Catálogo de publicaciones - libros

We presented “A small adaptive optical system on table for human retinal imaging” at the 3rd Workshop on Adaptive Optics for Industry and Medicine. In this system, a 19 element small deformable mirror was used as wavefront correction element. High resolution images of photo receptors and capillaries of human retina were obtained. In recent two years, at the base of this system a new adaptive optical system for human retina imaging has been developed. The wavefront correction element is a newly developed 37 element deformable mirror. Some modifications have been adopted for easy operation. Experiments for different imaging wavelengths and axial positions were conducted. Mosaic pictures of photoreceptors and capillaries were obtained. 100 normal and abnormal eyes of different ages have been inspected.The first report in the world concerning the most detailed capillary distribution images cover ±3° by ± 3° field around the fovea has been demonstrated. Some preliminary very early diagnosis experiment has been tried in laboratory. This system is being planned to move to the hospital for clinic experiments.

Adaptive optics for the human eye has two main applications: to obtain high-resolution images of the retina and to produce aberration-controlled retinal images to simulate vision. We built an adaptive optics prototype specially designed for visual testing based on the use of a liquid crystal spatial light modulator (Hamamatsu X8267). The system consists of a measurement channel with switchable red (633 nm) or infrared (780 nm) illumination, a real time Hartmann-Shack sensor (25 Hz), and an additional channel allowing subjects to perform visual tasks in green light through the adaptive optics system. We tested the modulator, both as aberration generator and as corrector, first in an artificial eye and then routinely in different living eyes. This device has advantages in terms of effective stroke and mode independence allowing production and compensation of a larger range of aberrations than with other correcting devices. This opens new possibilities for visual applications of adaptive optics. However, a low temporal response and diffraction effects may be important drawbacks of the modulator for some particular applications. Examples of the performance of the system and a discussion of its limitations and potential for performing visual optics experiments will be presented.

We have developed a prototype of a confocal scanning laser ophthalmoscope that incorporates adaptive optics to correct for the wavefront aberrations of the eye and those induced by the optical system. Two corrector devices were tried out in the experiments: a membrane deformable mirror and a liquid crystal spatial light modulator. We obtained high-resolution images of different parts of the retina with and without the wavefront correction. We also explored alternative adaptive optics configurations to improve on the imaging performance of the system.

The spatial resolution of retinal images is limited by the presence of time-varying aberrations present within the eye. A low-order adaptive optics system using a Shack-Hartmann wavefront sensor and a bimorph deformable mirror is described. Using a standard Hitachi video camera and a PC running Windows 2000, control of low-order aberrations within the eye can be achieved, enabling high resolution images to be obtained. The wavefront sensor employs a novel dithered reference technique to reduce speckle recorded in the hartmannogram. As a measurement of the residual errors within the image is known, it is possible to use post-processing techniques to further improve the resolution of the image. Example images of retinas taken with the system and then post-processed are shown, demonstrating the ability to calculate near diffraction-limited images. Data on the performance of the system are presented.

: The objective of this project was to apply an image restoration methodology based on wavefront measurements obtained with a Shack-Hartmann sensor and evaluating the restored image quality based on medical criteria.: Implementing an adaptive optics (AO) technique, a fundus imager was used to achieve low-order correction to images of the retina. The high-order correction was provided by deconvolution. A Shack-Hartmann wavefront sensor measures aberrations. The wavefront measurement is the basis for activating a deformable mirror. Image restoration to remove remaining aberrations is achieved by direct deconvolution using the point spread function (PSF) or a blind deconvolution. The PSF is estimated using measured wavefront aberrations. Direct application of classical deconvolution methods such as inverse filtering, Wiener filtering or iterative blind deconvolution (IBD) to the AO retinal images obtained from the adaptive optical imaging system is not satisfactory because of the very large image size, dificulty in modeling the system noise, and inaccuracy in PSF estimation. Our approach combines direct and blind deconvolution to exploit available system information, avoid non-convergence, and time-consuming iterative processes. : The deconvolution was applied to human subject data and resulting restored images compared by a trained ophthalmic researcher. Qualitative analysis showed significant improvements. Neovascularization can be visualized with the adaptive optics device that cannot be resolved with the standard fundus camera. The individual nerve fiber bundles are easily resolved as are melanin structures in the choroid. : This project demonstrated that computer-enhanced, adaptive optic images have greater detail of anatomical and pathological structures.

We present the novel instrument combining the Shack-Hartman wavefront sensor, compensator of low order aberrations and bimorph adaptive mirror for high order aberration correction. We have tested the developed instrument in the clinical environment on human subjects. The measurement rate was 77 frames per second. The typical residual error of correction was between 0.1-0.15 microns was achieved. The wavefront can be reconstructed in form of up to 36 Zernike polynomials. Automatic low order aberration compensator allows introduction of spherical and astigmatic terms with amplitudes up to ±15D and ±6D correspondingly and accuracy of 0.05 D. An 18 electrode bimorph deformable mirror corrector makes it possible to model Zernike aberrations up to the 3-th order + Spherical aberration with RMS amplitudes up to 1 micron. The results of measurements were compared with clinical refraction and good correspondence was found.

In the classical Hartmann test analysis, the transverse aberrations are represented by small straight segments joining two consecutive measured points in the pupil. In the analysis presented in this paper we propose the wavefront to be synthesized by many nonflat functions whose domains are the squares defined by the square unit cell defined by an array of holes in the Hartmann screen placed at the exit pupil of the system. Two advantages of this method are that a higher precision in the wavefront retrieval is obtained and second that the local curvatures and astigmatism with their corresponding axes are obtained.

This paper reports on the results of wavefront curvature sensor measurements over horizontal paths of 66m and 4 km. The wavefront curvature sensor used has been developed at QinetiQ and is based on the use of a quadratically distorted diffraction grating to enable the simultaneous recording of two symmetrically separated planes about the entrance pupil of a telescope. The measurements allow us to characterize the spatio-temporal nature of the wavefront errors and therefore enable us to estimate the wavefront sensor (WFS) and deformable mirror (DM) requirements for the development of an adaptive optic system (AOS). For the 66m path the dynamic range and frame-rate of the WFS camera was found to be adequate to drive the AOS, although the software based control resulted in intermittent performance. The data for the 4 km path suggested that the frame-rate of the WFS camera was at least a factor of 3 slower than would be necessary to either drive the AOS or make any detailed conclusions about the spatial analysis.

Based on the fact that the returned photon numbers are too low in Lunar Laser Ranging (LLR) and the new method that effects of atmospheric tiptilt are considered, the two algorithms for computation of atmospheric tip-tilt, absolute difference and cross correlation, are investigated. Programs are applied to the extended target of the lunar surface. The atmospheric tip-tilt is computed in different conditions, the best algorithm is selected. A new method is put forward to reduce the time in atmospheric tip-tilt computing, and the possibility on real-time compensation in Lunar Laser Ranging is positive.