Application of bispectral speckle imaging to near-diffraction-limited imaging in the presence of unknown aberrations

A laboratory experiment that demonstrates near-diffraction-limited imaging of a detailed object in the presence of unknown fixed aberrations in the imaging system is described. A random-phase plate is introduced in a pupil plane of the imaging system to eliminate the effect of fixed aberrations in the system. We employ a bispectral speckle imaging technique to recover the object from speckled images affected by both the random-phase fluctuations induced by the random-phase plate and the fixed aberrations present in the imaging system. For the case where the random phase is assumed to obey Gaussian statistics an approximate form of the bispectral speckle transfer function is obtained with an asymptotic expansion. This approximate form of the transfer function shows the diffraction-limited nature of bispectral speckle imaging when the standard deviation of the random-phase fluctuations is of the order of a wavelength of light. Experimental results are presented for fixed aberrations associated with lens tilt and defocus in the imaging system.     

用斑点成像消除光学系统像差的影响

在详细分析用斑点成像消除目标图像中随机扰动影响的基础上,提出了在有像差光学系统中,应用斑点成像消除图像中像差影响的方法。通过在光学系统的光路中引入随机相位屏,采集物的短曝光像,用斑点成像处理,恢复目标图像的功率谱和相位谱,可以得到目标近衍射极限的图像。实验结果表明,这种方法可以消除图像中光学系统像差的影响。     

Effects of Aberrations on Transfer Functions Used in High Angular Resolution Astronomical Imaging

Abstract

In this paper we present the results of the effect of aberrations on the transfer functions used in the high-angular resolution astronomical imaging techniques of speckle interferometry, Knox-Thompson and bispectral imaging. The analyses are based on a computer simulation of imaging through atmospheric turbulence. The results show that as the seeing becomes worse, its effect dominates the behaviour of the transfer functions which tend to be independent of (small) optical aberrations. However, if the wavefront variation due to fixed aberrations is significant over r ₀-sized regions in the pupil (where r ₀ is the Fried parameter), the above transfer functions do depend on the aberration: in particular, the bispectral transfer function is relatively sensitive to odd aberrations, such as coma.     

Analytical and Numerical Examination of the Quantitative Imaging Properties of Optical Diffraction Tomography

Abstract

The imaging properties of two-dimensional optical diffraction tomography are examined analytically and numerically. Taking into account lens aberrations in the optical system used to image the scattered light, we first derive an expression for the overall coherent transfer function. Then we use computer simulations to examine the influence of lens aberrations by comparing images obtained by a system with lens aberrations with images obtained by an aberration-free system. We also discuss the significance of systematic positioning errors in the set-up.     

Improving retinal image resolution with iterative weighted shift-and-add

High-resolution retinal imaging requires dilating the pupil, and therefore exposing more aberrations that blur the image. We developed an image processing technique that takes advantage of the natural movement of the eye to average out some of the high-order aberrations and to oversample the retina. This method was implemented on a long sequence of retinal images of subjects with normal vision. We were able to resolve the structures of the size of single cells in the living human retina. The improvement of resolution is independent of the acquisition method, as long as the image is not warped during scanning. Consequently, even better results can be expected by implementing this technique on higher-resolution images.     

一种基于YC-rC-b颜色空间和视觉特性的彩色图像水印技术

根据图像离散余弦变换频谱图特征及结合人眼对亮度和颜色的对比度敏感视觉特性,提出了一种人眼感知图像最小误差即JND阈值的计算方法,通过计算出的JND阈值,提出了一种基于YCrCb颜色空间的彩色图像水印技术方案。方案依据JND阈值筛选彩色图像的3个分量亮度图变换域频谱系数来进行水印的嵌入和提取。从水印技术方案上看,JND阈值的引入使得水印的嵌入达到最大;嵌入算法使得水印嵌入的位置具有随机性,有力地保证了水印信息的安全性。仿真实验和攻击测试表明:含水印图经压缩攻击后的图像与原始图像基本一样,提取的水印信息与原始水印基本相同;攻击后的含水印图具有较好的透明性和较高的鲁棒性。提出的基于YCrCb颜色空间和人眼视觉特性的彩色图像水印技术方案,是一种可行的、较好的彩色图像技术。     

