 mm。然后利用Matlab 7.0编写程序,测量不同位置处的点扩散函数(PSF),并对PSF进行二维傅里叶变换得到调制传输函数(MTF)。结果:锥形束CT在等中心平面内的空间分辨率最可高达50%MTF=8.15 Lp/cm,并沿着X、Y轴方向偏离等中心时,分辨率逐渐下降。在X=±6 cm、Y=±12 cm时,空间分辨率分别降为50%MTF=5.24 Lp/cm和50%MTF=0.64 Lp/cm;同样的趋势也出现在Z轴方向,当Z=±5 cm时50%MTF=7.16 Lp/cm,Z=±8 cm时50%MTF=4.09 Lp/cm;在Z轴方向上空间分辨率这种变化显然没有X、Y轴方向明显。结论:对于附加在Varian 23 EX直线加速器上的锥形束CT,其图像空间分辨率因扫描野中不同位置而有所变化,因此建议在患者复位时尽量将靶区中心放在等中心左右前后10 cm的空间区域内,在此范围内边缘和中心点可获得接近一致的空间分辨率。