Studies of transverse chromatic aberration based on individual eye model

Two kinds of individual eye models, with and without involving the angle between visual axis and optical axis, are constructed according to the measurements of the corneal surfaces, the optical axis lengths and wavefront aberrations. The constructed eye models are then used to investigate the properties of the transverse chromatic aberration (TCA) and its influence on visual performance. The statistical distributions of the horizontal TCA, vertical TCA and the absolute magnitude of the TCA over the spectrum from 470 to 650 nm are provided, respectively. The variations of TCA with wavelength as referring to the central wavelength of 555 nm for five subjects are also provided. Then we evaluate the impact of TCA on image performance in comparison with longitudinal chromatic aberration (LCA). It is shown that LCA is more detrimental than foveal TCA in image quality. It is found that, while LCA was corrected well, the foveal TCA is simultaneously corrected. Finally, we investigate the correction of peripheral TCA with an achromatizing element. The correction of peripheral TCA of human eye strongly depends on the location of the achromatizing element, and with the element in front of the eye, the peripheral TCA cannot be corrected.     

Contributions of the cornea and the lens to the aberrations of the human eye

The relative contributions of optical aberrations of the cornea and the crystalline lens to the final image quality of the human eye were studied. The aberrations of the entire eye were obtained from pairs of double-pass retinal images, and the aberrations of the cornea were obtained from videokeratographic data. Third-order spherical aberration and coma were significantly larger for the cornea than for the complete eye, indicating a significant role of the lens in compensating for corneal aberrations. In a second experiment retinal images were recorded in an eye before and after we neutralized the aberrations of the cornea by having the subjects wear swimming goggles filled with saline water, providing a direct estimate of the optical performance of the crystalline lens.     

Evaluation of topographical variation in ocular surface temperature by functional infrared thermography

The objective of this work is to evaluate topographical variation in the ocular surface temperature (OST) among the young, elderly and the subjects wearing contact lens using thermographic methodology. We recorded thermographic sequence lasting of 25 s for each eye. The ocular region in each of the thermal images in the sequence was identified and warped into a standard form. Then, the warped sequence was divided into a number of sub-sequences. A differential image which is an image matrix was obtained from each of these sub-sequences, by subtracting thermal images within the sub-sequence. And the histogram of the differential image was modelled by Gaussian mixture model to discriminate eyelashes from the ocular surface for every thermal image in the sub-sequence. Later, thermal data of eyelashes were eliminated in every thermal image and statistical analysis was performed on the sequences. Finally, topographical profile of each subject group was approximated by equations and illustrated using various temperature profiles. The ocular surface of the young subject was observed to be the warmest, and tear film was determined to play a major role in the topographical and temporal variations in OST. Significant topographical variation was observed among subject groups. Based on our compiled average OST profile (AOSTP), the maximum predictability of the bioheat simulation on ocular model can reach up to 90%.     

双树复数小波变换的视网膜图像半盲解卷积复原

针对导致自适应光学视网膜图像降质退化的原因,提出了一种结合双树复数小波变换(DTCWT)和图像半盲解卷积复原算法的方法。首先,对经过自适应光学实时校正技术得到的视网膜图像进行DT-CWT分解,得到低频和高频部分应的图像。将自适应光学成像系统中残余像差重建的光学传递函数作为图像复原模型的初始估计点扩散函数(PSF),并对低频部分图像进行条件约束的迭代半盲解卷积复原;对高频部分的图像进行去噪处理。最后,将处理后的高频和低频部分图像进行双树复数小波逆变换,获得复原图像。实验和结果表明:由该方法处理的视网膜细胞图像质量得到明显提高,图像客观质量评价参数相对于原始图像提高了5倍多;在视网膜细胞的空间频率范围内(70~90(°)~1),复原图像功率谱平均值提高了5倍左右,有助于对视网膜细胞的高分辨率观察。     