An ex vivo rat eye model to aid development of high-resolution retina imaging devices for rodents

High resolution in vivo retinal imaging in rodents is becoming increasingly important in eye research. Development of suitable imaging devices currently requires many lengthy animal procedures. We present an ex vivo rat model eye with fluorescently labelled retinal ganglion cells (RGC) and nerve fibre bundles that reduces the need for animal procedures while preserving key properties of the living rat eye. Optical aberrations and scattering of four model eyes and eight live rat eyes were quantified using a Shack–Hartmann sensor. Fluorescent images from RGCs were obtained using a prototype scanning laser ophthalmoscope. The wavefront aberration root mean square value without defocus did not significantly differ between model and living eyes. Higher order aberrations were slightly higher but RGC image quality was comparable to published in vivo work. Overall, the model allows a large reduction in number and duration of animal procedures required to develop new in vivo retinal imaging devices.     

Degree of polarization as an objective method of estimating scattering

A new method of determining objectively the amount of scattered light in an optical system has been developed. It is based on measuring the degree of polarization of the light in images formed after a double pass through the system. A dual apparatus composed of a modified double-pass imaging polarimeter and a wave-front sensor was used to measure polarization properties and aberrations of the system under test. We studied the accuracy of the procedure in a system that included a lanthanum-modified lead zirconate titanate (PLZT) ceramic plate able to generate variable amounts of scattered light as a function of the applied voltage. Changes in the voltage applied to the ceramics plate modified significantly the scattering contribution while hardly altering the wave-front aberration. The degree of polarization was well correlated with the level of scattering in the system as determined by direct-intensity measurements at the tails of the double-pass images. This indicates that this polarimetric parameter provides accurate relative estimates of the amount of scattering generated in a system. The technique can be used in a number of applications, for example, to determine objectively the amount of scattered light in the human eye.     

Derivation of the Lorentzian probability model for use in constrained image restoration

The Lorentzian nonlinear restoration algorithm is based on the empirical observation that the gradients in a general image have a Lorentzian probability density. In this paper an attempt is made to justify this observation by modelling the scene as a series of random sized features each with a random change in intensity across them. The high spatial frequency content of an image exists largely at edges and sharp features and is often lost due to aberrations in the optics of imaging systems. Nonlinear image restoration may be used to recover these frequencies, the most effective methods being constrained. In many nonlinear restoration techniques the amount of high spatial frequency content introduced into the restored image is uncontrolled. This problem has been overcome through the use of the Lorentzian algorithm, which imposes a statistical constraint on the distribution of gradients within the restored image. Images are presented to demonstrate the effectiveness of this method.     

3D-image restoration technique using Genetic Algorithm to solve blurring problems of images

Most images may not be sharp and clear due to various reasons like noise interference and is said to be in a blurred condition. Image de-blurring is fundamental in making pictures sharp and useful. Normally, along with the input blurred image, Point Spread Function (PSF) of the original image is required for the process of restoration and de-blurring. In this paper, we introduce a technique for image restoration by Richardson–Lucy algorithm where the optimised PSF is generated by the use of Genetic Algorithm (GA). Use of optimised PSF ensures that our proposed technique does not need the original image for the de-blurring purpose and can be greatly beneficial in the real time scenario cases. The dataset used for the evaluation of the proposed technique are real 3D images and the evaluation metrics used are peak signal-to-noise ratio (PSNR), Second-Derivative like Measure of Enhancement (SDME) and mean squared error (MSE). The technique is compared with existing techniques such as de-convolution method, regularisation filter, Wiener filter and Richardson–Lucy algorithm. From the results, we can observe that our proposed technique has achieved higher PSNR and SDME values and lower MSE values when compared with other techniques. We have achieved average PSNR of 70·94, SDME of 71·46 and MSE of 0·0063. The values obtained show the superior performance of the proposed technique.     

Effect of aberrations and scatter on image resolution assessed by adaptive optics retinal section imaging

The effect of increased high-order wavefront aberrations on image resolution was investigated, and the performance of adaptive optics (AO) for correcting wavefront error in the presence of increased light scatter was assessed in a model eye. An AO section imaging system provided an oblique view of a model retina and incorporated a wavefront sensor and deformable mirror for measurement and compensation of wavefront aberrations. Image resolution was quantified by the width of a Lorentzian curve fitted to a laser line image. Wavefront aberrations were significantly reduced with AO, resulting in improvement of image resolution. In the model eye, image resolution was degraded with increased high-order wavefront aberrations (horizontal coma and spherical) and improved with AO correction of wavefront error in the presence of increased light scatter. The findings of the current study suggest that AO imaging systems can potentially improve image resolution in aging eyes with increased aberrations and scatter.     

RETRACTED: Implementation System of Human Eye Tracking Algorithm Based on FPGA

With the high-speed development of transportation industry, highway traffic safety has become a considerable problem. Meanwhile, with the development of embedded system and hardware chip, in recent years, human eye detection eye tracking and positioning technology have been more and more widely used in man-machine interaction, security access control and visual detection.
In this paper, the high parallelism of FPGA was utilized to realize an elliptical approximate real-time human eye tracking system, which was achieved by the series register structure and random sample consensus (RANSAC), thus improving the speed of image processing without using external memory. Because eye images acquired by the camera often generate a lot of noises due to uneven light and dark background, the preprocessing technologies such as color conversion, image filtering, histogram modification and image sharpening were adopted. In terms of feature extraction of images, the eye tracking algorithm in this paper adopted seven-section rectangular eye tracking characteristic method, which increased a section between the mouth and the nose on the basis of the traditional six-section method, so its recognition accuracy is much higher. It is convenient for the realization of hardware parallel system in FPGA. Finally, aiming at the accuracy and real-time performance of the design system, a more comprehensive simulation test was carried out.
The human eye tracking system was verified on DE2-115 multimedia development platform, and the performance of VGA (resolution: 640×480) images of 8-bit grayscale was tested. The results showed that the detection speed of this system was about 47 frames per second under the condition that the detection rate of human face (front face, no inclination) was 93%, which reached the real-time detection level. Additionally, the accuracy of eye tracking based on FPGA system was more than 95%, and it has achieved ideal results in real-time performance and robustness.     

Phase-aberration correction using signals from point reflectors and diffuse scatterers: measurements

A method for phase-aberration correction of phased-array images is tested using a model of near-field velocity inhomogeneities. A set of grooved room-temperature vulcanizing plates was constructed to simulate near-field aberrations encountered in clinical ultrasound imaging. As expected, large image distortion was experienced when grooved plates producing significant aberrations were placed near the surface of the array. An iterative aberration correction procedure based on cross-correlation measures between neighboring elements in a phased array, using signals reflected from diffuse scatterers, significantly reduced the effects of these aberrations, producing images nearly identical to those generated in the absence of aberrations. The results suggest that a practical phase-aberration correction system can be constructed for medical ultrasound imaging and possibly all coherent imaging systems by using a sampled aperture.     

Performance of focus measures in the presence of nondefocus aberrations

The purposes of the study were to compare the performance of ten representative focus measures in the presence of nondefocus aberrations and to evaluate their applicability to the eye. For fixed amounts of nondefocus aberrations, the amount of defocus was changed to generate a series of blurred images from which focus measure curves were derived. In the presence of small amounts of nondefocus aberrations, all focus measures showed unimodal and monotonic behavior, although there were large differences in their sensitivity to defocus and effective ranges. There were breakdowns in monotonicity and unimodality for some focus measures when applied to data from human eyes, while other focus measures could detect the shift in the best-focus plane in the blurred image series resulting from spherical aberration.     

Lossless compression-based progressive image transmission scheme

Abstract

The lossless compression of images is widely used in medical imaging and remote sensing applications. Also, progressive transmission of images is often desirable because it can reduce the transmission bits of an image. Therefore, combining the features of lossless compression and progressive transmission of images has been intensely researched. The bitplane method (BPM) is the simplest way to implement a lossless progressive image transmission system. In the present paper, a new block-based scheme for lossless progressive image transmission is proposed. This scheme will reduce the transmission load and improve the image quality of BPM. This method first performs a quantization operation upon the blocks of an image. Next, these blocks are encoded with fewer bits, and the bits are then transmitted phase by phase. The experimental results show that the image quality of this method is better than those in the BPM and improved BPM in related traditional works under the same transmission load. Moreover, during the first phase, the difference in peak signal-to-noise ratio between the present method and BPM is exactly equal up to 8.85 dB. This method is therefore effective for lossless progressive image transmission.     