基于球面透视投影约束的全景环形透镜畸变校正

全景环形透镜将折反射面集成到一起,能无扫描地瞬时得到围绕光轴的360度超大视场,在机器人导航、视频监控和虚拟现实领域得到了广泛的应用.其成像机理是将围绕光轴的视场二次反射投影到环形平面上,图像存在严重的切向和径向畸变.本文根据全景环形透镜的特点采用基于球面透视投影模型对图像进行校正.首先建立含有畸变参量的全景环形透镜校正模型,将空间直线点映射为球面点,然后使用遗传算法将球面点拟合为球面上的最佳大圆,求出变形校正参量,进而校正全景环形像.仿真和真实图像实验表明,环形图像的切向和径向畸变得到了很好的校正.     

考虑视轴方向的个性化眼模型的构建

包含更多人眼解剖学特性的个性化眼模型具有重要的实验和临床意义。由角膜地形图计算了8只人眼视轴与光轴之间的夹角,水平分量平均值为4.23°±1.51°,竖直分量平均值为-0.40°±1.27°。根据视轴与光轴之间的夹角、角膜地形数据、眼内各部分轴向间距和人眼波像差,运用光学设计软件Zemax分别为这8只人眼构建了考虑视轴方向的个性化眼模型。在此基础上,计算了波前引导的个性化角膜切削深度,并与光程差方法计算的切削深度进行了比较。在切削光区中心处,两者相差不大,平均值为(0.09±0.04)μm;随着半径增大,两者之间的差值逐渐增大。对于所研究的实例,光区外围处的最大差值为0.59μm。个性化眼模型为设计波前引导的个性化角膜切削方案提供了一个有效工具。     

光学系统的低对比分辨本领和象差的公差

具有大象差的光学系统象差公差问题一直还未得到很好解决。本文中提出以低对比分辨本领作为光学系统的质量指标,并将光学系统的传递函数近似表示为正交多项式之和,象差任意大小的光学系统的最佳校正方案和公差之值就可以求出。在小象差情况下,所得的结果与Strehl判断的结果基本一致。 关键词：     

Customized design and efficient machining of astigmatism-minimized progressive addition lens

Astigmatism is inevitable and inherent to progressive addition lenses(PALs), which are typically distributed in the lateral areas on both sides of the progressive corridor. In this study, we took into account the spectacle frame for the customized freeform PAL design with the variational-difference numerical approach. The PAL surface with minimized astigmatism, approximately equal to 84% of the added power, was numerically resolved without expending the zone for clear vision. We validated our approach by experimentally demonstrating the procedure from tool path generation to surface power measurement, thus providing an efficient solution to the personalization of astigmatism-minimized PAL design and manufacture.     

Analysis of Purkinje images as an effective method for estimation of intraocular lens implant location in the eyeball

The implantation of an artificial intraocular lens (IOL) into a capsular bag in place of an extracted natural, but nontransparent lens is a standard therapy method in the case of cataract. Any decentration or tilt of the IOL can cause a deterioration of the retinal image quality, thus decreasing the vision quality. The analysis of the relative position of Purkinje images being a reflection of the point light source from the refracting surfaces of the eyeball determines the location of IOL inside the eye and thus enables to state its wrong or correct location. The paper describes the Purkinje experimental setup consisting of an illuminator (several infrared LEDs arranged in a circle), an imaging telecentric lens and image recording CCD camera and reports details of the system calibration The usefulness of the experimental setup has been proven both in laboratory conditions using artificial model of the eyeball and in measurements on human subjects. Simulations of the retinal image with different decentrations or tilts are shown.     

Hybrid Deconvolution of Adaptive Optics Retinal Images from Wavefront Sensing

Adaptive optics can be used to compensate for the wave aberration of the human eyes to achieve high-resolution imaging in real time. However the correction ＆ partial due to the limitation of hardware. We propose a kind of hybrid image post-processing method, which uses the blind deconvolution combined with the residual data in wavefront sensor to restore the partially adaptive optics corrected retinal image. This method is applied in the image restoration of the vivid human retinal images. The results show that it is effective to improve the retinal image quality.     