Calculation of retinal image quality for polychromatic light

Although the retinal image is typically polychromatic, few studies have examined polychromatic image quality in the human eye. We begin with a conceptual framework including the formulation of a psychophysical linking hypothesis that underlies the utility of image quality metrics based on the polychromatic point-spread function. We then outline strategies for computing polychromatic point-spread functions of the eye when monochromatic aberrations are known for only a single wavelength. Implementation problems and solutions for this strategy are described. Polychromatic image quality is largely unaffected by wavelength-dependent diffraction and higher-order chromatic aberration. However, accuracy is found to depend critically upon spectral sampling. Using typical aberrations from the Indiana Aberration Study, we assessed through-focus image quality for model eyes with and without chromatic aberrations using a polychromatic metric called the visual Strehl ratio. In the presence of typical levels of monochromatic aberrations, the effect of longitudinal chromatic aberration is greatly reduced. The effect of typical levels of transverse chromatic aberration is virtually eliminated in the presence of longitudinal chromatic aberration and monochromatic aberrations. Clinical value and limitations of the method are discussed.     

Influence of the gaze-stabilizing eye movements on the quality of the retinal image of the human eye

Abstract

In this work we have studied the influence of the gaze stabilizing movements of the eye on the quality of the retinal image of the human eye obtained by double pass methods. The results obtained agree with the expected differences between the coherent and incoherent behaviour of the optical system of the eye. The movements-free retinal image is obtained from a typical retinal image by considering a filter function in the frequency domain which characterizes the effect of the considered movements.     

基于变分贝叶斯多图像超分辨的平面复眼空间分辨率增强

平面复眼成像系统利用多个子孔径对场景进行成像,由于子孔径大小和图像传感器空间采样率的限制,各子孔径图像质量较差。如何融合多个子孔径图像来获得高分辨率图像是亟需解决的问题。多图像超分辨理论利用多幅具有互补信息的图像来重构高空间分辨率图像,然而现有理论通常采用过于简化的运动模型,这种简化的运动模型对平面复眼成像并不完全适用。若直接把现有多图像超分辨理论用于平面复眼分辨率增强,不准确的相对运动估计将降低图像分辨率增强性能。针对这些问题,本文在变分贝叶斯框架下改进了现有多图像超分辨理论中的运动模型,并把导出的联合估计算法用于平面复眼分辨率增强。仿真数据实验和真实复眼数据实验验证了推荐方法的正确性和有效性。     

Holographic optical elements in the presence of spherical aberration and focus error: Some remarks on imaging quality

Abstract

When aberrations are small, it is possible to use wave aberration variance to obtain the position of the best image plane in both conventional and holographic optical systems. However, when aberrations increase, this measurement is not sufficient and it does not provide the correct plane position for the best image. In this paper, two alternative methods for evaluating image quality in spherically aberrated on-axis holographic lenses are compared. These two methods are the standard deviation of the distribution of light intensity and entropy. Both methods are based on the calculation of the diffraction integral and the subsequent analysis of the distribution of light intensity as a probability density function for each image plane being analysed. Even though the results obtained with both methods are identical when aberrations are small, only entropy provides the best image when the value of these aberrations increases.     

Double Passage Imaging Through a Random Screen Using a Non-redundant Aperture

Abstract

When a deterministic object situated in the near or far-field is coherently illuminated and viewed through the same random screen, the time-averaged image contains diffraction-limited information. We demonstrate that the use of a non-redundant aperture considerably simplifies the retrieval of this information. The effects of the number of sub-apertures, finite sub-aperture size and finite exposure time on the imaging process are discussed. The optimal procedure for recovery of object spatial frequencies is shown to be sequential, using a two pinhole or Michelson-Fizeau aperture. Computer-simulated reconstructions of simple object distributions are presented.