Prospective motion correction for 3D pseudo-continuous arterial spin labeling using an external optical tracking system

Head motion is an unsolved problem in magnetic resonance imaging (MRI) studies of the brain. Real-time tracking using a camera has recently been proposed as a way to prevent head motion artifacts. As compared to navigator-based approaches that use MRI data to detect and correct motion, optical motion correction works independently of the MRI scanner, thus providing low-latency real-time motion updates without requiring any modifications to the pulse sequence. The purpose of this study was two-fold: 1) to demonstrate that prospective optical motion correction using an optical camera mitigates artifacts from head motion in three-dimensional pseudo-continuous arterial spin labeling (3D PCASL) acquisitions and 2) to assess the effect of latency differences between real-time optical motion tracking and navigator-style approaches (such as PROMO). An optical motion correction system comprising a single camera and a marker attached to the patient's forehead was used to track motion at a rate of 60 fps. In the presence of motion, continuous tracking data from the optical system was used to update the scan plane in real-time during the 3D-PCASL acquisition. Navigator-style correction was simulated by using the tracking data from the optical system and performing updates only once per repetition time. Three normal volunteers and a patient were instructed to perform continuous and discrete head motion throughout the scan. Optical motion correction yielded superior image quality compared to uncorrected images or images using navigator-style correction. The standard deviations of pixel-wise CBF differences between reference and non-corrected, navigator-style-corrected and optical-corrected data were 14.28, 14.35 and 11.09 mL/100 g/min for continuous motion, and 12.42, 12.04 and 9.60 mL/100 g/min for discrete motion. Data obtained from the patient revealed that motion can obscure pathology and that application of optical prospective correction can successfully reveal the underlying pathology in the presence of head motion.     

人眼视网膜成像自适应光学系统的初步试验和改进

搭建了一套基于液晶空间光调制器的人眼视网膜成像自适应光学系统,进行了活体人眼视网膜的初步实验.经过系统闭环校正,PV值和RMS值分别从2.293λ降低到0.176 553λ,从0.55129λ降低到0.105 11λ,接近衍射极限的水平.获得了较为清晰的人眼视网膜细胞图像,验证了液晶空间光调制器在人眼视网膜高分辨率自适应成像中应用的可行性,并针对试验中的遇到的激光散斑以及照明控制等问题,对原系统提出了一些改进设计.     

高分辨率长焦广角低畸变光学成像系统设计

鉴于全视角高精度三维测量仪中现有光学成像系统无法同时满足大视角、高分辨率和低畸变等技术指标,为此设计了一种能够同时克服上述缺陷的光学成像和畸变校正系统。采用复杂化双高斯结构形式进行f-θ镜头设计,引入非球面提高系统成像质量。实验结果表明,设计的光学系统为长焦广角低畸变高分辨率光学系统,在环境温度-10 ℃~70 ℃下,视场角达到90°,畸变小于-0.001 67%,传递函数达到0.4@100 lp/mm,可实现工作距离3 m~100 m成像清晰。同时,光学系统中非球面镜片的面型精度会对成像质量产生很大的影响,根据公差分析,非球面的面型精度PV值小于0.17 μm时系统成像质量满足要求,实际加工过程中非球面面型PV值达到0.158 μm,传递函数达到设计指标要求,提高了系统的成像质量。     

数码相机原理与系统设计研究

介绍了数码相机的工作原理 ,分析了数码相机的系统结构 ,提出了设计数码相机的处理流程。典型的数码相机系统有镜头、闪光灯、光学取景器、LCD显示屏、图像数据存储扩展设备接口、图像数据传输接口、供电系统以及核心处理器等八个主要模块。数码相机的数据流向从图像传感器开始 ,止于图像数据的存储和传输。数据流的处理主要包括模数转换、光学黑电平钳位、针对镜头的边缘畸变的运算修正、坏像素处理、白平衡处理、伽马校正、色彩合成处理、边缘检测 (锐度检测 )和伪彩色检测 (伪彩色抑制 )、JPEG压缩和图像存储器等模块     

Lucky averaging: quality improvement of adaptive optics scanning laser ophthalmoscope images

Adaptive optics (AO) has greatly improved retinal image resolution. However, even with AO, temporal and spatial variations in image quality still occur due to wavefront fluctuations, intraframe focus shifts, and other factors. As a result, aligning and averaging images can produce a mean image that has lower resolution or contrast than the best images within a sequence. To address this, we propose an image postprocessing scheme called "lucky averaging," analogous to lucky imaging [J. Opt. Soc. Am. 68, 1651 (1978)] based on computing the best local contrast over time. Results from eye data demonstrate improvements in image quality.     

Capability of a Liquid-Crystal Adaptive Optics System Based on Feedback Interferometry for Retinal Imaging

A modified arrangement of the adaptive optical retinal imaging system that we described previously is proposed to reduce the intensity loss in the system, so that it works properly even when the intensity of light incident on the eye is very weak. Experiments to verify the system performance were conducted using a conventional artificial eye with a specular reflector as a model retina. We observed that an image of a test target (mimicking a retina) blurred by an aberration plate (mimicking the ocular aberrations) was successfully restored in the adaptive optics fashion even when the intensity of the incident light probing the aberration of the eye became about 1.5% of that required in the previous system. Effect of a more realistic artificial eye with a scattering object as a model retina was also examined experimentally. We found that not only the ocular aberrations, but also the retinal scattering cause the wave-front deformations and that our adaptive optics system compensates for both of them simultaneously.     

Design of optical system for head-mounted micro-display

Assembling one optical imaging system with 0.56″ liquid crystal on silicon (LCOS), we achieved one practical head-mounted micro-display system for civilian applications. The optical system consists of three refractive elements and one binary surface. It has a 32° field of view (FOV), a 10 mm exit pupil diameter, and a 20 mm eye clearance distance. Compared with the traditional optical system, the hybrid refractive/diffractive system has a longer working distance and can be used in higher resolution display with better image quality. Designing results indicate that the introducing of binary element in the system with smaller FOV could improve the image quality dramatically.     

The study of correction for chromatic aberrations of human eye with diffractive–refractive hybrid elements

A chromatic corrector with two refractive–diffractive hybrid elements is designed over a reasonable wide field-of-view of 7° to correct the chromatic aberrations of human eye. We investigate the performance of the corrector using several optical criteria in a comparison with the chromatic corrector with single refractive–diffractive element. Both the correctors can correct the longitudinal chromatic aberration (LCA) fairly. It is demonstrated that the chromatic corrector with single element introduces serious eccentric transverse chromatic aberration (TCA), which deteriorates the image quality at the edge field-of-view. On using the chromatic corrector with two refractive–diffractive elements, not only the LCA but also the TCA can be corrected very well. The designed chromatic corrector can be used to improve the retinal image quality and has potential application in the design of visual instruments.     

The importance of coherence of coupling when viewing through visual aids

It is shown that there may be major problems in the use of mtf's in assessing visual systems because of the coherence of coupling between the visual aid and the refraction optics of the eye. A simple computer program has been set up to help study the errors in predictions of retinal image quality made from mtf measurements. This can be used to produce complex amplitude spread functions cheaply for each element of a system and for cascaded systems. Mtf's derived from multiplying the individual mtf's of a visual aid and of the eye's refraction optics are compared with those derived from complex amplitude wavefront analysis     

全景环形透镜二维平面成像展开算法研究

针对基于平面圆柱透视法PCF(Plat Cylinder Perspective)设计的全景环形透镜PAL(Panoramic Annular Lens)的环行像存在畸变失真的现象,对全景环形透镜PAL的二维平面环形像设计了展开算法,分别在切向和径向进行线性化处理.算法采用了线性插值,使展开后图像无盲区,同时建立在FCP成像原理上的转化,实现了对二维平面PAL像进行真实恢复,且效果良好.